^^ »lJII.08OPHICAL TRANSACTIONS. [aNNO 1763. 



earth is also sometimes much heated by the sun; and such heated surface not 

 being changed, heats the air that moves over it. Seas, lakes, and great bodies 

 of water, agitated by the winds, continually change surfaces; the cold surface 

 in winter is turned under, by the rolling of the waves, and a warmer turned up; 

 in summer, the warm is turned under, and colder turned up. Hence the more 

 equal temper of sea-water, and the air over it. Hence in winter, winds from 

 the sea seem warm, winds from the land cold. In summer the contrary. There- 

 fore the lakes north-west of us,* as they are not so much frozen, nor so apt to 

 freeze as the earth, rather moderate than increase the coldness of our winter 

 winds. The air over the sea being warmer, and therefore lighter in winter than 

 the air over the frozen land, may be another cause of our general north-west 

 winds, which blow off to sea at right angles from our North American coast; 

 the warm light sea air rising, the heavy cold land air pressing into its place. 



■'Heavy fluids descending frequently form eddies, or whirlpools, as is seen in a 

 funnel, where the water acquires a circular motion, receding every way from a 

 centre, and leaving a vacancy in the middle, greatest above, and lessening down- 

 wards, like a speaking trumpet, its large end upwards. Air descending, or as- 

 cending, may form the same kind of eddies, or whirlings, the parts of air ac- 

 quiring a circular motion, and receding from the middle of the circle by a cen- 

 trifugal force, and leaving there a vacancy, if descending, greatest above, and 

 lessening downwards; if ascending, greatest below, and lessening upwards; 

 like a speaking trumpet, standing its large end on the ground. When the air 

 descends with violence in some places, it may rise with equal violence in others, 

 and form both kinds of whirlwinds. The air in its whirling motion receding 

 every way from the centre or axis of the trumpet, leaves there a vacuum, which 

 cannot be filled through the sides, the whirling air as an arch preventing; it 

 must then press in at the open ends. The greatest pressure inwards must be at 

 the lower end, the greatest weight of the surrounding atmosphere being there. 

 The air entering rises within, and carries up dust, leaves, and even heavier 

 bodies that happen in its way, as the eddy or whirlpool passes over land. If it 

 passes over water, the weight of the surrounding atmosphere forces up the 

 water into the vacuity; part of which, by degrees joins with the whirling air, 

 and adding weight, and receiving accelerated motion, recedes still farther from 

 the centre or axis of the trump, as the pressure lessens; and at last, as the 

 trump widens, is broken into small particles, and so united with air as to be 

 supported by it, and become black clouds at the top of the trump. Thus these 

 eddies may be whirlwinds at land, waterspouts at sea. A body of water so 

 raised may be suddenly let fall, when the motion, &c. has not strength to sup- 



* In Pennsylvania. — Orig. 



