﻿434 Mr. J. K. Laughton on Barometric Differences 



with the difference in pressure which is registered. If a certain 

 number of cubic feet of air are each minute driven up one shaft 

 or down another, the velocity of the ascending or descending 

 current must be approximately the same throughout the shaft at 

 all depths, and the influence which the motion exercises on the 

 barometer or gauge must be constant. That the observed dif- 

 ference, far from being constant, increases as the depth increases, 

 shows at once that it is not to the motion that we must look for 

 its cause, but rather to the difference of the weight of the columns 

 of air. But the difference shown by the gauge is much less 

 than what is due to the difference of weight at the several depths, 

 calculated for the mean temperatures : near the top this is partly 

 to be accounted for by the temperature of the upcast being lower 

 than the mean, and the remaining portion of the column being 

 thus heavier than is calculated ; but near the bottom, where the 

 temperature is higher than the mean, it would seem that expan- 

 sion does not take place quickly enough to balance the increase 

 of elastic force caused by the heat. A large part of the pressure 

 due to the difference of weight is absorbed by the " drag of the 

 air ;" it is the remainder only which forces the heated air up the 

 shaft ; and the velocity is thus much less than what is properly 

 due to the difference of temperature. It is, however, impossible 

 to say how much of the relative increase of elasticity is due to 

 heat, how much to mechanical compression ; but near the top of 

 the shaft, where the weights of the columns become the same 

 and the compression ceases, though the velocity remains neces- 

 sarily unchanged, the gauge no longer shows any difference. 



It appears to me, therefore, that, so far as we can apply the 

 circumstances of coal-pit ventilation, there is absolutely no evi- 

 dence whatever that any diminution of barometric pressure is 

 caused by an ascending current, or, conversely, any increase by 

 a descending one. The belief in such an effect appears, in fact, 

 to be due to the confusion between weight and elastic force, 

 already spoken of; for where such a confusion of idea exists, it 

 is natural to suppose that an ascending weight will exert a less 

 pressure than a stationary or a descending one. 



But even if we were to admit that a barometric difference may 

 be due to such a cause, to the fullest extent that can be claimed 

 for it, it would still be quite impossible to attribute to it any ap- 

 preciable decrease of pressure in the centre of a cyclone. Mr. 

 Buchan conceives that in a cyclone the air rushing towards the 

 area of least pressure rarely makes a complete circuit*; and 

 since the force of the wind is greatest in the immediate neigh- 

 bourhood of the central calm, it follows, according to this view, 

 that air with its maximum velocity is entering into the central 

 * Handy Book, p. 281. 



