Monday, October 29, 2007

Bush's Dangerous liaisons

Op-Ed Contributor
Bush’s Dangerous Liaisons




By FRANÇOIS FURSTENBERG
Published: October 28, 2007


Montreal


MUCH as George W. Bush’s presidency was ineluctably shaped by Sept. 11, 2001, so the outbreak of the French Revolution was symbolized by the events of one fateful day, July 14, 1789. And though 18th-century France may seem impossibly distant to contemporary Americans, future historians examining Mr. Bush’s presidency within the longer sweep of political and intellectual history may find the French Revolution useful in understanding his curious brand of 21st- century conservatism.


Soon after the storming of the Bastille, pro-Revolutionary elements came together to form an association that would become known as the Jacobin Club, an umbrella group of politicians, journalists and citizens dedicated to advancing the principles of the Revolution.


The Jacobins shared a defining ideological feature. They divided the world between pro- and anti-Revolutionaries — the defenders of liberty versus its enemies. The French Revolution, as they understood it, was the great event that would determine whether liberty was to prevail on the planet or whether the world would fall back into tyranny and despotism.


The stakes could not be higher, and on these matters there could be no nuance or hesitation. One was either for the Revolution or for tyranny.


By 1792, France was confronting the hostility of neighboring countries, debating how to react. The Jacobins were divided. On one side stood the journalist and political leader Jacques-Pierre Brissot de Warville, who argued for war.


Brissot understood the war as preventive — “une guerre offensive,” he called it — to defeat the despotic powers of Europe before they could organize their counter-Revolutionary strike. It would not be a war of conquest, as Brissot saw it, but a war “between liberty and tyranny.”


Pro-war Jacobins believed theirs was a mission not for a single nation or even for a single continent. It was, in Brissot’s words, “a crusade for universal liberty.”


Brissot’s opponents were skeptical. “No one likes armed missionaries,” declared Robespierre, with words as apt then as they remain today. Not long after the invasion of Austria, the military tide turned quickly against France.


The United States, France’s “sister republic,” refused to enter the war on France’s side. It was an infuriating show of ingratitude, as the French saw it, coming from a fledgling nation they had magnanimously saved from foreign occupation in a previous war.


Confronted by a monarchical Europe united in opposition to revolutionary France — old Europe, they might have called it — the Jacobins rooted out domestic political dissent. It was the beginning of the period that would become infamous as the Terror.


Among the Jacobins’ greatest triumphs was their ability to appropriate the rhetoric of patriotism — Le Patriote Français was the title of Brissot’s newspaper — and to promote their political program through a tightly coordinated network of newspapers, political hacks, pamphleteers and political clubs.


Even the Jacobins’ dress distinguished “true patriots”: those who wore badges of patriotism like the liberty cap on their heads, or the cocarde tricolore (a red, white and blue rosette) on their hats or even on their lapels.


Insisting that their partisan views were identical to the national will, believing that only they could save France from apocalyptic destruction, Jacobins could not conceive of legitimate dissent. Political opponents were treasonous, stabbing France and the Revolution in the back.


To defend the nation from its enemies, Jacobins expanded the government’s police powers at the expense of civil liberties, endowing the state with the power to detain, interrogate and imprison suspects without due process. Policies like the mass warrantless searches undertaken in 1792 — “domicilary visits,” they were called — were justified, according to Georges Danton, the Jacobin leader, “when the homeland is in danger.”


Robespierre — now firmly committed to the most militant brand of Jacobinism — condemned the “treacherous insinuations” cast by those who questioned “the excessive severity of measures prescribed by the public interest.” He warned his political opponents, “This severity is alarming only for the conspirators, only for the enemies of liberty.” Such measures, then as now, were undertaken to protect the nation — indeed, to protect liberty itself.


If the French Terror had a slogan, it was that attributed to the great orator Louis de Saint-Just: “No liberty for the enemies of liberty.” Saint-Just’s pithy phrase (like President Bush’s variant, “We must not let foreign enemies use the forums of liberty to destroy liberty itself”) could serve as the very antithesis of the Western liberal tradition.


On this principle, the Terror demonized its political opponents, imprisoned suspected enemies without trial and eventually sent thousands to the guillotine. All of these actions emerged from the Jacobin worldview that the enemies of liberty deserved no rights.


Though it has been a topic of much attention in recent years, the origin of the term “terrorist” has gone largely unnoticed by politicians and pundits alike. The word was an invention of the French Revolution, and it referred not to those who hate freedom, nor to non-state actors, nor of course to “Islamofascism.”


A terroriste was, in its original meaning, a Jacobin leader who ruled France during la Terreur.


François Furstenberg, a professor of history at the University of Montreal, is the author of "In the Name of the Father: Washington’s Legacy, Slavery and the Making of a Nation."




Powered by ScribeFire.

Thursday, October 25, 2007

CONCRETO

O Concreto é uma mistura, em determinadas proporções, de quatro componentes básicos: cimento, pedra, areia e água.

Tipos de concreto: simples, armado e magro.

