Dr. Croll's Theory of Glacial and Warm Periods. 85 



wind thus lags behind, these places come up, as it were, 

 against it, the result being an east wind. Since, therefore, 

 the wind north of the equator is under the influence of two 

 forces — one, the low pressure near the equator, drawing it 

 southwards, and the other, the rotation of the earth, deflect- 

 ing it eastwards — it will by the law of the composition of 

 forces take an intermediate direction, and blow from north- 

 east. For the same reason, south of the equator the south 

 is deflected into a south-east wind " (E. B. XVI., 103.) 



While the rotation of the earth gives the two trade winds 

 a certain westerly motion, it for the same reason gives the 

 anti-trades an easterly movement, since in this case the 

 winds are moving from a latitude where the earth has a 

 great, to one where it has a small rotatory movement. That 

 this is so we can tell by the direction taken by the volcanic 

 ashes carried by the anti-trades to which we have already 

 referred. The result, then, is that the trades and anti- 

 trades, instead of blowing directly to and from the equator, 

 blow at a certain angle to it. 



Between the north-east and south-east trades is the well 

 known belt of equatorial calms. These calms Dr. Croll, 

 following other writers, attributes to the conflict of the two 

 sets of winds, which, according to him, create a neutral belt 

 between them. To this view I cannot at all assent. 



If the effect of heat and the addition of moisture, upon 

 air were to leave its density and specific gravity unaltered, 

 and merely to induce it to move laterally, and thus form a 

 wind, the argument might hold good, but the immediate 

 effect of adding heat and moisture to air is to make it lighter, 

 and also to expand it, and it must begin to move in the direc- 

 tion of least resistance, that is to ascend. As the cold air from 

 the pole gradually moves towards the equator, it gets more 

 and more heated, and more and more full of moisture, and 

 more and more expanded and buoyant. On the contrary, 

 that which proceeds from the equator towards the poles 



