﻿442 On Barometric Differences and Fluctuations. 



ciples of fluids in motion, but that, when once formed, the as- 

 cending current, supplied by the vorticose movement due to 

 the action of the centrifugal tendency in a confined space, gives 

 rise to such excessive condensation that the depression of the 

 barometer near the centre is very much increased, and most so 

 within the tropics, where the absolute quantity and elastic force 

 of vapour in the air are greatest. This increase of depression 

 increases the force of the wind, which again increases the de- 

 pression, the velocity of the ascending current, and the con- 

 densation. The various agencies thus continue to act and react 

 on each other, so that it is impossible to separate the effects 

 of each ; but as the whirl is commenced and the centre of low 

 pressure first formed by mechanical action, it is to it that I at- 

 tribute the origin of the storm. Without the mechanical action 

 such storms never occur, notwithstanding even excessive disturb- 

 ances in the thermometric or hygrometric conditions of the atmo- 

 sphere ; and, On the other hand, gentler whirls and barometric 

 disturbances within considerable limits frequently do occur with- 

 out any corresponding manifestation of meteorological changes. 

 But this conclusion is, in its essential points, the same as 

 that at which we previously arrived (namely, that differences of 

 barometric pressure are in many instances caused by the winds), 

 and justifies us in saying that the meteorological axiom, that 

 all winds are caused by differences of barometric pressure, 

 although presumably true on hydrostatic principles, is not sup- 

 ported, and is frequently contradicted, by positive evidence. 



The prevailing winds of different localities, which thus, regu- 

 larly or irregularly, produce many of the barometric variations, 

 may be traced, more or less distinctly, to their geographical 

 causes, as deflections of the great westerly wind which we find 

 ruling at the surface of the earth in temperate latitudes, and 

 which, from extreme north to extreme south, so far as our expe- 

 rience extends, is permanent in the upper regions of the atmo- 

 sphere*. But the origin of this westerly wind is obscure. Re- 

 cent observations seem to point towards the conclusion that 

 the sun's atmosphere has a general movement from west to east, 

 independent of, and additional to his proper movement of ro- 

 tation, and almost exactly similar to that which, it appears to 

 me, affects our own. If this is confirmed, it is at least a stri- 

 king coincidence, and would seem to lend a very strong sup- 

 port to the various other reasons for believing that this domi- 

 nant motion of our atmosphere is due to cosmical forces. 



* I have treated of this question in detail in my ' Physical Geography ;' 

 to say more about it here would be merely to repeat the arguments and 

 illustrations which I have there put forward. 



