the Distribution of Heat over the Globe. 85 



at the equator, are nowhere below the snowline; they must 

 therefore lie in a region actually below the freezing-point. In 

 fact, if those currents were warm, they would elevate the snow- 

 line above themselves. The heated air rising off the hot burn- 

 ing ground at the equator, after ascending a few miles, becomes 

 exposed to the intense cold of the upper regions of the atmo- 

 sphere ; it then very soon loses all its heat, and returns from 

 the equator much colder than it went thither. 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 snowline 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 tempera- 

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

 where the temperature does not fall below 38° or 39°. The heat 

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

 but from the warm water of the Atlantic — in fact from the Gulf- 

 stream. The upper current acquires its heat after it descends to 

 the earth. There is one way, however, whereby heat is indi- 

 rectly conveyed from the equator by the anti-trades ; that is, 

 in the form of aqueous 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 derived from the ocean in temperate regions. The 

 upper current receives the greater part of its moisture after it 

 descends to the earth, whilst the moisture received at the equator 

 is in great part 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 

 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, the possibility of conveying the sun's heat 

 from the equatorial regions by means of ocean-currents is pre- 

 vented. The transference of heat could then be effected only by 

 * Heat as a Mode of Motion, art. 240. 



