200 FERGUSON'S LECTURES. 



LECT. in at the key-hole of the door, or any small chink, when 

 ^~. ^^ there is a fire in the room. So we may take it in ge- 

 neral, that the air will press towards that part of the 

 world where it is most heated. 



The trade- Upon this principle, we can easily account for the 

 trade-winds, which blow constantly from east to west 

 about the equator. For, when the sun shines perpen- 

 dicularly on any part of the earth, it will heat the air 

 very much in that part, which air will therefore rise 

 upward, and when the sun withdraws, the adjacent air 

 will rush in to fill its place ; and consequently will 

 cause a stream or current of air from all parts towards 

 that which is most heated by the sun. But as the sun, 

 with respect to the earth, moves from east to west, the 

 common course of the air will be that way too ; conti- 

 nually pressing after the sun : and therefore, at the 

 equator, where the sun shines strongly, there will be a 

 continual wind from the east ; but, on the north side, it 

 will incline a little to the north, and on the south-side, 

 to the south. 



This general course of the wind about the equator, is 

 changed in several places, and upon several accounts ; 

 as, 1. By exhalations that rise out of the earth at cer- 

 tain times, and from certain places ; in earthquakes, 

 and from volcanos. 2. By the falling of great quantities 

 of rain, causing thereby a sudden condensation or con- 

 traction of the air. 3. By burning sands, that often 

 retain the solar heat to a degree incredible to those who 

 have not felt it, causing a more than ordinary rarefaction 

 of the air contiguous to them. 4. By high mountains, 

 which alter the direction of the winds in striking 

 against them. 5. By the declination of the sun towards 

 the north or south, heating the air on the north or south 

 side of the equator. 



To these and such like causes is owing, 1. the irre- 

 gularity and uncertainty of winds in climates distant 

 from the equator, as in most parts of Europe. 2. Those 



