402 Hydrodynamics, 



The air being thus expanded by means of heat, it is displaced 

 by means of the denser air which rushes in from higher latitudes. 

 The winds which are occasioned in this manner are called the 

 trade winds, and are so uniform in their action, that they render 

 the greatest service to those who navigate the tropical seas. 

 From October to May, the north-east trade wind continues to 

 blow, and from April to October the south-west. 



M. De Luc has advanced a very ingenious explanation of the 

 oblique direction in which these winds approach the equator. 

 This he has attributed to the rotation of the earth. The air at 

 any place, when apparently calm, or in a state of rest, possesses 

 obviously the same velocity of rotation with which that place 

 revolves. Now the rotatory velocity of any point on the earth's 

 surface is proportional to the parallel of latitude which passes 

 through it, that is, to the cosine of latitude, as may be easily 

 shown; hence, were the whole atmosphere in a tranquil state, 

 the actual velocity of every section of it would be proportional 

 to the cosine of its angular distance from the equator. Let us 

 now suppose, that from the influence of heat the condensed air 

 of the north begins to move towards the south. Its rotatory mo- 

 tion being the same with that of the region which it has left, is 

 less than that of the place into which it has come. Hence, as 

 the direction of the earth's motion is from west to east, it will 

 appear to blow from east to west with a relative velocity equal 

 to the difference between the velocities of the parallel left and 

 that come to. TJie same explanation obviously applies to the 

 westerly direction of the wind which rushes towards the equator 

 from the south. In confirmation of this theory, it may be ob- 

 served, upon the authority of Mr. Dalton, that in sailing north- 

 wards, the trade-winds veer more and more from the east towards 

 the north, so that about their limit they become nearly N. E. ; 

 and vice versa, in sailing southwards they Become at least almost 

 S. W. These winds are seldom perceptible in a higher latitude 

 than 30. Their place of concourse appears to be a little to the 

 north of the equator, from the circumstance that the sun, the 

 source of heat, is longer in the northern than in the southern 

 hemisphere. Near the equator they blow about. 4 from the 

 east or west points. 



