﻿438 Mr. J. K. Laughton on Barometric Differences 



tion a large addition must be made on account of the excessive 

 condensation that takes place. 



It is very commonly objected to the view which I have here 

 formed of the effect of centrifugal motion, that if the depression 

 were so caused, the tendency of the air being to fly outwards, 

 the wind would necessarily tend outwards, whilst careful obser- 

 vation shows that in such storms the wind, on the contrary, 

 tends inwards*. That the wind does frequently tend inwards is a 

 well-established fact; but the objection is founded on an incor- 

 rect idea of the motion which results from a centrifugal tendency 

 under the peculiar circumstances. The air whilst rotating 

 is held in by the elastic force and weight of the compara- 

 tively quiescent air outside the whirl, the friction against which 

 checks its motion; it cannot fly off as water from a twirling 

 mop ; it is confined within close limits ; but as the velocity above 

 is very much greater than that at the surface, the air is there 

 able to force its way much further, and to make a section of re- 

 lative low pressure in the column of low pressure. Towards this 

 the air from the surface forces itself, and generates an ascend- 

 ing current; towards the place left vacant by the ascending air 

 other surface air moves and forms this vorticose movement, which 

 is to be observed not only in cyclones and storms, but in the 

 dust whirlwinds of every arid plain, in the little whirls of sand 

 and straw at the corner of a street on a March day, and may 

 be shown experimentally in a tumbler of water with a few grains 

 of sand in it ; if the water be stirred briskly round, the vorti- 

 cose movement which goes on at the bottom will be distinctly 

 visible, and the sand will finally arrange itself in a sharp-pointed 

 conical heap in the middle. The vorticose movement is a di- 

 stinct, clearly marked, and invariable consequent of a centrifu- 

 gal tendency manifesting itself in a fluid revolving in a confined 

 space ; and its presence in a cyclone would be very far from in- 

 validating the opinion I have just stated, even if the excessive 

 condensation did not in itself produce a sufficient depression to 

 meet the objection. 



Independently of the arguments I have already adduced, there 

 are several phenomena exhibited by storms which appear to me 

 capable of ready explanation when referred to the mechanical 

 action of the wind, but are exceedingly difficult to understand 

 if that action is denied : these have been observed principally 

 in tropical cyclones, where, the violence being greatest, irregula- 

 rities are, as it were, magnified, and rendered less likely to 

 escape notice. Such a phenomenon is the rise in the barometer 

 which is very frequently found on the outskirts of a hurricane, 

 and which would seem due to the outward pressure of the air 

 * Handy Book of Meteorology; p. 280. 



