46 OUR HERITAGE THE SEA 



year, but sufficiently steady especially as it is wind 

 we are considering to be called a permanent current 

 of air. Now, it will certainly be asked, Why do these 

 great air currents act thus why, indeed, are they in 

 being at all ? 



Well, without pretending to be scientific, but at 

 the same time keeping closely to fact as far as it has 

 been ascertained, the reason of the Trade Wind is this. 

 Within the tropics the sun's rays pour down fervently 

 and heat the air, rarefy it, in fact, so that it rises 

 higher and higher above the sea, leaving room for the 

 colder, heavier air from the poles to rush in and fill 

 up the partially vacated space. Now, if the globe did 

 not revolve upon its axis, the direction of these inrush- 

 ing currents of air would be from due north and south 

 towards the equator. But the girth of the revolv- 

 ing globe increases from pole to equator ; the tropical 

 surface often, therefore, like the outside of a wheel is 

 moving from west to east faster than the incoming air 

 from nearer the poles, which, so to say, gets left behind 

 and is deflected in the direction of east to west. So 

 that northward of the equator the north wind acquires 

 an easterly trend, and to the southward of the equator 

 takes the same bias. Hence these two main streams of 

 moving air or wind travel more or less steadily in a 

 north-east or south-easterly direction, and from their 

 dependency and steadiness they have received the 

 names they bear of the North-East and South-East 

 Trades. Of course, there are other factors which enter 

 into the production of these two mighty air currents, 

 such as the changeable influence of the heat over the 

 land, configuration of the land, etc. But these are 

 the main causes, and, since this is in no sense a 



