Page:Popular Science Monthly Volume 91.djvu/107

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Popular Science Monthly

��91

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��Counting the Moisture Drops in a Fog

MEASUREMENTS of fog have hitherto been crude.

But an example of more re- fined measurements of fog has recently been afforded by experts of the United States Bureau of Standards. The measurements were made in the most notorious- ly foggy region of the world — the Grand Banks of Newfoundland.

The tiny drops that constitute a fog are smaller than raindrops. They are formed by the conden- sation of the gaseous water in the air, known as water vapor. Each drop condenses about a "nucleus" — consisting of some substance other than water. The air always contains an immense number of nuclei, ready to form centers of condensa- tion, when the conditions of temperature and moisture are right for this process. A method of counting these invisible nuclei was devised by John Aitken. It consists in causing a drop to form around each nucleus in a sample of air, and then counting the drops through a microscope.

Another process, devised by Carl Barus, makes it possible to determine the size of the drops. When a light is viewed through a cloud or fog it is seen to be surrounded by a colored ring, called a "corona You have seen such rings around the moon and around street lamps at night. The angular di- ameter, or aperture, of these rings depends up- on the size of the drops. Small drops produce big rings, and vice versa.

The apparatus of Ba- rus was installed in the pilot-house of the Seneca, and the number of nuclei present in a given volume of air was measured three times a day, whether the weather was foggy or otherwise. A sample of air was drawn through a pipe, projecting from the port bow. It was admitted to a "fog chamber," saturated with water vapor, suddenly expanded, to con- dense water on the nuclei,

���Instrument for measuring the tiny drops of fog. Sixty billion drops equal one -seventh of a glass of water

��forming an artificial fog. The corona around a source of light, viewed through this fog, was measured, and the size of the drops was determined from the known amount of moisture in the chamber.

���Martha's combination umbrella and rain -cape. Even the dog can crawl under cover

��A Twelve-Year-Old Girl's Combina- tion Umbrella and Rain-Cape

A LITTLE girl, Miss Martha Bachman, who lives in Chattanooga, Tenn., has evidently suffered the discomfort of wet stockings caused by the flapping of her just-so-long rain-cape against her legs on her way to school, as so many other little girls have done in days gone by. But Martha has an idea for eliminating such discomfort from future rainy days.

Her idea is to attach a cape of oil-cloth or rubberized material to the rim of the umbrella with snap-fasteners, but- toning it down the front in ordinary rain-cape fash- ion. An isinglass window at about the level of the eyes would prevent the wearers of such an umbrella-cape from bumping into each other on the street, like pilotless ships. In this way books could be protected from the rain also and the hand holding the umbrella.

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