O concreto simples é preparado com os 4 componentes básicos e tem grande resistência aos esforços de compressão, mas baixa resistência aos esforços de tração.

Já o concreto armado tem elevada resistência tanto aos esforços de tração como aos de compressão, mas para isso precisa de um quinto componente: armadura ou ferro.

O concreto magro é na verdade um concreto simples com menos cimento. Ele é mais econômico mas só pode ser usado em partes da construção que não exijam tanta resistência e impermeabilidade.

COMPONENTES DO CONCRETO

1.Cimento

As matérias primas do cimento são calcário, argila, gesso e outros materiais enominados adições. A sua fabricação exige grandes e complexas instalações industriais, como um possante forno giratório que chega a atingir temperaturas próximas a 1500ºC.

No mercado existem diverso tipos de cimento. A diferença entre eles está na composição, mas todos atendem às exigências das Normas Técnicas Brasileiras. Cada tipo tem o nome e a sigla correspondente estampada na embalagem, para facilitar a identificação. Os tipos de cimento adequados aos usos gerais no meio rural são os seguintes:



NOME
SIGLA (estampada na embalagem)

CIMENTO PORTLAND comum com adição
CP I-S-32
CIMENTO PORTLAND composto com escória
CP II-E-32
CIMENTO PORTLAND composto com pozolana
CP II-Z-32
CIMENTO PORTLAND composto com filer
CP II-F-32
CIMENTO PORTLAND de alto forno
CP III-32
CIMENTO PORTLAND pozolânico
CP IV-32
Existem ainda outros tipos de cimento para usos específicos.

Em sua embalagem original - sacos de 50 kg - o cimento pode ser armazenado por cerca de 3 meses, desde que o local seja fechado coberto e seco. Além disso, o cimento deve ser estocado sobre estrados de madeira, em pilhas de 10 sacos, no máximo.



2. Pedra

A pedra utilizada no concreto pode ser de dois tipos:

- seixo rolado de rios, cascalho ou pedregulho;
- pedra britada ou brita.

Os seixos rolados são encontrados na natureza. A pedra britada é obtida pela britagem mecânica de deterninadas rochas duras.

Independentemente da origem, o tamanho das pedras varia muito e tem influência na qualidade do concreto. Por isso, as pedras são classificadas por tamanhos medidos em peneiras (pela abertura da malha). As Normas Técnicas brasileiras estabelecem 6 tamanhos:



TAMANHO DAS PEDRAS
Pedra zero (ou pedrisco)
4,8mm a 9,5mm

Pedra1
9,5mm a 19mm

Pedra2
19mm a 25mm

Pedra3
25mm a 38mm

Pedra4
38mm a 76mm

Pedra-de-mão
O concreto das benfeitorias rurais pode ser feito com pedras 1 ou 2, as mais encontradas no comércio de materiais de construção.

Se forem utilizados seixos rolados, cascalho ou pedregulho, das propriedades, convém classificar esse material antes de seu uso. A forma mais simples, porém menos precisa, de fazer isso é apanhar um punhado de pedras do monte a ser usado e medir a maior dimensão de cada uma com uma régua milimitrada. A maioria das pedras medidas deverá se enquadrar da faixa de pedra 1 (9,5mm a 19mm) e pedra 2 (19mm a 25mm).

Caso o material disponível não esteja de acordo com essas medidas, consulte um proffisional especializado a respeito.

Tanto os seixos rolados como a pedra britada devem estar limpos antes de seu uso. O pó de britagem, o barro da jazida, galhos, folhas, raízes, devem ser retirados à mão ou por lavagem.



3-Areia

A areia utilizada no concreto é obtida em leitos e margens de rios, ou em portos e bancos de areia.

A areia deve ter grãos duros. E, assim como a pedra, ela também precisa estar limpa e livre de torrões de barro, galhos, folhas e raízes antes de ser usada.

As Normas Técnicas Brasileiras classificam a areia, segundo o tamanho de seus grãos, em: muito fina, fina, média, grossa.

Mas isso só tem importância em obras de maior porte. Nesses casos, é necessário consultar um profissional especializado, pois essa classificação só pode ser feita, com precisão, em laboratório.



4- Água

A água a ser utilizada no concreto deve ser limpa - sem barro, óleo, galhos, folhas e raízes. Em outras palavras, água boa para o concreto é água de beber. Nunca use água servida ( de esgoto humano ou animal, de cozinha, de fábricas, etc.) no preparo do concreto.



5- Armadura

A armadura é conposta de barras de aço, também chamadas de ferro de construção ou vergalhões. Eles têm a propriedade de se integrar ao concreto e de apresentar elevada resistência à tração. Por isso, são colocados nas partes da peça de concreto que vão sofrer esse esforço. Por exemplo, numa viga apoiada nas extremidades, a parte de cima sofre compressão e a de baixo, tração. Nesse caso, os
vergalhões devem ficar na parte debaixo das vigas.


Os vergalhões que compõem a armadura são amarrados uns aos outros com arame recozido.

