328 PHILOSOPHICAL TRANSACTIONS. [aNNO iSsS. 



there are such constant calms in that part of the ocean called the Rains : for 

 this tract being placed in the middle, between the westerly winds blowing on 

 the coast of Guinea, and the easterly trade-winds, blowing to the westward of 

 it, the tendency of the air here is indifferent to either, and so stands in equi- 

 librio between both ; and the weight of the incumbent atmosphere being dimi- 

 nished by the continual contrary winds blowing from hence, is the reason that 

 the air here holds not the copious vapours it receives, but lets them fall in so 

 frequent rains. 



But as the cool and dense air, by reason of its greater gravity, presses on the 

 hot and rarefied, it is demonstrable, that this latter must ascend in a continued 

 stream, as fast as it rarefies, and that being ascended, it must disperse itself to 

 preserve the equilibrium ; that is, by a contrary current, the upper air must 

 move from those parts where the heat is greatest : so by a kind of circulation 

 the north-east trade wind below will be attended with a south-westerly above, 

 and the south-easterly with a north-west wind above. And that this is more 

 than a bare conjecture, the almost instantaneous change of the wind to the op- 

 posite point, which is frequently found in passing the limits of the trade winds, 

 seem to assure us ; but that which above all confirms this hypothesis, is the 

 phenomenon of the monsoons, by this means most easily solved, and without 

 it hardly explicable. 



Supposing therefore such a circulation as above, it is to be considered that to 

 the northward of the Indian ocean there is every where land within the usual 

 limits of the latitude of 30°, viz. Arabia, Persia, India, &c. which for the 

 same reason as the midland parts of Africa, are subject to excessive heats when 

 the sun is to the north, passing nearly vertical ; but yet are temperate enough 

 when the sun is removed towards the other tropic ; because of a ridge of moun> 

 tains at some distance within the land, said to be frequently in winter covered 

 with snow, over which the air, as it passes, must needs be much chilled. Hence 

 it comes to pass, that the air, coming according to the general rule out of the 

 north-east in the Indian seas, is sometimes hotter, sometimes colder, than that 

 which by this circulation is returned out of the south-west, and by consequence, 

 sometimes the under current or wind is from the north-east, sometimes from the 

 south-west. 



That this has no other cause, is clear from the times when these winds set in, 

 viz. in April, when the sun begins to warm those countries to the north, the 

 south west monsoon begins, and blows during the heats till October ; when the 

 sun being retired, and all things growing cooler northward, and the heat in- 

 creasing to the south, the north-east winds enter, and blow all the winter till 

 April again. 



