GENEKAL OBSEEVATIONS ON THE WINDS. 101 



or compensating current ; and that the crossing of the winds at the Tropic 

 is a physical impossibihty. 



Another view of the cause of atmospheric circulation is that by Professor 

 Laughton, of Portsmouth, who contends that it is due to aero-tidal or 

 mechanical causes. He supposes that the prevailing Westerly wind of 

 high latitudes is caused by the influence of the moon, and that the modi- 

 fications of this wind, caused by the coasts which bound the sea, are the 

 sources of all other winds. For instance, he says wh«n our Westerly wind 

 strikes the coast of Portugal, it will branch to the North and South as any 

 fluid would do under similar circumstances, and that eventually it will 

 recurve to the Westward to fill up the space from which the air forming 

 the original Westerly wind had moved. Thus eddying, as it were, in two 

 great circuits, and carrying the surface waters with it, causing the main 

 Currents of the Ocean.* 



There can be no doubt that land frequently offers an obstruction to a 

 current of air, and diverts its course, but this theory does not take into 

 account the changes which come with the seasons ; they seem to be due 

 to Temperature. 



(10.) The Honourable E. Abercromby considers that the popular idea 

 about the circulation of the atmosphere over the Equator is certainly 

 erroneous. It is usually supposed that the N.E. and S.E. Trades meet 

 over the Doldrums, and that the air then rises and pours back to the 

 nearest Pole. He says : — " The result of my own observations in the 

 Doldrums of both the Atlantic and Indian Oceans, points to the conclusion 

 that at high levels the two Trades rather tend to coalesce into a single 

 Easterly current, and that the poleward motion of air near the Equator is 

 very small. The propagation of the dust ejected by the great eruption of 

 Krakatoa, in 1883, confirmed this view. The dust went round the world 

 in a belt near the Equator in about four days, and did not reach middle 

 latitudes till two or three months later. "f As this dust, however, is con- 

 sidered to have been projected far above the region influenced by the 

 action of the Winds (12), this argument is not conclusive. 



(11.) There is one feature of the atmosphere which has been involved in 

 some obscurity, or, at least, has been the subject of controversy. It is the 

 condition of Aqueous Vapour, at all times present in the air. It is a very 

 important question, as upon this water-bearing property of the air, evapo- 

 ration, condensation, and rain depend, and consequently climate and fer- 

 tility to the earth. Air charged with vapour is specifically lighter than dry 

 air, thus diminishing the atmospheric pressure where it exists, and this is 

 well illustrated by the low pressure existing over the moist Equatorial 

 Calm Belt. 



The doubts may be briefly stated. The eminent chemist, Dalton, demon- 

 strated that one gas (and aqueous vapour is such) could permeate or exist 

 in connection with another gas without displacing its bulk, and that water 

 was thus diffused through the atmosphere without increasing its volume. 



♦ •' Physical Geography in its relation to prevailing Winds and Currents." 



t "Seas and Skies in Many Latitudes," London, 1888, page 427 ; "Observations on 



Cloud Movements near the Equator," by the same author, in the Quarterly Journal ot 



the Royal Meteorological Society, 1888, pp. 231 — 295. 



