244 Mr. W. Brown on the Oscillations of the Barometer. 



but in the greatest degree towards B, because of the resistance 

 given to the upper current in its How northwards after passing 

 tliis point, and the increasing density of the air in the colder 

 current. In my first paper on this subject (vol. xx.), I gave an 

 illustration by a figure, in which for want of due consideration 

 arise of the barometer was conceived to occur at the point C; 

 but in a subsequent paper on the " Storms of the Tropics" (vol. 

 xxiii.), it is assumed that this is, as here expressed, the point 

 of the greatest barometric depression, though I omitted to 

 notice the discrepancy. As this part of the subject is of 

 great importance, it may be proper to give a further explana- 

 tion of it. The upper current is supposed to descend to the 

 surface of the earth between A and C, on which it will flow 

 to a certain distance dependent on its power to overcome the 

 opposite one from B, and C is the point at which it meets 

 this current and advances upon it; here therefore there will 

 be an influx of air from both sides at the surface, whilst that 

 above is carried away in a continuous current from eg to b. 

 Now it will at once be evident that in this position of the cur- 

 rents, any change produced in the atmospheric pressure will 

 depend upon their relative velocities; if that of the upper one 

 is so much the greatest, that notwithstanding the check given 

 it by the opposite force, more air is carried off from the higher 

 parts of the atmosphere above C towards b than is brought to 

 that point in the lower, the pressure at C must diminish. But 

 the diminution of pressure thus begun by the force of the de- 

 scending current, will go on until this force is reduced, by the 

 loss of pressure sustained, to an equality with that of the op- 

 posite one, and then its momentum being destroyed it will 

 cease to advance, and the latter will begin to advance upon 

 it, to restore the equilibrium of the atmosphere; hence the 

 point of its furthest advance and first cessation must be near 

 that where the diminution of the atmospheric pressure is 

 greatest, or the point C; and at this point, in great storms, 

 there will be a comparative calm throughout a certain extent 

 of the atmosphere. That the conclusion resulting from this 

 reasoning is in consonance with observed fact, may be seen 

 from the observations of P. J. Espy, who has shown that the 

 space between the opposite sides of a storm is in reality the 

 place of minimum pressure in those storms of America which 

 he has investigated*. 



* This, according to this observer, is the position of the fall of rain which 

 occurs during storms, the fact upon which he has founded his theory; but 

 it will be seen that in this case the conditions are precisely such as, accord- 

 ing to the general opinion of meteorologists, are requisite to produce rain, 

 — these are, the meeting of two currents in precisely opposite conditions 



