Free printable SSAT practice test for Grade 9

In the last days of August 1854, cholera tore through a few crowded blocks of Soho, in central London. Within roughly ten days, hundreds of people who lived near Broad Street were dead, many of them within a day or two of falling ill. Families fled. Shops closed. Those who stayed waited to learn who would be next.

Most physicians of the time believed that cholera spread through "miasma," a poisonous vapor thought to rise from filth and rotting matter. The theory had obvious appeal in a city where sewage collected in cellars and the summer air could be foul. It also suggested a remedy: clear away the smell, and the disease would go with it.

John Snow, a London physician then best known for his work with anesthesia, was not convinced. He had argued for several years that cholera was carried in contaminated water and entered the body through the mouth. The Soho outbreak gave him a chance to test the idea. He obtained the addresses of the dead and went door to door, asking where each household got its water.

The pattern he found was striking. The deaths clustered around a public pump on Broad Street and thinned out with distance from it. Just as revealing were the exceptions. A large workhouse nearby, home to hundreds of people, had its own well and suffered few deaths. Workers at a local brewery, who were given beer to drink, were largely spared. Meanwhile, a widow who lived miles away in Hampstead died of cholera; she had preferred the taste of the Broad Street water and had it carried to her house.

On September 7, Snow presented his evidence to local officials, and the next day the handle was removed from the pump. Snow himself later acknowledged that the outbreak was already in decline, partly because so many residents had fled, so removing the handle cannot be credited with ending it. His larger achievement lay elsewhere. In a revised edition of his work on cholera, published the following year, he included a map that marked each death as a small bar beside the address. Seen at a glance, the map made an argument that columns of numbers could not.

Even so, many officials remained skeptical. A local clergyman, Henry Whitehead, set out to disprove Snow and ended by supporting him, tracing the likely source to a cesspool a few feet from the well, in which a sick infant's diapers had been rinsed. Acceptance of the waterborne theory came slowly, and the germ responsible for cholera would not be widely recognized for decades.

Today, Snow's map is often described as a founding document of epidemiology, the study of how diseases spread through populations. Its lesson is not simply that maps are useful. It is that careful attention to the cases that do not fit can be as persuasive as the cases that do.

Far below the reach of sunlight, in water only a few degrees above freezing, it is always snowing. The flakes are not ice. They are clumps of dead and dying matter: the remains of microscopic algae, the shed shells of tiny animals, waste pellets and grains of dust blown off distant deserts, all bound together by a sticky film of mucus that many sea creatures produce. Ocean scientists call this steady drift "marine snow," and although a single flake is hardly worth noticing, the snowfall as a whole is one of the planet's most important means of moving carbon.

The process begins at the sunlit surface. Phytoplankton, single-celled organisms that float near the top of the sea, absorb carbon dioxide and use sunlight to turn it into living tissue, much as land plants do. Phytoplankton multiply quickly and die quickly, and they are eaten by small grazing animals that in turn release compact waste pellets. Each of these fragments contains carbon that was, not long before, dissolved in the water and in contact with the air above it.

Most of that material never travels far. Bacteria break much of it down long before it leaves the upper ocean, releasing the carbon back into the water, where it can return to the atmosphere. But some particles collide, stick together and grow heavy enough to sink faster than they can be consumed. A clump may spend weeks falling, and along the way it serves as a moving meal. Animals of the deep, from small crustaceans to the vampire squid, which gathers drifting particles with long, sticky filaments, depend on this rain from above in a place where almost nothing grows.

Only a small fraction of the original carbon reaches the deep ocean or the seafloor. That fraction matters enormously, however. Carbon carried into the deep can remain isolated from the atmosphere for centuries, and some of it is buried in sediment for far longer. Scientists refer to this whole chain, from surface growth to deep storage, as the "biological pump." Without it, the amount of carbon dioxide in the air would be considerably higher than it is.

Researchers study marine snow with sediment traps, which are containers anchored at various depths to catch what falls, and increasingly with cameras and robotic floats that can count particles as they pass. Their measurements remain uncertain, partly because the ocean is vast and the snowfall varies with season and place. A spring bloom of phytoplankton can send down a heavy burst; a nutrient-poor stretch of open ocean may produce only a thin drizzle.

What the research makes clear is that the deep sea, often imagined as remote and unchanging, is closely tied to the surface world. The carbon in a flake drifting past a vampire squid may have been in the air above the waves only a season earlier.