﻿and Fluctuations. 435 



space along its entire circumference. In violent cyclones the 

 maximum velocity of the wind is estimated as certainly not less 

 than 120 miles an hour, and is probably a good deal more. The 

 height to which the storm extends is absolutely unknown ; Cap- 

 tain Maury has estimated the height of the Trade- Winds as about 

 three miles ; I should be inclined to agree with Redfield, that 

 one mile is probably the extreme height of a cyclone. If, then, 

 the diameter of the central space be 12 miles, the quantity of air 

 that is poured in in the course of an hour is ir X 12 X 120 cubic 

 miles, which, disappearing upwards from an area of ir X 36 square 

 miles, gives us an ascensional velocity of 40 miles an hour — 

 that is, of 66 feet a second. Even then, if we were to suppose 

 that the greatest difference observed at Killingworth, which is 

 equivalent to a barometric depression of *06 of an inch, corre- 

 sponds to the aggregate velocity of 46 feet a second, and that 

 the depression increases in proportion to the velocity, we should 

 still have no more than *09 of an inch as the measure of that 

 component of the depression in the centre of a cyclone which 

 could be attributed to this cause. But such a supposition would 

 be wild, to the verge of absurdity ; and not less so is the esti- 

 mate of 66 feet a second as the velocity of the ascending cur- 

 rent. Such a velocity represents itself a gale of wind, a wind 

 of force 7 on the Beaufort scale ; and most certainly nothing 

 like it was ever observed in the most violent cyclone. That the 

 air towards the centre of these storms has a sensible upward mo- 

 tion has been noticed ; but even an approximation to such a 

 velocity as 66 feet a second is out of the question. A velocity of 

 5 feet a second, or even 10, may perhaps be admitted ; and if, 

 for the sake of the argument, we were to allow that the upper 

 difference observed at Killingworth, or '10 of an inch of the 

 water-gauge, denotes the depression caused by a velocity of 46 

 feet, we might so account for a central barometric depression of 

 about *0015 of an inch, a depression which no instrument prac- 

 tically used at sea could be depended on to register. 



If, then, the barometric depression in the centre of a cyclone 

 cannot be referred to condensation, if it is not due either to the 

 withdrawal of the elastic force of the aqueous vapour or to the 

 liberation of heat above, if it is not due to the ascensional velociy 

 of an upward current, meteorological reasons for its existence 

 fail us, and we are thrown back on that which I have already 

 suggested as its primary cause — the centrifugal tendency esta- 

 blished by the force of the whirl itself. This is constantly said 

 to be utterly inadequate to produce such a depression as is ob- 

 served; as I have just shown the insufficiency of other alleged 

 causes, I will attempt to form some estimate of what may be the 

 capability of this mechanical agent. 