Existem também armaduras pré-fabricadas, que ja vêm com os vergalhões unidos entre si: são as telas soldadas, que servem de armadura para lajes e pisos.

A maioria dos vergalhões tem saliências na superfície.

As Normas Técnicas Brasileiras classificam os vergalhões para concreto de acordo com a sua resistência e padronizam as bitolas. Há 3 categorias no mercado: aço CA 25, aço CA 50, aço CA 60.

Os números 25, 50 e 60 referem-se á resistência do aço : quanto maior o número, mais resistente será o vergalhão.

Os vergal`ões são vendidos em barras retas ou dobradas, com 10m a 12m de comprimento. Eles são cortados e dobrados no formato necessário, no próprio local da obra. O uso de telas soldadas em lajes e pisos reduz a mão-de-obra e elimina as perdas do método de montagem da armadura no local da obra ( pontas cortadas que sobram).

Prefira marcas de vergalhões fabricados em usinas siderúrgicas que tenham um rigoroso controle de qualidade e que respeitem as exigências das Normas Técnicas Brasileiras.



PREPARO DO CONCRETO

A qualidade das benfeitorias executadas com concreto não depende apenas das características dos seus componentes. As sete etapas, explicadas a seguir, também contribuem muito para garantir a qualidade e a economia desejadas.



1- Dosagem do concreto

O concreto é uma mistura dos vários componentes, em determinadas proporções, chamadas de dosagem ou traço, na linguagem da construção civil.

O traço varia de acordo com a finalidade de uso e com as condições de aplicação. A tabela seguinte apresenta os traços mais adequados para os principais usos no meio rural. Se nenhum deles se alicar ao seu caso específico, consulte um profissional habilitado.



TRAÇOS DE CONCRETO
Aplicações Traço Rendimento por saco de cimento
Para base de fundações e
1 saco de cimento
14 latas ou 0,25 metros cúbicos

para contrapisos (concreto
8 latas e meia de areia

magro)
11 latas e meia de pedra

2 latas de água


Concreto para fundações
1 saco de cimento
9 latas ou 0,16 metros cúbicos

5 latas de areia

6 latas e meia de pedra

1 lata e meia de água

Concreto para pisos
1 saco de cimento
8 latas ou 0,14 metros cúbicos

4 latas de areia

6 latas de pedra

1 lata e meia de água


Concreto para pilares,
1 saco de cimento
8 latas ou 0,14 mertos cúbicos

vigas, vergas, lajes e
4 latas de areia

produção de pré-moldados
5 latas e meia de pedra

em geral
1 lata e um quatro de água



Atenção: 1) A lata de medida deve ser de 18 litros.
2) As pedras devem ser 1 ou 2.



2- Cálculo estrutural
O traço define a proporção dos componentes do concreto simples. Caso seja utilizado o concreto armado, é preciso definir também a posição, o tipo, a bitola e a quantidade dos vergalhões que vão compor a armadura. Essa determinação chama-se cálculo estrutural e deve ser feita, obrigatoriamente, por um profissiona habilitado.


3- Execução das fôrmas

Como já dito, o concreto é moldável. Por isso, é preciso prever a montagem dos moldes - chamados de fôrmas, na linguagem da construção civil. As fôrmas devem ser muito bem feitas, travadas e escoradas, para que a estrutura de concreto tenha boa qualidade e não ocorram deformações ( só para se ter uma idéia, o peso do concreto é duas vezes e meia maior que o da água).

As fôrmas também devem ser estanques (sem fendas ou buracos) para evitar o vazamento do concreto.

As formas podem ser feitas de diversos materiais: madeira, alumínio, fibra de vidro, aço, plástico.

As fôrmas são compostas de 2 elementos:

- o caixão da fôrma, que contém o concreto e, portanto, fica em contato com ele;
- a estruturação da fôrma, que evita a deformação e resiste ao peso do concreto.

O caixão da fôrma é feito com chapas de madeira compensada. Na estruturação podem ser usadas peças de madeira serrada ou madeira bruta.

Quanto ao acabamento da superfície, existem dois tipos de chapas no mercado: plastificadas e resinadas.

O aproveitamento médio das plastificadas é de 15 vezes, enquanto o das resinadas é de 4 a 5 vezes.

O travamento e o escoramento das fôrmas requerem muito cuidado. Dependendo do tamanho do vão ou do peso do concreto a ser suportado, é necessário usar peás mais robustas de madeira serrada, como tábuas, vigas ou até pranchões. As madeiras brutas podem substituir as serradas no escoramento e, eventualmente, no travamento. Mas é desaconselhável o seu uso em outras funções, como o encaibramento das lajes, por exemplo.



O travamento, o alinhamento, o prumo e o nivelamento das fôrmas devem ser conferidos antes da concretagem, para evitar deformações no concreto.

As ferramenta necessárias para a execução de uma fôrma são : serrote, martelo de carpinteiro, prumo, linha, maangueira de nível e, eventualmente, uma bancada para "bater"as fôrmas.


