/. Croll — Influence of the Gulf-stream. 161 



upper regions of the atmosphere. It then very soon loses all its 

 heat, and returns from the equator much colder than it came. It is 

 impossible that we can receive any heat directly from the equatorial 

 regions by means of aerial currents. It is perfectly true that the 

 south-west wind, to which we owe so much of our warmth in this 

 country, is a continuation of the anti-trade. But the heat which this 

 wind brings to us is not derived from the equatorial regions. This 

 will appear evident, if we but reflect that, before the upper current 

 descends to the snow-line after leaving the equator, it must traverse 

 a space of at least 2000 miles ; and to perform this long journey 

 several days will be required. During all this time the air is in a 

 region below the freezing-point ; and it is perfectly obvious that by 

 the time it begins to descend it must have acquired the temperature 

 of the region in which it has been travelling. 



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

 is below 32'^ could never warm a country such as ours, whose tem- 

 perature 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 aqueous vapour. In the forma- 

 tion of one pound of water from aqueous vapour, as Professor Tyn- 

 dall 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 derived 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 equator 

 is condensed and falls as rain in those regions. 



These, as well as many other considerations which might be 

 stated, lead to the conclusion that, in order to raise the mean tem- 

 perature 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 crust, if any, can have no sensible effect on climate. 

 But these currents, as we have just seen, are ill adapted for con- 

 veying 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 

 1 Heat as a Mode of Motion, article 240. 



VOL. YI. — NO. LVIII. 11 



