122 Philosophy of Storms. 



the height of 4800 feet before it begins to form cloud, the whole 

 column would then be 1 00 feet of air hghter than surrounding 

 columns ; and if the column should be very narrow, its velocity 

 of upward motion would follow the laws of spouting fluids, which 

 would be 8 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 is 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 70°. 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 degree in ascending one hundred yards. 



Now it has been ascertained by aeronauts and travellers on 

 mountains, that the atmosphere itself 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 perpendicular 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 

 to that of the surrounding air at the same elevation, may be cal- 

 culated 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 elasticity given out by this condensation is known, and also 

 the effect this caloric has in expanding the air receiving it, be- 

 yond the volume it would have, if no caloric of elasticity was 

 evolved in the condensation of the vapor. 



For example, according to the experiments of Prof. Walter R. 

 Johnson, of Philadelphia, a pound of steam at the temperature of 

 212° contains 1030° of caloric of elasticity, and as the sum of 

 the latent and sensible caloric of steam is the same at all tempe- 