4- Execução da armadura

A execução da armadura compreende as seguintes operações: corte, dobramento, amarração, posicionamento, conferência.

As principais peças de concreto armado das benfeitorias de pequeno porte têm formato ou função de : fundações, vigas, pilares, lajes.

A armadura das fundações das obras de pequeno porte consiste, em geral, de dois ou três vergalhões.

Os pilares e as vigas têm armadura composta de vergalhões longitudinais e estribos. Estes, mantém os vergalhões longitudinais na posição correta e ajudam o conjunto a aguentar esforços de torção e flexão. As extremidades dos vergalhões longitudinais devem ser dobradas em forma de gancho, para garantir sua ancoragem ao concreto.

As lajes concretadas no local têm vergalhões nos sentidos de comprimento e da largura,formando uma tela.


ex1.JPG (10361 bytes)

O conjunto de pilares, vigas e lages é submetido ainda a outros esforços. Por isso, o cálculo estrutural determina também a colocação de uma armadura complementar, chamada de ferro negativo.



ex2.JPG (8651 bytes)

Em geral, as armaduras são montadas no local da obra, sobre cavaletes onde os vergalhões são amarrados uns aos outros com arame cozido.

Emendas de vergalhões devem ser evitadas. Caso ejam necessárias, devem ficar desencontradas (ou desalinhadas). O transpasse (ou trespasse) da emenda deve ter um comprimento de oitenta vezes o diâmetro do vergalhão.

Quando são usadas telas soldadas, uma tela deve cobrir 2 malhas da outra.

Tanto os vergalhões como as telas devem ser firmemente amarrados nas emendas.

O concreto resiste bem ao tempo mas a armadura pode sofrer corrosão se não ficar bem protegida por uma camada de cobrimento de, no mínimo, 1 cm de concreto. Para garantir que a armadura fique a essa distância mínima da superfície, são usados espaçadores (pequenas peças de argamassa de cimento e areia, fixadas na armadura).

As ferramentas necessária para a confecção de armaduras são: tesourão, serra de arco, Torquês, alavanca para dobrar, bancada com pinos.



5- Mistura do concreto

O concreto pode ser misturado de três modos: manualmente, em betoneiras, em usina ( central de concreto ou concreteira).

- Mistura manual do concreto:

a) Espalhe a areia formando uma camada de uns 15 cm;
b) Sobre a areia, coloque o cimento;
c) Com uma pá ou enxada mexa a areia e o cimento até formar uma mistura bem uniforme;
d) Espalhe a mistura formando uma camada de 15cm a 20 cm;
e) Coloque a pedra sobre essa camada, misturando tudo muito bem;
f) Faça um monte com um buraco (coroa) no meio;
g) Adicione e misture a água aos poucos, evitando que escorra.

É muito importante que a quantidade de água da mistura esteja correta. Tanto o excesso quanto a falta são prejudiciais ao concreto. Se a mistura ficar com muita água, a resistência do concreto pode diminuir bastante, porque os componenentes, em geral, se separam. Ao contrário, se a mistura ficar seca, ele será difícil de adensar. Além disso, a peça concretaa ficará cheia de buracos, com a aparência ruim e com baixa resistência.

A mistura do concreto deve ser uma tentativa de acertar o traço a ser adotado nas misturas seguintes com o mesmo material. Sempre que a areia, a pedra ou o cimento mudar, será necessário ajustar o traço novamente.

Caso seja difícil saber, pela observação visual, se a quantidade de água da mistura está correta, a solução é alisar a supefície da mistura com uma colher de pedreiro para ver o que acontece:

a) Se a superfície alisada ficar úmida, mas não escorrer água, a quantidade de água está certa;

b) Se escorrer há excesso de água. Isso deve ser imediatamente corrigido: coloque mais um pouco de pedra e areia na mistura e mexa tudo de novo, até não escorrer mais água;

c) Se a superfície alisada nem ficar úmida, é sinal de que falta água. Nesse caso, continue misturando a massa, pois, em geral, com mais algumas mexidas o concreto costuma ficar mais mole. Se a mistura ainda ficar muito seca, adicione cimento e água, na poção de cinco partes de cimento para cada três de água. Para isso, use um recipiente pequeno (por exemplo, uma lata limpa de óleo de cozinha). Nunca adicione apenas água na mistura, pois isso diminui muito a resistência do concreto.



- Concreto misturado em betoneira

A betoneira é uma máquina que agiliza a mistura do concreto.

a) Coloque a pedra na betoneira;
b) Adicione metade de água e misture por um minuto;
c) Ponha o cimento;
d) Por último, ponha a areia e o resto da água.

A betoneira precisa estar limpa (livre de pó, água suja e restos da última utilização) antes de ser usada. Os materiais devem ser colocados com a betoneira girando e no menor espaço de tempo possível. Após a colocação de todos os componentes do concreto, a betoneira ainda deve girar por, no mínimo, 3 minutos.

