410 PHYSICAL GEOGPtAPHY OF THE SEA, AND ITS METEOROLOGY. 



leaves and dust. These little whirlwinds are, I take it, the great 

 storms of the sea in miniature ; and a proper study of the minia- 

 ture on land may give us an idea of the great original on the ocean. 



798. The unequally heated plain is thought to be the cause of 

 A vera causa, the ouo. But there are no unequally heated plains 



at sea; nevertheless, the prmw??i mohile there is said, and rightly 

 said, to be heat. Electricity, or some other imponderable, may 

 be concerned in the birth of the whirlwind both ashore and afloat. 

 But that is conjecture ; the presence of heat is a fact. In the 

 middle of the cyclone there is generally rain, or hail, or snow; 

 and the amount of heat set free during the process of condensing 

 the vapour for this rain, or hail, or snow, is sufficient to raise from 

 the freezing to the boiling point more than five times the 

 whole amount of water that falls. This vast amount of heat is 

 set free, not at the surface of the sea, it is true, but in the cloud- 

 region, and where the upward tendency of the indraught is still 

 farther promoted. What sets the whirlwind a-brewing is another 

 question ; but its elements being put in motion, there is a dimi- 

 nished barometric pressure, first, on account of centrifugal ten- 

 dency ; next, on account of the ascending column of air, which 

 expands and ascends, — ascends and expands on account of such 

 diminished pressure ; — and next, though not least, on account of 

 the heat which is set free by the condensation of the vapour 

 which forms the clouds and makes the rain. This heat expands 

 and pushes aside the upper air still more. 



799. After much study, I find some difficulties about the 

 Objections to the cyclono thcor}^ that I cannot overcome. They are 

 ^^""■y- of this sort: I cannot conceive it possible to have 

 a cyclone with a revolving and travelling disc 1000, or oOO 

 miles in diameter, as the expounders of the theory have it. 

 Is it possible for a disc of such an attenuated fluid as common 

 air, having 1000 miles of diameter with its less than w^afer-like 

 thickness in comparison, to go travelling over the earth's surface 

 and revolving about a centre with tornado violence ? With the 

 log-books of several vessels before me that are supposed to have 

 been in different parts of the same cyclone I have a number 

 of times attempted to project its path, but I always failed to 

 bring out exactly such a storm as the theory calls for.* I make a 

 distinction between the hauling of the wind in consequence of 



* Since this was written, I liave had the privilege of examining at the Meteor.o 

 logical Department of the Board of Trade, Admiral Fitzroys admirable diagranis, 



