864 Handbook of Nature-Study 



There are two forces in constant operation on each gas that surroimds 

 the earth, viz., expansive force and gravity. Expansive force pushes the 

 gas up and gravity pulls it down, but the force of gravity decreases as the 

 distance from the center of the earth increases, so there is a point at a cer- 

 tain distance above the earth where the two forces just balance each other, 

 and each gas will expand upward to that point but will not rise beyond it. 

 Therefore, if we know the expansive force of a gas and the rate at which 

 gravity decreases, it is possible to calculate the height to which the differ- 

 ent gases that compose the air will rise. 



In this way it has been determined that carbon dioxide, which is one of 

 the heavier gases, extends upward about ten miles, water vapor about 12 

 miles, oxygen about 30 miles and nitrogen about 35 miles while hydrogen 

 and helium, the lightest gases known, do not appear at the surface at all, but 

 probably exist at a height of from 30 miles to possibly 200 miles. 



£ 



The zone of twilight in midwinter. 

 From Todd's New Astronomy. 



There are other ways in which we are able to gain some idea of the 

 approximate height at which there is an appreciable atmosphere. When 

 the rays of light from the sun enter our atmosphere they are broken up or 

 scattered — diffracted — so that the atmosphere is partially lighted for 

 some time before sunrise and after sunset. This is called twilight. If there 

 were no atmosphere, there would be no twilight, and darkness would fall the 

 instant the sun passed below the horizon. Twihght, which is caused by the 

 sun shining on the upper atmosphere, is perceptible until the sun is about 

 16° below the horizon.' ' From this it is calculated that the atmosphere has 

 sufficient density at a height of 40 miles to scatter, or diffract, sunlight. 



Observations of meteors, commonly called shooting stars, indicate that 

 there is an appreciable atmosphere at a height of nearly 2 00 miles. Meteors 

 are solid bodies flying with great velocity through space. Occasionally 

 they enter our atmosphere. Their velocity is so great that the slight 

 resistance offered by the air generates enough heat by friction, or by the 

 compression of the air in the path of the meteor, to make it red hot or to 

 bum it up before it reaches the bottom of the atmosphere. Only the largest 

 meteors reach the earth. 



When a meteor is observed by two or more persons at a known distance 

 from each other, and the angle which the line of vision makes with the 

 horizon is noted by each, it is a simple matter to calculate the distance from 

 the earth where the lines of vision intersect, and thus determine the height 

 of the meteor. In this way, reliable observations have given the height at 

 which there is sufficient density in the atmosphere to render meteors lumin- 

 ous as 188 miles. 



