﻿and Fluctuations. 431 



the conclusion that storms are due in great measure to the geo- 

 graphical conditions under which they occur, and that, though 

 possibly originated by meteorological disturbances, the part 

 played by these must be comparatively trifling, since in other 

 localities, where similar disturbances must frequently occur, 

 they do not give rise to similar storms. 



If, then, storms of a cyclonic nature (and it is to these only 

 that I here refer) are caused by the mechanical clashing of atmo- 

 spheric currents either against each other or against more mate- 

 rial obstacles, it is evident that the remarkable barometric de- 

 pression in their centre is, in the first instance, occasioned by the 

 force of the whirl, and that the whirl is not occasioned by the 

 barometric depression. This, however, is contrary to the opinion 

 now in favour with many meteorologists, who maintain that the 

 barometric depression is first formed, and that the storm, or the 

 rush of air towards the area of minimum pressure, is more or 

 less violent according to the extent to which the pressure has 

 been reduced. That there is a very intimate connexion between 

 the violence of the storm and the depression of the barometer 

 would seem to be certainly established \ but they are so mixed 

 up one with the other, each operating on and increasing the 

 other, that it is difficult to assign to each its exact value as agent 

 and as patient. But this is not what is meant by those who teach 

 that the barometric depression is the absolute cause of the storm, 

 and that the depression is due to other agencies, the capabilities 

 of which they seem to me to overrate. 



Of these agencies, that on which most stress is laid is the con- 

 densation of aqueous vapour, and the sudden manifestation of 

 heat in the upper regions of the atmosphere. That excessive 

 rain is the invariable concomitant of storms of this class, and 

 more distinctly of tropical hurricanes, is not to be disputed ; but 

 there are many places in the world where rain falls at times 

 quite as freely as in the very centre of a hurricane without pro- 

 ducing any depression such as is here spoken of. I have already 

 examined into the effect of heavy rain on the barometer in India 

 and other places ; but I nowhere find any grounds for assigning 

 to it a capability of diminishing the pressure by 2*5 inches, a 

 diminution which has been observed in the centre of a cyclone ; 

 neither are the localities in which cyclones take their rise locali- 

 ties of excessive rain. If the aqueous vapour in these districts 

 is not liable to free condensation, and if the very freest conden- 

 sation which we have satisfactory means of measuring, as at 

 Cherrapongee, produces no such excessive effect on the barometer 

 — no effect, indeed, at all approaching to it — it is difficult to ad- 

 mit that condensation is the cause of the depression observed. 

 The whole of the depression, however, is not attributed to this ; 



