266 METROROI.OGV, 



ihe increasing elevation. This statement of the varying temperature of 

 the atmosphere refers only to the mass in a quiescent state, uninfluenced 

 by ascending columns, or by any variation in its hygrometric condition. 

 These are modifying influences, which will be noticed hereafter. Now 

 vapor is always condensible by a certain degree of cold, and as the va- 

 por in the atmosphere must lise in company with the ascending col- 

 umn of air into regions that become colder and colder as it ascends, it 

 must ultimately reach a point, if it continues to ascend, where the di- 

 minished temperature will condense it. This brings us to a point — the 

 formation of cloud — which is at once the most bcauiiful and simple in 

 the whole range of our subject, and which, aside from other concurrent 

 phenomena, gives to the theory of Professor Espy, a pre-eminence over 

 every other. 



In illustrating the formation of clnud 1 wonld ask attention, for a 

 moment, to a few facts that force theniselves almost daily upon liie ob- 

 servation of every one. On breathing upon glass, or other substance 

 colder than the breath, moisture will be (h posited upon it; the windows 

 of crowded rooms, particularly in winter, become dripping wet in a 

 short time ; and a glass of sparivling watei-, cool from the fountain, siveats 

 profusely in a hot summer's day. These results are produced by the 

 condensation of the vapor in the air upon llic colder bodies with which 

 it comes in contact. The formation of dew, as explained by Dr. Wells 

 in his excellent treatise on that subject, is a phenomenon of a similar 

 character. Substances covered with dew are always colder than the 

 contiguous strata of air, or than those bodies on which dew is not de- 

 posited. In fact, dew is a deposition of water previously existing in the 

 air as vapor, and which loses its gaseous form only in consequence of 

 being chilled by contact with colder bodies. Another not unusual oc- 

 currence of a like nature must, from its sudden and remarkable appear- 

 ance, have attracted the attention of every observer. After the continu- 

 ance of a cold spell of weather so as to cool the walls of buildings, the 

 earth, and all bodies on its surface, there sometimes suddenly springs 

 up a spell of warm weather accompanied with a southerly wind. Im- 

 mediately the walls of stone buildings begin to sweat, stones, rocks, &c. 

 become quite wet, and although no clouds are seen, yet every thing feels 

 damp in consequence of the cold surfaces condensing the vapor of the 

 warmer air. I might multiply such facts, but these are suflicient for il- 

 lustration. 



There are several circumstances, connected with these facts, which 

 have an important bearing upon Professor Espy's theory. I shall, how- 

 ever, at present, ask attention to only two points, viz : — Thai the atmos- 



