§ 833. THE WINDS OF THE SOUTHERN HEMISPHERE. 447 



the northern one may be called. These yapor-bearing winds 

 which brought the rains to Patagonia are — I wish to keep this 

 fact in the reader's mind — the counter-trades of the southern hemi- 

 sphere. As such they have to perform their round in the grand 

 system of aerial circulation, and as, in every system of aerial cir- 

 culation there must be some point or place at which motion ceases 

 to be direct and commences to be retrograde, so there must be a 

 place somewhere on the surface of our planet where these winds 

 cease to go forward, stop, and commence their return to the north ; 

 and that place is, in all 2yrohability, xoitliin the antarctic regions. Its 

 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 winds, and constant precipitation. 



833. But, be that as it may, the air which these vapor-bearing 

 Ako of a low barom- wiuds — vapor-bcariug 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, condensation of its 

 vapor commences ; with this, vast quantities of latent heat, which 

 converted the water out at sea into vapor 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 high- 

 er and higher, while the barometer sinks lower and lower. This 

 reasoning is suggested not only by the facts and circumstances al- 

 ready stated as well known, but it derives additional plausibility 

 for correctness by the low barometer 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 press- 

 ure 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 re- 

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

 the mean of 2472 barometric observations made along that part 

 only of the route to Australia which lies between the meridians 



* Maury's Sailing Directions, 6th ed., 1854, p. 692 ; ditto, 8th ed., 1859, vol. ii., p. 

 450. 



