20 PHILOSOPHICAL TRANSACTIONS. [aNNO 1735. 



or else have applied it to the explication of these phaenomena, on insufBcient 

 principles. 



That the action of the sun is the original cause of these winds, it seems all 

 are agreed ; and that it does it by causing a greater rarefactio-i of the air in 

 those parts on which its rays, falling nearly perpendicular, produce a greater 

 degree of heat there, than in other places ; by which means the air becoming 

 specifically lighter than the rest round about, the cooler air will, by its greater 

 density and gravity, remove it out of its place, and make it rise upwards. But 

 it seems that this rarefaction will have no other effect, than to cause the air to 

 rush in from all parts, into the place where it is most rarefied, especially from 

 the north and south, where the air is coolest, and not more from the east than 

 the west, as is commonly supposed. So that, setting aside the diurnal motion 

 of the earth, the tendency of the air would be from every side towards that 

 part where the sun's action is most intense at the time, and so a n. w. wind be 

 produced in the morning, and a n. e. in the afternoon, by turns, on this side 

 of the parallel of the sun's declination, and a s. w. and s. e. on the other side. 



That the perpetual motion of the air towards the west, cannot be derived 

 merely from the action of the sun upon it, appears more evidently from this : 

 if the earth be supposed at rest, that motion of the air will be communicated to 

 the superficial parts, and by little and little produce a revolution of the whole 

 the same way, except there be the same quantity of motion given the air in a 

 contrary direction in other parts at the same time ; which is hard to suppose. 

 But if the globe of the earth had before a rotation towards the east, this by the 

 same means must be continually retarded: and if this motion of the air be sup- 

 posed to arise from any action of its parts on each other, the consequence will 

 be the same. For this reason it seems necessary to show how these phaenomena 

 of the trade-winds may be caused, without producing any real general mo- 

 tion of the air westwards. This will readily be done by taking in the considera- 

 tion of the diurnal motion of the earth: for, let us suppose the air in every part 

 to keep an equal pace with the earth in its diurnal motion ; in which case there 

 will be no relative motion of the surface of the earth and air, and consequently 

 no wind : then by the action of the sun on the parts about the equator, and the 

 rarefaction of the air thence proceeding, let the air be drawn down thither from 

 the N. and s. parts. The parallels continually enlarge, as they approach to the 

 equator, and the equator exceeds the tropics, nearly in the ratio of 1000 to 

 gi7 ; consequently their difference in circuit is about 2083 miles, and the sur- 

 face of the earth at the equator moves so much faster than the surfiice of the 

 earth with its air at the tropics. From which it follows, that the air, as it 

 moves from the tropics towards the equator, having a less velocity than the 

 parts of the earth it arrives at, will have a relative motion contrary to that of 