Para verificar se a quantidade de água está correta, pode ser feirto o mesmo teste da colher de pedreiro, já descrito na mistura manual do concreto. Se houver necessidade, o ajuste da quantidade de água deve ser feito da mesma forma.

Existem no mercado betoneiras com diferentes capacidades, de produção de concreto. A maioria é movida a energia elétrica. Essas máquinas podem ser alugadas ou compradas dos seus fabricantes ou distribuidores.

As ferramentas necessárias para a mistura do concreto são: enxada, pá, carrinho de mão, betoneira, lata de 18 litros, colher de pedreiro.



- Concreto misturado em usina (central de concreto ou concreteira)

O concreto também pode ser comprado pronto, já misturado no traço desejado e entregue no local da obra por caminhões-betoneira. Esse tipo de fornecimento só é viável para quantidades acima de 3 metros cúbicos e para obras não muito distantes das usinas ou concreteiras, por questão de custo.


6- Concretagem

A concretagem abrange o transporte do concreto recém misturado, o seu lançamento nas fôrmas e o seu adensamento dentro delas. A concretagem deve ser feita no máximo uma hora após a mistura ficar pronta. Nessa etapa é importante a presença de um profissional experiente.

O transporte pode ser feito em latas ou carrinho de mão, sem agitar muito a mistura, para evitar a separação dos componente.

As fôrmas devem ser limpas antes da concretagem. Quaiquer buracos ou fendas que possam deixar o concreto vazar precisam ser fechados. Em seguida as fôrmas têm de ser molhadas para que não absorvam a água do concreto. Esse não deve ser lançado de grande altura, para evitar que os componentes se separem na queda. o certo é despejar o concreto da altura da borda da fôrma.

Errado Certo

ex5.jpg (9615 bytes)



A concretagem nunca deve parar pela metade, para evitar emendas, que ficarão visíveis depois da desforma.

O concreto deve ser adensado em camadas, à medida que é lançado nas fôrmas. Isso pode ser feito manualmente, com um soquete (haste feita de madeira ou barra de aço) ou com a ajuda de vibradores elétricos. O adensamento é necessário para que o concreto preencha toda a fôrma, sem deixar vazios ou bolhas. Quanto mais adensado (compactado) for o concreto, maior será sua resistência e durabilidade.

As ferramentas necessárias para a concretagem são: pá, enxada, carrinho de mão, lata
de 18 litros e colher de pedreiro.

7- Cura e desforma do concreto

Cura é a fase de secagem do concreto, na linguagem da contrução civil. Ela é importantíssima: se não for feita de modo correto, o concreto não terá a resistência e a durabilidade desejadas.

Ao contrário do que se possa pensa, para uma boa cura não basta deixar o concreto simplesmente secar ao tempo. O sol e o vento secam o concreto depressa demais. Na verdade, ele deve ser mantido úmido por uma semana. Isso pode ser feito regando o concreto pelo menos uma vez por dia ou cobrindo a sua superfície com sacaria ou capim molhados.

Mas cuidado: o concreto fresco não pode ficar encharcado nas orimeiras seis horas aós a mistura, quando ainda está mole. Caso haja o risco de cair uma chuva forte após o término da concretagem de uma peça de grande superfície, (uma laje ou um piso) o concreto fresco deve imediatamente ser coberto com uma lona plástica.

A desforma, ou seja, a retirada das fôrmas, deve ser feita depois que o concreto atingir
uma boa resistência, geralmente três dias após a concretagem.

Primeiro, são retiradas as peças laterais, com cuidado, evitando choques ou pancadas, para não estragar as fôrmas e para não transmitir vibrações ou esforços ao concreto. O escoramento das fôrmas de lajes ou vigas só deve ser retirado 3 semanas após a concretagem.

As ferramenta necessárias para a desforma são: Martelo de carpinteiro, pé-de-cabra e serote.

© Copyright 1998 B@NET, All rigths
reserved. Todos os direitos reservados.

Powered by ScribeFire.

Wednesday, October 24, 2007

How to Cool the Globe

New York Times
Opinion


Op-Ed Contributor
How to Cool the Globe


By KEN CALDEIRA
Published: October 24, 2007

DESPITE growing interest in clean energy technology, it looks as if we are not going to reduce emissions of carbon dioxide anytime soon. The amount in the atmosphere today exceeds the most pessimistic forecasts made just a few years ago, and it is increasing faster than anybody had foreseen.

Even if we could stop adding to greenhouse gases tomorrow, the earth would continue warming for decades — and remain hot for centuries. We would still face the threat of water from melting glaciers lapping at our doorsteps.

What can be done? One idea is to counteract warming by tossing small particles into the stratosphere (above where jets fly). This strategy may sound far-fetched, but it has the potential to cool the earth within months.

Mount Pinatubo, a volcano in the Philippines that erupted in 1991, showed how it works. The eruption resulted in sulfate particles in the stratosphere that reflected the sun’s rays back to space, and as a consequence the earth briefly cooled.

