VOL. LV.] PHILOSOPHICAL TfiANSACT10N8. !*27 



The earth turning on its axis in about 24 hours, the equatorial parts must 

 move about 1 5 miles in each minute. In northern and southern latitudes this 

 motion is gradually less to the poles, and there nothing. If there was a general 

 calm over the face of the globe, it must be by the air's moving in every part as 

 fast as the earth, or sea, it covers. 



He that sails, or rides, has insensibly the same degree of motion as the ship, 

 or coach, with which he is connected. If the ship strikes the shore, or the 

 coach stops suddenly, the motion continuing in the man, he is thrown forward. 

 If a man were to jump from the land into a swift sailing ship, he would be 

 thrown backward, or towards the stern, not having at first the motion of the 

 ship. He that travels, by sea or land, towards the equinoctial, gradually acquires 

 motion; from it, loses. But if a man were taken up from latitude 40 (where 

 suppose the earth's surface to move 12 miles per minute) and immediately set 

 down at the equinoctial, without changing the motion he had, his heels would 

 be struck up, he would fall westward. If taken up from the equinoctial, and set 

 down in latitude 40, he would fall eastward. 



The air under the equator, and between the tropics, being constantly heated 

 and rarefied by the sun, rises. Its place is supplied by air from northern and 

 southern latitudes, which coming from parts where the earth and air had less 

 motion, and not suddenly acquiring the quicker motion of the equatorial earth, 

 appears an east wind blowing westward, the earth moving from west to east, and 

 slipping under the air. Thus, when we ride in a calm, it seems a wind against 

 us. If we ride with the wind, and faster, even that will seem a small wind 

 against us. The air rarefied between the tropics, and rising, must flow in the 

 higher region north and south. Before it rose, it had acquired the greatest mo- 

 tion the earth's rotation could give it. It retains some degree of this motion, 

 and descending in higher latitudes, where the earth's motion is less, will appear 

 a westerly wind, yet tending towards the equatorial parts, to supply the vacancy 

 occasioned by the air of the lower regions flowing thitherwards. Hence our ge- 

 neral cold winds are about north-west, our summer cold gusts the same. 



The air in sultry weather, though not cloudy, has a kind of haziness in it, 

 which makes objects at a distance appear dull and indistinct. This haziness is 

 occasioned by the great quantity of moisture equally diffused in that air. When, 

 by the cold wind blowing down among it, it is condensed into clouds, and falls 

 in rain, the air becomes purer and clearer. Hence, after gusts, distant objects 

 appear distinct, their figures sharply terminated. 



Extreme cold winds congeal the surface of the earth, by carrying off^ its fire. 

 Warm winds, afterwards blowing over that frozen surface, will be chilled by it. 

 Could that frozen surface be turned under, and a warmer turned up from be- 

 neath it, those warm winds would not be chilled so much. The surface of the 



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