Vlll 



INTRODUCTION. 



NEPHELESCOPE. calculations made on chemical principles. 

 Mr. Espy exhibited the mode of operating 

 with this instrument. 



By means of the condensing pump a, 

 air may be forced into the glass vessel &, 

 and its degree of condensation can be mea- 

 sured by the barometer gage c. 



After the instrument is charged, the 

 stop-cock is turned, and the pump removed. 

 When the air within acquires the tempera- 

 ture of the air without, a measure is care- 

 fully applied to the barometer gage to as- 

 certain how much higher the mercury 

 stands in the outer leg than in the inner ; 

 the cock is then turned, and the air per- 

 mitted to escape, and at the moment of 

 equilibrium, the cock is closed again. 

 Now as the cock is closed at the moment 

 I the greatest cold is produced by expan- 

 jsion, the mercury in the outer leg will be- 

 gin to ascend, and that in the inner leg to descend, because the air 

 within receives heat from without, and the difference of level being 

 measured as before, will indicate the number of degrees cooled 

 by a given expansion. 



Mr. Espy shewed that when dry air is used in the experiment, 

 the temperature is reduced about twice as much as when moist 

 air is used, on account of latent caloric evolved in the latter case 

 by the formation of cloud which is plainly visible. (59.) Mr. 

 Espy then proceeded to give the following synopsis of his theory, 

 premising that the numbers he should introduce were not intended 

 to be strictly accurate, and would be subject to many corrections 

 one in particular, in which no notice had been taken of the 

 specific heat of air under different pressures. 



SYNOPSIS. 



When the air near the surface of the earth becomes more heated 

 or more highly charged with aqueous vapor, which is only five- 

 eighths of the specific gravity of atmospheric air, its equilibrium 

 is unstable, and up-moving columns or streams will be formed. 

 As these columns rise, their upper parts will come under less pres- 

 sure, and the air will therefore expand ; as it expands, it will grow 

 colder about one degree and a quarter for every hundred yards of 

 its ascent, as is demonstrated by experiments on the nephelescope, 

 (58 to 68.) The ascending columns will carry up with them the 

 aqueous vapor which they contain, and, if they rise high enough, 