If we could pour a five-gallon bucket’s worth of sulfate particles per second into the stratosphere, it might be enough to keep the earth from warming for 50 years. Tossing twice as much up there could protect us into the next century.

A 1992 report from the National Academy of Sciences suggests that naval artillery, rockets and aircraft exhaust could all be used to send the particles up. The least expensive option might be to use a fire hose suspended from a series of balloons. Scientists have yet to analyze the engineering involved, but the hurdles appear surmountable.

Seeding the stratosphere might not work perfectly. But it would be cheap and easy enough and is worth investigating.

This is not to say that we should give up trying to reduce greenhouse gas emissions. Ninety-nine percent of the $3 billion federal Climate Change Technology Program should still go toward developing climate-friendly energy systems. But 1 percent of that money could be put toward working out geoengineered climate fixes like sulfate particles in the atmosphere, and developing the understanding we need to ensure that they wouldn’t just make matters worse.

Think of it as an insurance policy, a backup plan for climate change.

Which is the more environmentally sensitive thing to do: let the Greenland ice sheet collapse and polar bears become extinct, or throw a little sulfate in the stratosphere? The second option is at least worth looking into.

Ken Caldeira is a scientist at the Carnegie Institution’s department of global ecology.


Powered by ScribeFire.

Tuesday, October 23, 2007

Sono

New York Times

Health


An Active, Purposeful Machine That Comes Out at Night to Play
Jonathan Rosen

Some neuroscientists say that at least one vital function of sleep is tied to learning and memory, and new findings suggest that sleep plays a crucial role in flagging and storing important memories.


By BENEDICT CAREY
Published: October 23, 2007


The task looks as simple as a “Sesame Street” exercise. Study pairs of Easter eggs on a computer screen and memorize how the computer has arranged them: the aqua egg over the rainbow one, the paisley over the coral one — and there are just six eggs in all.
Skip to next paragraph
Night Life
A special issue of Science Times examines a cascade of research into the science of sleep.

Most people can study these pairs for about 20 minutes and ace a test on them, even a day later. But they’re much less accurate in choosing between two eggs that have not been directly compared: Aqua trumped rainbow but does that mean it trumps paisley? It’s hazy.

It’s hazy, that is, until you sleep on it.

In a study published in May, researchers at Harvard and McGill Universities reported that participants who slept after playing this game scored significantly higher on a retest than those who did not sleep. While asleep they apparently figured out what they didn’t while awake: the structure of the simple hierarchy that linked the pairs, paisley over aqua over rainbow, and so on.

“We think what’s happening during sleep is that you open the aperture of memory and are able to see this bigger picture,” said the study’s senior author, Matthew Walker, a neuroscientist who is now at the University of California, Berkeley. He added that many such insights occurred “only when you enter this wonder-world of sleep.”

Scientists have been trying to determine why people need sleep for more than 100 years. They have not learned much more than what every new parent quickly finds out: sleep loss makes you more reckless, more emotionally fragile, less able to concentrate and almost certainly more vulnerable to infection. They know, too, that some people get by on as few as three hours a night, even less, and that there are hearty souls who have stayed up for more than week without significant health problems.

Now, a small group of neuroscientists is arguing that at least one vital function of sleep is bound up with learning and memory. A cascade of new findings, in animals and humans, suggest that sleep plays a critical role in flagging and storing important memories, both intellectual and physical, and perhaps in seeing subtle connections that were invisible during waking — a new way to solve a math or Easter egg problem, even an unseen pattern causing stress in a marriage.

The theory is controversial, and some scientists insist that it’s still far from clear whether the sleeping brain can do anything with memories that the waking brain doesn’t also do, in moments of quiet contemplation.

Yet the new research underscores a vast transformation in the way scientists have come to understand the sleeping brain. Once seen as a blank screen, a metaphor for death, it has emerged as an active, purposeful machine, a secretive intelligence that comes out at night to play — and to work — during periods of dreaming and during the netherworld chasms known as deep sleep.

“To do science you have to have an idea, and for years no one had one; they saw sleep as nothing but an annihilation of consciousness,” said Dr. J. Allan Hobson, a psychiatry professor at Harvard. “Now we know different, and we’ve got some very good ideas about what’s going on.”

The evidence was there all along. Infants make sucking motions when asleep, and their closed eyelids quiver, as if the eyeballs beneath had a life of their own. But it wasn’t until the early 1950s, in a lab at the University of Chicago, that scientists recorded and identified what was happening.

Eugene Aserinsky, then a graduate student in physiology, reportedly was monitoring sleep and waking in his 8-year-old son, using electronic leads stuck to the boy’s head, connected to a brain-wave detecting machine. He had attached two leads to the boy’s eyelids as well, so he could tell whether his son woke up. One night he noticed percolating wave patterns that showed the boy had awoken. But he hadn’t.

Dr. Aserinsky confirmed the activity in others, and in 1953 he and his adviser, Nathaniel Kleitman, published the finding in a now-famous paper in Science. They later called the odd, unconscious state rapid eye movement, or REM, sleep.

