and its Physical Relations to the Glacial Epoch. 129 



If such be the case, it is evident that a wind whose tempera- 

 ture is below 32° could never warm a country such as ours, 

 whose temperature does not fall below 38° or 39°. The heat of 

 our south-west winds is derived, not from the equator but from 

 the warm water of the Atlantic — in fact, from the Gulf-stream. 

 The upper current derives its heat after it descends to the earth. 

 There is one way, however, whereby heat is indirectly conveyed 

 from the equator by that current ; that is, in the form of aque- 

 ous vapour. In the formation of one pound of water from 

 aqueous vapour, as Professor Tyndall strikingly remarks, a 

 quantity of heat is given out sufficient to melt five pounds of cast 

 iron*. It must, however, be borne in mind that the greater 

 part of the moisture of the south-west and west winds is de- 

 rived from the ocean in temperate regions. The upper current 

 receives the greater part of its moisture after it descends to the 

 earth. The greater part of the moisture received at the equa- 

 tor is condensed and falls as rain in those regions. 



These, as well as many other considerations which might be 

 stated, seem to lead to the conclusion that, in order to raise the 

 mean temperature of the whole earth, water should be placed 

 along the equator — and not land, as is generally believed. 

 For if land is placed at the equator, we prevent the possibility 

 of conveying the sun's heat from the equatorial regions by 

 means of ocean-currents. The transference of heat could only 

 then be effected by means of the upper currents of the trades ; 

 for the heat conveyed by conduction along the solid coast, if any, 

 can have no sensible effect on climate. But these currents, as 

 we have just seen, are ill adapted for conveying heat. 



The surface of the ground at the equator becomes intensely 

 heated by the sun's rays. This causes it to radiate off its 

 heat more rapidly into space than a surface of water heated 

 under the same conditions. Again, the air in contact with the 

 hot ground becomes also more rapidly heated than in contact 

 with water ; and consequently the ascending current of air carries 

 off a greater amount of heat. But if the heat thus carried away 

 were transferred by means of the upper currents to high latitudes 

 and there employed to warm the earth, then the heat thus con- 

 veyed might to a considerable extent compensate for the absence 

 of ocean-currents, and land at the equator might in this case be 

 nearly as well adapted as water for raising the temperature of the 

 whole earth. But such is not the case ; for the heat carried up by 

 the ascending current at the equator is not employed in warming 

 the earth, but is thrown off into cold stellar space above. 

 This ascending current, instead of being employed in warming 

 the globe, is in reality one of the most effectual means that the 

 * Heat as a Mode of Motion, article 240. 



Phil. Mag. S. 4. Vol. 33. No. 221. Feb. 1867. K 



