﻿and Fluctuations. 335 



the much greater difference of temperature, but of the immediate 

 contact of the two bodies of air. In a coal-mine, for instance, 

 between. the air that, passing through or over an enormous fire, is 

 driven up the upcast shaft and the air that drives it up, there is 

 a difference in temperature of perhaps not less than 200° F. 

 The difference in the weight of a cubic foot of air corresponding 

 to such a difference of temperature is about 184 grains ; and the 

 weight of a cubic foot of the hotter air would be something less 

 than 400 grains. Thus we have the acceleration produced as 

 about 15 feet per second. Such a calculation has, of course, no 

 pretensions to accuracy ; my object here is, not to arrive at the 

 velocity with which air issues from the mouth of a coal-pit, but 

 to show that, between the pressure which causes the current of 

 air in a coal-pit and the pressure which has been supposed to 

 cause the trade-winds, there is practically no comparison, and that 

 the latter force is utterly inadequate to produce any appreciable 

 effect. 



I have been obliged to dwell on this point in order to establish 

 the fact that there can be no perceptible barometric difference 

 due to the lifting effect of the interchange resulting from differ- 

 ences of density. A low barometer caused by a high temperature 

 is therefore really caused by a further expansion consequent on 

 the withdrawal of a pressure from above, by the overflowing of 

 such air as, by the expansion of the lower strata, is lifted above 

 the strata of equal pressure. A method of calculation similar to 

 that already adopted enables us to form some idea of the nume- 

 rical value of this overflow. A cubic foot of moist air at 72° F. 

 is increased at 80° F. to 1*008 foot, which gives a linear expan- 

 sion of from 1 foot at 72° to 1-0027 at 80°. If, therefore, we 

 suppose this expansion to extend through a height of 10,000 

 feet (a very exaggerated estimate), the superior strata on the 

 parallel of 9° are raised 27 feet higher than the strata of equal 

 pressure on the parallel of 30°. The incline down which the 

 overflow pours is therefore one of 27 feet in 21 x 60 x 6000, or 

 1 foot in 280,000. The motion generated by gravity on such a 

 slope, with the very intense friction which masses of air exercise 

 on each other against it, cannot possibly be perceptible ; it can 

 neither become a sensible wind, nor can it show itself by any 

 difference of barometric pressure. 



I have already called attention to the fact that geographical 

 observations directly contradict the statements that a high tem- 

 perature or a humid atmosphere cause a low barometer, and that, 

 on the other hand, a low temperature or a dry atmosphere cause 

 a high one. So far as geography is concerned, we may regard 

 it as clearly established that there is no necessary relation between 

 the mean reading of the barometer and the mean readings of the 



