58' SCIENTIFIC RECREATIONS. 



represented by air. A gas is an elastic fluid, and differs very considerably, 

 from water ; for a gas will fill a large or small space with equal convenience^ 

 like the genii which came out of the bottle and obligingly retired into it again 

 to please the fisherman. We have seen that the pressure of the air is 1 4.-^ 

 per square inch at a temperature of 32. It is not so easy to determine the 

 pressure of air at various times as that of water. We can always tell the 

 pressure of a column of water when we find the height of the column, as it is 

 the weight of so many cubic inches of the liquid. But the pressure of the 

 atmosphere per square inch at any point is equal to the weight of a vertical 



Fig: 56. The "Ludion." 



column of air one inch square, reaching from that point to the limit of the 

 atmosphere above it. Still the density is not the same at all points, so we 

 have to calculate. The average pressure at sea level is 14*7 per square 

 inch, and sustains a column of mercury i square inch in thickness, 29*92, or 

 say 30 inches high. These are the data upon which the barometer is based, 

 as we have seen. 



In our article upon "Chemistry" we will speak more fully of the 

 atmosphere and of its constituents, etc. 



