INTRODUCTION. xi 



the atmosphere, to make one cubic foot of water, if this quantity 

 be subtracted from 8,000, it will leave 6,700 cubic feet of actual 

 expansion of the air in the cloud, for every cubic foot of water 

 generated there by condensed vapor. When hail or snow is 

 formed, the caloric of fluidity given out will produce about an 

 eighth greater expansion. This great expansion of the air in the 

 forming cloud will cause the air to spread outwards in all direc- 

 tions above, causing the barometer to rise on the outside of the 

 cloud, above the mean, and to fall below the mean under the mid- 

 dle of the'cloud as much as it is known to do in the midst of great 

 storms. For example, if the dew point should be very high, say 

 78 D , then the quantity of vapor in the air would be about one fif- 

 tieth of its whole weight, and if the upmoving column should rise 

 high enough to condense one half its vapor into cloud, it would 

 heat the air containing it 45, and the air so heated would be ? 4 ? 5 F 

 larger than it would be if it was at zero, and not so heated. And 

 if we assume a case within the bounds of nature, and suppose the 

 cloud and the column under the cloud to occupy three fourths of 

 the whole weight of the atmosphere, or in other words, if we sup- 

 pose the top of the cloud to reach a height where the barometer 

 would stand at 7 inches, and the mean temperature of the whole 

 column 40 warmer than the surrounding air, (which we may sup- 

 pose, for the sake of illustration, to have a mean temperature of 

 zero,) then would the barometer fall under the cloud at the sur- 

 face of the earth & f 22.5 inches or a little more than an inch 

 and eight tenths. 1 



Though the air will be driven up much higher than the point 

 here assumed, and of course, increase the depression of the ba- 

 rometer, from its specific levity, the cloud will cease to form at 

 greater heights, because the dew point, at these great elevations, 

 falls by a further ascent as rapidly as the temperature ; and at 

 greater elevations, it will even fall more rapidly. If, for instance, 

 the air should rise from where the barometer stands at 6 inches, to 

 where it stands at 3 inches, the dew point would fall about 20, 

 but the temperature would fall less than 20, and therefore no va- 

 por would be condensed by such ascent. When a cloud begins to 

 form from an ascending column of air, it will be seen to swell out 



1 Sittings of the French Academy of Science, 1839, page 715. M. Four- 

 net says, that the parasite clouds which are formed over Mount PiJat 

 do not always redissolve immediately after having been carried beyond 

 the place ot their birth, and that the formation of this kind of cloud is ac- 

 companied by a very considerable local depression of the barometer. If the 

 formation of a small parasitic cloud produces a considerable depression of the 

 barometer, what ought a great storm cloud to do ? 



