112 PHILOSOPHICAL TRANSACTIONS, [aNNO 1685. 



east to west in both seas within the tropics, because the eastern winds prevail 

 there, yet returning from west to east they must seek the westerly winds with- 

 out the tropics, in the latitude of 27° . 



Now the reason of this seems to be clearly deducible from the former; for 

 the pressure of the air between the tropics being continually less than that of 

 the neighbouring parts of the atmosphere, and so consequently by them pressed 

 westward; its motion from west to east is proportionably increased beyond that 

 uniform motion it would have, if the whole atmosphere were of an equal den- 

 sity, and consequently there will blow a constant wind from west to east, for 

 some degrees beyond the tropics. 



Again, the easterly winds between the tropics it seems do not blow constantly 

 from the same point, nor] directly from the east; but for the one half of the 

 year, viz. from about April till November, they come from the south-east, and 

 for the other half of the year, viz. from November to April, they blow from 

 the north-east. And these are called the monsoons and trade winds. Hence it 

 is that those who sail from China, Japan, &c. to Bantam, must wait for the 

 northerly monsoon, which falls between November and April; and those who 

 return from Bantam must go back again when the southerly monsoon comes, 

 which is between April and November. And the currents of the seas are said 

 to observe the same motions and changes with the winds. 



Now these monsoons may be easily accounted for from what has already been 

 said about the cause of the continual easterly winds between the tropics; for 

 since the diminution of the pressure of the air under the line, and that of the 

 neighbouring parts of the atmosphere, cause these continual breezes, if the 

 sun were constantly in the equinoctial, it is probable the wind would always 

 blow directly from the east ; but as he is the one half of the year on the one 

 side of the line, and the other half on the other, there must necessarily follow 

 a change of these breezes into stated monsoons. For if we imagine the atmos- 

 phere to be divided into two equal hemispheres by the equinoctial plane, and if 

 the sun were always in the plane, there would be constantly an equal pressure 

 from both these hemispheres on the air under the line, and the breeze would be 

 directly from the east. But when the sun comes on the north side of the line 

 as far as the tropic of Cancer, and back again, there is not an equal balance, 

 but the pressure of the southern hemisphere of the air must needs be greatest 

 and consequently the breeze must blow all that season from the south-east; 

 and when the sun returns again to the southward of the line, as far as Capri- 

 corn, and back again, the pressure of the northern hemishphere must prepon- 

 derate, and cause the wind to blow all that half year from the north-east. And 

 this seems to accord very well with experience, for their northern monsoons are 



