Popular Science Monthly 



91 



these 



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. 



