PRESSURE OJ^ AIR AXD WATER. 13 



consist of a certain number of strata, it is obvious 

 that the lowest of these must bear the weight of all 

 above it, and is therefore more compressed than the 

 next higher, and so on till we reach the surface. So 

 with the atmosphere. Its entire weight presses on 

 the lowest stratum which touches tlie sea, and that 

 weight of pressure in the torrid and temperate zones 

 is marked by the barometer at 30 inches. If we 

 take the barometer 87 feet above the level of the 

 sea, it will mark the diminished pressure by 29*9, 

 showing that it is one-tenth less. To show a diminu- 

 tion of another tenth it would be necessary to go 

 through a second space of more than 87 feet, because 

 the {)ressure of the whole atmosphere is less by the 

 height already attained. Thus, we shall find it 

 necessary to rise higher and higher for every succes- 

 sive tenth, until we reach a point when the pressure 

 altogether ceases and is marked by 0. This would 

 be at the top of the atmosphere. 



The reader will now understand how it is that a 

 barometer serves to indicate the height of any part of 

 the earth's surface; indeed, it is the only possible 

 means at our command in many cases for ascertaining 

 height. A process analogous to this would be em- 

 ployed with advantage to measure the depths of the 

 sea. Suppose an instrument to be sunk in the water. 

 The depth of water through which it had fallen 



