X INTRODUCTION. 



upward motion would follow the laws of spouting fluids, which 

 would be eight times the square root of 100 feet a second, that is, 

 80 feet a second, and the barometer in the centre of the column at 

 its base would fall about the ninth of an inch. As soon as cloud 

 begins to form, the caloric of elasticity of the vapor or steam i& 

 given out into the air in contact with the little particles of water 

 formed by the condensation of the vapor. This will prevent the 

 air, in its further progress upwards, from cooling so fast as it did 

 up to that point ; and, from experiments on the nephelescope, it is 

 found to cool only about one half as much above the base of the 

 cloud as below ; that is, about five eighths of a degree for one 

 hundred yards of ascent, when the dew point is about seventy de- 

 grees. If the dew point is higher, it cools a little less, and if the 

 dew point is lower, it cools a little more than five eighths of a de- 

 gree in ascending one hundred yards. 



Now, it has been ascertained by aeronauts and travellers on 

 mountains, that the atmosphere itself, free from clouds, is about 

 one degree colder for every hundred yards in height above the 

 surface of the sea ; therefore, as the air in the cloud above its 

 base is only five eighths of a degree colder for every hundred 

 yards in height, it follows, that when the cloud is of great per- 

 pendicular height above its base, its top must be much warmer 

 than the atmosphere at that height, and consequently much lighter. 

 Indeed, the specific gravity of a cloud of any height, compared 

 with that of the surrounding air at the same elevation, may be 

 calculated, when the dew point is given ; for its temperature is 

 known by experiments with the nephelescope, and the quantity of 

 vapor condensed by the cold of diminished pressure at every point 

 in its upward motion, and of course the quantity of caloric of elas- 

 ticity given out by this condensation is known, and also the effect 

 this caloric has in expanding the air receiving it, beyond the vol- 

 ume it would have if no caloric of elasticity was evolved in the 

 condensation of the vapor. (175.) For example, according to the 

 experiments of Professor W. R. Johnson, of Philadelphia, a pound 

 of steam, at the temperature of 212, contains 1,030 of caloric 

 of elasticity ; and as the sum of the latent and sensible caloric of 

 steam is the same at all temperatures, it follows, that a pound of 

 steam being condensed in 1,210 pounds of water at 32, would 

 heat this water up one degree ; and, as the specific caloric of air 

 is only 0.267, if a pound of vapor should be condensed in 1,210 

 pounds of air, it would heat that air nearly 4, or, which is the 

 same thing, it would heat 100 pounds of air about 45. And in 

 all these cases it would expand the air about 8,000 times the bulk 

 of water generated ; that is, 8,000 cubic feet for every cubic foot 

 of water formed out of the condensed vapor. And as it requires 

 about 1,300 cubic feet of vapor, at the ordinary temperatures of 