“This was really the beginning of modern sleep research, though you wouldn’t have known it at the time,” said Dr. William Dement, then a medical student in Dr. Kleitman’s lab and now a professor of psychiatry and sleep medicine at Stanford University. “It took years for people to realize what we had.”

Dr. Dement, infatuated with Freud’s theories about dreams, quickly threw himself into the study of REM. He found that it was universal and occurred periodically through the night, alternating with other states. He gave them names: Stages 3 and 4, or deep sleep, when electrical waves roll as slow as mid-ocean swells; Stage 2, an intermediate stage between REM and deep sleep; and Stage 1, light sleep.

He also confirmed the link between REM and dreaming, and for a time hopes for sleep research — and money for it — soared.

Yet Drs. Dement, Hobson and others found in their studies scant evidence to confirm that dreams were the disguised, forbidden wishes described by Freud. They found instead a tangle of apparent anxieties, fantasy and vivid, often nonsensical replays of events that showed few verifiable patterns or measurable function.

They had hit a wall, and sleep research, like its nocturnal subjects, dropped from REM excitement back into a void. “You had this great excitement, basically followed by 40 years of nothing; it was just horrible,” said Robert Stickgold, a cognitive neuroscientist at Harvard. “Just a period of darkness.”

The sun came up in 1994, in Rehovot, Israel. There, a research team led by Avi Karni found that depriving people of REM sleep undermined memory of patterns they had learned the day before, while depriving them of deep sleep did not.

This result raised more questions than it answered — Were the participants simply sleepy, or stressed? Why just REM? What was the purpose of the other sleep states? — but it was an invitation to researchers interested in sleep.

“I called Karni immediately, and he sent me all his protocols, everything,” Dr. Stickgold said.

Others called, too. The field was waking up, and now turning its focus to a long-neglected area: learning and memory.

Since then the study findings have come almost too fast to digest, and they suggest that the sleeping brain works on learned information the way a change sorter does on coins. It seems first to distill the day’s memories before separating them — vocabulary, historical facts and dimes here; cello scales, jump shots and quarters over there. It then bundles them into readable chunks, at different times of the night. In effect, the stages of sleep seem to be specialized to handle specific types of information, the studies suggest.

On a recent Monday afternoon in Dr. Stickgold’s lab at Beth Israel Deaconess Medical Center in Boston, a postdoctoral student, Matthew Tucker, was running a study of the effect of naps on memorized words. In a neighboring room, a Boston University student was cramming on a list of 48 word-pairs; in another, a stubbly University of Massachusetts student had finished studying and was reclining for a nap, his face covered with electrode patches, like leeches sprouting antenna.

“College students are always ready for nap; we have no problems there,” Dr. Tucker was saying, as he moved back and forth, checking his watch, timing one student’s nap and the other’s study period.

He sat down for a moment. “We are finding that if a person takes a nap that contains slow-wave sleep — deep sleep — that performance on declarative memory tasks, which require the memorization of fact-based information like word-pairs, is enhanced compared to a person who doesn’t take a nap,” Dr. Tucker said.

Previous studies of nocturnal sleep have found the same thing. Memory of learned facts, whether they are names, places, numbers or Farsi verbs, seems to benefit in part from deep sleep. Healthy sleepers usually fall into deep sleep about 20 minutes or so after head meets pillow. They might spend an hour or more in those lolling depths early in the night, and typically less time later on. When cramming on facts, in short, it may be wiser to crash early at night and arise early, than to burn the candle until 2 a.m., the research suggests.

REM sleep, the bulk of which comes later in the night, seems important for pattern recognition — for learning grammar, for example, or to bird-watch, or play chess.

In one 2003 study, Sara Mednick, then at Harvard and now at the University of California, San Diego, led a team that had 73 people come into the lab at 9 a.m. and learn to discriminate between a variety of textured patterns. Some of the participants then took a nap of about an hour at 2 p.m. and the others did not.

When retested at 7 p.m. the rested group did slightly better. When tested again the next morning, after everyone had slept the night, the napping group scored much higher. The naps included both REM and deep sleep.

“We think that a nap that contains both these states does about the same for memory consolidation as a night’s sleep,” when it comes to pattern recognition learning, Dr. Mednick said.

Not that Stage 2 is an empty corridor between destinations. In series of experiments that he began in the early 1990s, Dr. Carlyle Smith of Trent University in Canada has found a strong association between the amount of Stage 2 sleep a person gets and the improvement in learning motor tasks. Mastering a guitar, a hockey stick or a keyboard are all motor tasks.

Musicians, among others, have sensed this for ages. A piece that frustrates the fingers during evening practice often flows in the morning. But only in recent years has the science caught up and given their instincts a practical shape.

For instance, Dr. Smith said that people typically got most of their Stage 2 sleep in the second half of the night. “The implication of this is that if you are preparing for a performance, a music recital, say, or skating performance, it’s better to stay up late than get up really early,” he said in an interview. “These coaches that have athletes or other performers up at 5 o’clock in the morning, I think that’s just crazy.”

