556 Principles of Weather Forecasting. 



forms which depend upon it, for this is the moving power 

 of the world and all life implies motion. Deep in the 

 solid earth no life exists. In the greatest depths of the 

 ocean, where the air changes are slow and where little or no 

 light can come, life is nearly absent ; and high in the atmos- 

 phere only latent forms of life, like the spores and germs 

 of microscopic forms are drifted by the winds, 



In brief the life zone is that portion of the three spheres 

 where the largest amount of sunshine is transformed into 

 heat motion and therefore where there is the largest 

 amount of energy available for the use of plants and ani- 

 mals. 



703. Depth of the Atmosphere. We are living at the bot- 

 tom of an ocean of air whose depth is at present unknown. 

 Judging from the rate of decrease of pressure, as measured 

 by the barometer, its depth would be placed at something 

 less than 50 miles, for at 30 miles, could an instrument be 

 placed at that level, it is calculated that its reading would 

 be only .005 of an inch of mercury. Observations which 

 have been made upon the hight at which shooting stars or 

 meteors become visible shows that this is even more than 

 100 miles and it is believed that these bodies become visible 

 only after they have traversed enough of our atmosphere 

 to develop sufficient heat by friction and compression to 

 make them white-hot; and although the velocity of these 

 bodies is very great yet the upper air is so rarified they 

 must pass through great depths before sufficient heat can 

 be developed to make them white-hot. From these consid- 

 erations it appears likely that air may be found at highta 

 even exceeding 500 miles. 



704. Composition of the Atmosphere The air at differ- 

 ent times and in different places contains a great variety 

 of gases and volatile products but there are certain con- 

 stituents which are found everywhere in the explored reg- 

 ions and in pretty constant ratios. These are, for dry air : 



1. Nitrogen, forming about 77.18 per cent, by volume. 



