THE WINDS OF THE SOUTHERN HEMISPHEKE. 431 



vapour commences ; witli tliis, vast quantities of latent heat, which 

 converted the water out at sea into vapour for these winds, are set 

 free in the upper air. There it reacts by warming the ascending 

 columns, causing them still farther to expand, and so to rise higher 

 and hiQ:her, while the barometer sinks lower and lower. This 

 reasoning is suggested not only by the facts and circumstances 

 already stated as well known, but it derives additional plausibility 

 for correctness by the low bai'ometer of these regions. In the 

 equatorial calm belts the mean barometric pressure is about 0.25 

 inch less than it is in the trade-winds, and this diminution of 

 pressm-e is enough to create a perpetual influx of the air from either 

 side, and to produce the trade-winds. Off Cape Horn the mean 

 barometric pressure* is 0.75 inch less than in the trade-wind 

 regions. This is for the parallel say of 57° — 8° S. According to 

 the mean of 2,472 barometric observations made along that part 

 only of the route to Australia which lies between the meridians of 

 the Cape of Good Hope and Melbourne, the mean barometric 

 pressure on the polar side of 42° S. has been shown by Lieutenant 

 Van Gough, of the Dutch Navy, to be 0.33 inch less than it is in 

 the trade-winds. The mean pressure in this part of the South 

 Indian Ocean is, under winds with easting in them, 29.8 inches: 

 ditto, under winds with westing, 29.6 inches. Plate I. shows a 

 supposed mean pressure in the polar calms of not more than -28.75 

 inches. The barometric curve, page 468, shows with a higher 

 degree of probabihty that the mean pressure there is about 28.14 

 inches. 



834. To what, if not to the effects of the condensation of vapour 

 Aqueous vapour the bomo by thoso Surcharged winds, and to the immense 

 cause of both. precipitation in the austral regions, shall we ascribe 



this diminution of the atmospherical pressm-e in high south lati- 

 tudes ? It is not BO in high north latitudes, except about the 

 Aleutian Islands of the Pacific, where the sea to windward is also 

 wide, and where precipitation is frequent, but not so heavy. The 

 steady flow of " brave " winds towards the south would seem to call 

 for a combination of physical conditions about their stopping-place 

 in the antarctic regions, exceedingly favourable to rapid, and heavy, 

 and constant precipitation there. The rain-fall at Cherraponjie and 

 on the slopes of the Patagonian Andes reminds us what those 

 conditions are. There mountain masses seem to perform in the 



* Maury's Sailing Directions, 6th ed., 1854, p. 692 ; ditto, Sth ed., 1859, vol. ii., 

 p. 450. 