For all these nighttime fireworks, memory researchers have yet to work out a complete picture of how all the pieces fit together. Each has a theory, but they differ: Dr. Smith focuses on Stage 2, others on deep sleep, still others on REM or a combination of REM and deep sleep. And no one knows how individual differences, between night owls and early birds, for instance, affect nighttime learning.

In addition, said Jerome Siegel, a professor of psychiatry at the University of California, Los Angeles, millions of people have taken drugs that suppress REM without reporting serious memory problems. “I wouldn’t rule out the possibility that sleep contributes to learning and memory consolidation, but the claim is that it’s essential, that it’s doing something the waking brain won’t, and the research hasn’t shown that,” Dr. Siegel said.

Even the college all-nighter provides evidence that some consolidation occurs during waking, he said. “College students know that the best way to learn stuff isn’t to stay up all night because it’s going to impair your judgment,” Dr. Siegel said, “but it doesn’t matter how good your judgment is if the information isn’t in there. And students know from experience that a lot of it is.”

One reason some neuroscientists are confident that the sleeping brain is actively working on the day’s streaming video of information is because they have seen it with their own eyes — or heard it, at least.

In his lab at the Massachusetts Institute of Technology, Matthew Wilson has been studying rats and mice wearing what look like Carmen Miranda hats. These are ultralight implants through which researchers thread hairlike wires to record the activity of single cells deep in the brain, in the left and right hippocampus, where the day’s memories are recorded.

Past research has shown that the hippocampus is spatially sensitive: it seems literally to pair the firing of individual neurons with locations outside the body. These systems are thought to function in similar ways in humans and rodents.

Computers record the cells’ firing in real time and can broadcast it over speakers. “I would listen to this background music of the brain sometime when the animals were asleep, and I started hearing this section that sounded very much like the pattern when the animals were in the maze,” Dr. Wilson said in an interview. “I recognized the firing pattern.”

The maze route is an important memory for these animals; it’s about all they know. In a paper published last December, Dr. Wilson and Daoyun Ji reported that in sleeping animals they had recorded chatter in neurons in the visual center of the neocortex, followed by an apparent response in the hippocampus — and not just any response, but a replay of the activity in the hippocampus that occurred during a maze task.

Dr. Wilson thinks of this as a kind of off-line conversation between the neocortex, which is involved in conscious learning during waking, and the hippocampus. “What we notice is that the light goes on in the neocortex a fraction of a second before it goes on in the hippocampus, as if the cortex is asking for information,” he said.

He said that this process was probably similar to what goes on when people take a moment to reflect, without distractions, sifting through the experiences of the day, also flagging important details, replaying events. “The question is not whether this is an essential process; it is,” Dr. Wilson said. “The question is whether there is something going on during this process that is unique to sleep.”

Subimal Datta, a neuroscientist across the river at Boston University School of Medicine, thinks so. In his studies of animals, he has documented that during sleep the brain is awash in a chemical bath unlike any during waking. Levels of inhibitory transmitters increase sharply, and levels of many activating messengers drop, or shut down entirely.

Even before REM is detectable, Dr. Datta said, a small pocket of cells in the brainstem spurs a surge in glutamate — an activating chemical — which leads to protein synthesis and other changes that support long-term memory storage.

“During waking we have a thousand things happening at once, the library is filling up, and we can’t possibly process it all,” Dr. Datta said. While awake the brain is also gathering lots of valuable information subconsciously, he said, without the person’s ever being aware of it.

“It’s during sleep that we have this special condition to clear away this overload, and these REM processes then help store what’s important,” Dr. Datta said.

In the jargon of the field, the “signal to noise ratio” becomes much stronger. The neural trace of the trivia has weakened, and crucial details are replayed and reinforced.

Dreams still defy scientific measurement but they, too, have a place in the evolving theory of sleep-dependent learning.

It is likely during REM, some scientists argue, that the brain proceeds to mix, match and juggle the memory traces it has preserved, looking for hidden connections that help make sense of the world. Life experience is cut up and reordered, sifted and shuffled again. This process could account for the cockeyed, disjointed scenes that occur during dreams: the kaleidoscope of distilled experience is being turned.

It also might account for that golden gift often attributed to a night’s sleep: inspiration.

To hear some people tell it, a night’s sleep changed their world. It was reportedly during sleep that the Russian scientist Dmitri Mendeleev’s periodic table of the elements tumbled into place. Friedrich August Kekule, a 19th-century chemist, said he worked out the chemical structure of the benzine ring — an important discovery — when he dreamed of a snake biting its tail. Athletes, including the golfer Jack Nicklaus, have also talked about insight coming during sleep.

Slight corrections in technique are revealed; sand traps are averted; mountains move.

“It does make sense these insights come during REM,” Dr. Walker said. “I mean, what better time to play out all these different scenarios and solutions and ideas than in dreams, where there are no consequences?”

The problem, he and others say, is how to study it. That, most neuroscientists agree, will take some very creative thinking — both of the daytime and nighttime kind.




Powered by ScribeFire.