The Phenomena of the Atmosphere. 81 



pressed by all the former fifteen, and its own 

 weight added, gives it a density of sixteen grains; 

 and so on, descending downwards to the bottom. 

 The first inch has a density of one, the second 

 inch a density of two, the third inch a density of 

 four, the fourth inch of eight, the fifth of sixteen, 

 and so on. Thus the inches of air increase in 

 density as they descend from the top, at the rate 

 of one, two, four, eight, sixteen, thirty- two, sixty- 

 four, &c. which is called a geometrical progres- 

 sion. Or if we reverse this, and begin at the 

 bottom, we may say, that the density of each of 

 these inches becomes less upwards in a geometri- 

 cal progression. If, instead of inches, we sup- 

 pose the parts -into which this pillar of air is 

 divided to be extremely small, like those of air, 

 the rule will hold equally good in both. So that 

 we may generally assert, that the density of the 

 air, from the surface of the earth, decreases in a 

 geometrical proportion. 



This being understood, should we now desire 

 to know the density of the air at any. certain 

 height, we have only first to find out how much 

 the density of the air is diminished to a cer-. 

 tain standard height, and thence proceed to tell 

 how much it will be diminished at the greatest 

 heights that can be imagined. At small heights 

 the diminution of its density is by fractional or 

 broken numbers. We will suppose at once that 

 at the height of five miles, or a Dutch league, 

 the air is twice less dense than at the surface of. 



