PHILOSOPHY OF STORMS, 6 



100 yards ; and that the dew-point falls about one quarter of a degree, 

 on account of the greater space occupied by the air and vapor, for' ev- 

 ery hundred yards of ascent. Now suppose the temperature of the air 

 upon the surface of the earth to be 70°, the dew-point being 60°, there 

 will then be a difl'erence of 10° between the temperature of the dew- 

 point and that of the air. This difFerence is called the complement nf 

 the deiD-point. The temperature of an ascending column of air, there- 

 fore, cooling 11° for every 300 feet of its ascent, and the dew-point fall- 

 ing one quarter of a degree under the same circumstances, it follows 

 that one degree is equivalent to every hundred yards, and that such a 

 column will begin to form cloud when it rises about as many times 300 

 feet as there are degrees in the complement of the dew-point. Now 

 the complement of the dew-point at the surface of the earth being 

 10°, the ascending column of air, after it has gone up 3000 feet, 

 will have cooled 12|°, and will thereby be reduced to 57^", or the 

 estimated dew-point at that elevation. At this temperature and height, 

 vapors cannot at this time exist in a gaseous form in the air, and, 

 therefore, must be condensed into cloud. Hence the height of the 

 base of a cloud, forming under these circumstances, must be 3000 feet, 

 and its temperature 57|°. That this calculation is correct, has been 

 proved by a number of gentlemen in Philadelphia, who instituted a se- 

 ries of expeiiments for this and other purposes. They raised kites ten 

 feet in diameter, and attached them to wires three miles long, wound 

 upon a reel. While the kites were hovering in the base of a cloud their 

 height was taken by one set of men by means of the sextant, while an- 

 other set took the height of the base of the cloud with the thermome- 

 ter. The results of both observations were put down separately, and, 

 when compared, were found to agree, far within the limits of the errors 

 of observation. It may, therefore, be considered as established that the 

 difference between the temperature of the de7c-])oint, and that of the air, 

 (the complement of the deic-point,) multiplied hy 300 will always give 

 the height^ in fcet^ of the base of a cloud. 



In accordance with my promise in the last number, I will now ex- 

 plain the manner of calculating the quantity of vapor in a given amount 

 of air, although it will considerably lengthen this communication. This 

 discovery was made almost simultaneously by Dalton and Gay Lussac, 

 and afterwards was more fully investigated by Dulong and Petit. I may 

 here pause to observe that the principles, upon which Professor Espy 

 establishes his theory, were discovered by other philosophers without 

 any reference, even the most remote, to their bearing upon storms, and 

 that he is merely applying them to the explanation of certain natural 



