PHILOSOPIIV OF STORMS. 53 



instriiincnl was employed by Professor Espy by which he could get the 

 temperature below or above the dew-point in quick succession, and thus 

 make and unmake cloud almost in the same instant. This was riierely 

 a strong glass tube with a piston, originally constructed for igniting tin- 

 der. By drawing back the piston, expanding the air, and thus reducing 

 the temperature below the dew-point, cloud was formed ; and by push- 

 ing in the piston, condensing the air, and thus elevating the temperature 

 again above the dew-point, the cloud disappeared — proving not only the 

 ralionale of the formation of cloud, but also that the quantity of vapor 

 is proportionate to the temperature. All clouds are formed in nature in 

 this way, whenever the air is caused to ascend high enough to reduce 

 its temperature down to the dew-point ; and the cloud becomes denser 

 and denser as the air continues to ascend above its base. 



Now wlien vapor in a gaseous state is cooled to a point sufliciently 

 low to convert it to water, it parts with its heat of elasticity, which is 

 set free or beconies sensible. Tliis heat of elasticity, or latent caloriCf 

 as it is more frequently called, is contained in large quantities in vapor, 

 and it is given out or rendered sensible, whenever the vapor or steam in 

 the air is condensed into cloud. The heat, therefore, which begins to 

 be discharged at the base of the cloud, must change the rate of cooling 

 in the up-moving column, from the base of the cloud upwards, from that 

 which exists from its base downwards. If, however, the up-moving 

 column of air were free from vapor, then the ratio of cooling would 

 not be affected ; but the nephelescopic experiments have shown that so 

 soon as an ascending column of vapor is condensed, iJie law of cooling 

 in its upward motion is changed, by the heat set free, Jrom a degree to 

 about half that quanlily for every hundred yards above the base of the 

 cloud ; that is, about five-eighths of a clegree for one hundred yards of 

 ascent, when the dew-point is about 70 degrees. If the dew-point is 

 higher, it cools a little less, and if the dew-point is lower, it cools a 

 little more than live-eighths of a degree in ascending one hundred yards. 

 Now as it is known that the atmosphere, free from clouds, on the oul,- 

 side of the ascending column is colder about one degree for every hun- 

 dred yards in height, and the ascending column itself becomes. only five- 

 eighths of a degree colder for every hundred yards above the base of the 

 cloud, it is plain that the dry air on the outside of a cloud must he near- 

 ly double as cold as the moist air within it, and it follows that the cloud 

 must be of a less specific gravity than the surrounding air at the same 

 height. If the top of a cloud, therefore, be six thousand yards higher 

 than its base, the aii outside of its summit must be twenty-two degrees 

 colder than the uir iu the cloud \ if it be eight thousand yards higher, it 



