THE WINDS OF THE SOUTHERN HEMISPHERE. 447 



precise locality has not been determined, but I suppose it to be a 

 band or disc— an area — within the polar circle, which, could it 

 be explored, would' be found, like the equatorial calm belt, a 

 place of light airs and calms, of ascending columns of air, — a 

 region of clouds, of variable wdnds, and constant precipitation. 



833. Also of a low barometer . — But, be that as it may, the air 

 which these vapour-bearing winds — vapour -bearing because they 

 blow over such an immense tract of ocean — pour into this 

 stopping-place has to ascend and flow off as an upper current, to 

 make room for that which is continually flowing in below. In 

 ascending it expands and grows cool, and, as it grows cool, con- 

 densation of its vapour commences ; with this, vast quantities of 

 latent heat, Avhich 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 higher, while the barometer 

 sinks lower and loAver. This reasoning is suggested not only by 

 the facts and circumstances already stated as well known, but it 

 derives additional plausibility for correctness b}' the low baro- 

 meter 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 pressure is enough to create 

 a perpetual influx of the air from either side, and to produce the 

 trade-winds. Oif 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 baro- 

 metric 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 ti-ade-wdnds. 

 The mean pressure in this part of the South Indian Ocean is, 

 under winds with easting in them, 29.8 inches : ditto, under 

 wdnds 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 wdth a higher degree of 

 probability that the mean pressure there is about 28.14 inches. 



834. Aqueous vapour the cause of hoth. — To wdiat, if not to the 



* Maury's Sailing Directions, Gtli cd., lSo4, p. 692; ditto, Sfcli cil., 1859, 

 vol. ii., p. 450. 



