448 PHILOSOPHICAL TRANSACTIONS. [anNO 1740. 



the land-breeze begins, and continues till 8 next morning. The interval be- 

 tween these two breezes, at morning and evening, are the hottest parts of 

 the day. 



The way of accounting for this vicissitude of sea and land-wind, is thus : 

 the sun, as it ascends, sheds its heat equally on the land and sea ; but the 

 earth receives the heat sooner than the water, or else reflects it stronger. For 

 one or both of these reasons, the air that hangs over the land, is heated more 

 than the sea air, it becomes thereby more rarefied, and consequently lighter : 

 and therefore the sea air, with its superior weight, flows in upon it every way. 

 The intervals between are owing to the air of both places being in an equal de- 

 gree of heat, and consequently of equal weight. 



The trade-wind never varies, which is thus accounted for: the air just 

 under the sun is the hottest : the cold air presses upon the hot, as the 

 hot air follows the sun ; and therefore it makes a perpetual flow of wind be- 

 tween the tropics from Africa to America, and from thence to the East- 

 Indies. 



With regard to the wind influencing the weather ; though air be an un- 

 mixed fluid, yet it is capable of receiving many vapours, which float in it, as 

 we see other bodies float in water. Sometimes the vapour ascends, and some- 

 times it falls to the ground. All which is probably effected by heat and cold 

 in this manner : heat separates water into small particles, and the incorporated 

 air, rarefied by the same heat, blows up those particles into bubbles ; by which 

 means the swoln vapour, becoming specifically lighter than a like space of 

 ambient air, ascends, swift at first, affording a pleasant sight in a warm sum- 

 mer's day, and then gradually slower, till it gets up to that part of the air 

 which is of equal lightness with itself; and there it remains, as long as the air 

 continues in the same state. But whenever the air cools, in which these 

 watery bladders float, the cold contracts the bladder, which becoming thereby 

 specifically heavier than the air, down it falls in dew, or rain. A common 

 alembic sufficiently shows the operation of heat and cold on the ascending and 

 descending vapour. 



Thus in a calm evening, when there is no wind to waft the air, as the heat 

 of the sun declines, the cold arrests some few of the last ascending vapours, 

 and, by its own force, without any other change in the state of the air, com- 

 pels them to return, in dew, to the very spot from whence they arose; whilst 

 their brethren escape, who go out of the reach of the cold a little before the 

 approach of night. 



Since therefore the same air, in different states of heat and cold, affects 

 vapour in this manner, it thence follows, that vapour, wafted from air of one 



