PRESIDENTIAL ADDRESS. LXV 



tropics, and this aqueous vapour being a good absorbent of heat 

 increases the differential heating at the equator. Further, the 

 atmospheric pressure at the equator is lessened by the presence of 

 this aqueous vapour, the largest amount found anywhere in the 

 earth's atmosphere, and aqueous vapour, being lighter than air in 

 the proportion of 1-1.623 for similar temperatures and pressure, 

 the aqueous vapour at the equator has a greater ballooning or 

 flotation effect there than elsewhere. Both these causes then, to- 

 gether with the adiabatic expansion of the equatorial air, contri- 

 bute to produce those convection currents which are the prime 

 cause of the trade winds and of the anti-trades. 



We can now follow the atmospheric circulation on the poleward 

 side of the high pressure belts (or " horse latitudes," as they are 

 popularly called). 



First of all, it may be explained that not all the air which re- 

 turns to the earth's surface at the base of the high pressure belt 

 flows back again towards the equator. A large part of it divides 

 off and blows polewards, but like the anti-trade winds, from 

 which it has been derived, it, too., undergoes a deflection to the left, 

 and becomes at first the north-west, then west-north-west, and 

 ultimately a westerly wind. These winds, known as the " Roaring 

 Forties," are the most persistent and violent winds in the world. 

 The position of their northern boundary varies a good deal with 

 the seasons, but may be described as approximately nearly coinci- 

 dent with the south coast of Australia. The exact mean southern 

 boundary is as yet unknown, but it may be stated that it extends 

 at times down to about latitude 60 deg. south. It is here that there 

 has been proved to exist that great gully or gutter in the earth's 

 atmosphere^ forming a complete channel around the earth from 

 east to west. The Roaring Forties blow into it from the north, the 

 blizzards from the Antarctic blow into it from the south. A pos- 

 sible reason for its existence will be suggested presently; but we 

 must first follow the probable course of the earth's atmosphere at 

 a high level from the southern anticyclone belt to the south pole. 

 Very little, indeed, is known as yet about this subject, chiefly on 

 account of the fact that nearly the whole of this area is covered 

 by water. Hence the great importance of meteorological observa- 

 tions such as those that are being taken now in the sub-Antarctic 

 at Macquarie Island, by Mawson's Australasian expedition, and 

 in the Antarctic by Filchner, Scott, and Mawson. We do, how- 

 ever, know something of the circulation of the upper atmosphere 

 in the northern hemisphere, though even there there is still much 

 difference of opinion as to the directions of movement of the upper 

 air currents. We have occasionally actual experiences such as the 



' This is called by Hann " Die Rinne " or " Die Luftfurche ", 

 6117. c 



