114 MOTIONS AND PEESSUEE OF THE ATMOSPHEEE. 



that a higher barometer sometimes exists over the water than over the 

 land, which is probably the cause of so much North-Easterly wind at that 

 season in England. 



*' Again, to the Northward of the N.B. Trades, in the Calms of Cancer, 

 there is a region of very high barometer which does not extend up to the 

 coast of Africa or America (see the French 'Atlas des Mouvements 

 G6n^raux de 1' Atmosphere '). It is also well known to seamen that this 

 area of high barometer extends farther North in the summer ; which, 

 taken into consideration with the heated land and consequent lower 

 pressure on the coasts of Spain and Portugal, seems sufficient to account 

 for the prevalence of Northerly winds on those coasts. The N.W. wind, 

 which so very generally precedes the N.E. Trades with ships outward 

 bound from England, seems to result from the draught of air, in accord- 

 ance with Buys-Ballot's Law, between this area of high pressure and the 

 lower pressure to the Bast and N.E. of it. For the same reason the South- 

 Easterly tendency of the N.E. Trades, on the Western side of the Atlantic, 

 between 20° and 30° N. (see the Pilot Charts for the Atlantic Ocean), is 

 probably due to the air drawing round this area of high pressure. In fact, 

 it seems most probable that this area of high pressure is a perpetual ' Anti- 

 Cyclone,' the name given by Mr. Francis Galton, in his ' Meteorographica,'* 

 to patches of high pressure round which the wind revolves in a contrary 

 direction to that of a Cyclone, which is an area of low pressure ; and that 

 this area of high pressure is a downward current of air, which has over- 

 flowed from the various high temperatures and low pressures around it, 

 out of which the air seems to issue in various directions, being governed 

 by the neighbouring low and high pressures ; whilst in it there is a large 

 extent of equal pressure and consequent calm ('the Horse Latitudes') 

 which should be avoided by saiUng ships as a kind of atmospheric shoal. 

 It will be noticed that the route of West Indian Hurricanes is round this 

 area of high pressure. The effect of the earth's revolution upon these dov^n- 

 ward and upward currents of air is supposed to be the cause of their 

 revolving in contrary directions. f 



" In working up the meteorological data for that part of the Atlantic 

 between 20° N. and 10° S., I notice that in the winter there is a tendency 

 for the N.E. Trades to draw to the S.E., on the S.W. side of the Cape Verde 

 Islands, which probably arises from the air being cooler and heavier over the 

 land, thus bringing Buys-Ballot's Law into play to deflect the wind. 



"It is well known to those who navigate the South Atlantic, that an 

 outward bound ship on the Western side of that ocean has a N.E. wind 

 towards the Southern verge of the S.E. Trades, whilst a homeward bounder 

 in the same latitude, but near the African coast, has a S.W. wind. These 

 winds seem to result from the same cause, which has been alluded to in 

 the North Atlantic, viz., an area of high pressure over the central part of 

 the ocean, which decreases as you approach the land on either side. 



" In the Proceedings of the Meteorological Society, February 17th, 1869, 

 are given ' Eesults from Meteorological Observations, made at the Eoyal 

 ■Observatory, Cape of Good Hope, for twenty years, by Sir T. Maclear,' 



• " Meteorographica," by Francis Galton, F.R.S., F.R.G.S. — Macmillan & Co. 

 f Full descriptions of the Cyclone and Anti-Cyclone axe given hereafter. 



