-1859] WRITINGS OF JOSEPH HENRY. 271 



on the one hand, and the increased pressure of the whole 

 atmosphere by the addition of vapor on the other. 



It is probable however that in portions of the earth in 

 which the air is constantly saturated at a uniform tempera- 

 ture and at which the diffusion is permanently uniform, if 

 the elastic force of the vapor is subtracted from the whole 

 height of the mercurial column it will give the pressure of 

 an atmosphere of dry air. 



On the supposition that the vapor is uniformly distributed 

 through the atmosphere, (which will not be far from the 

 truth if considered with reference to the principal zones of 

 the earth,) we can calculate the whole weight of water con- 

 tained. If the water were at the boiling point its elastic 

 tension or pressure would be equal to the pressure of the 

 atmosphere, and in this case it would support 30 inches of 

 mercury, or its equivalent, 407*4 inches of water; and since 

 transparent vapor observes the same law of expansion and 

 contraction by variations of pressure and temperature that 

 dry air does, it is clear that we shall have the following re- 

 lation for any other temperature, namely, as 30 inches is to 

 the quantity of mercury expressing the elasticity of the air 

 at any temperature, so is 407'4 inches of water to the whole 

 weight of the aqueous vapor, provided the weight of vapor 

 were the same as that of the air. It has however been proved 

 by the experiments we have described that vapor is only five- 

 eighths of the density of air, and therefore the quantity 

 found by the foregoing relation must be reduced in this 

 ratio. 



If we assume that the dew-point is on an average 6° below 

 the temperature of the air, and allowing the temperature of 

 the tropical regions to be 82°, we shall have the following 

 proportion,— 30 : 0-897 : : 407*4 : 12'181. This last number 

 must however be multiplied by f , and this will give us 7*013 

 inches. From this it will appear that if the atmospheric 

 columns at the equator were to discharge their whole watery 

 store the moisture precipitated would cover the earth to the 

 small depth of 7*613 inches; and from a similar calculation 

 we find that if the column of air resting upon the city of 



