<i'i,S PHILOSOPHICAL TEANSACTIONS. [aNNO I766.' 



fore, will bear any load inferior to the force of these repulsions. Hence the 

 support of fogs, mists, clouds. 



Very warm air, clear, though supporting a very great quantity of moisture, 

 will grow turbid and cloudy on the mixture of a colder air, as foggy turbid air 

 will grow clear by warming. Thus the sun shining on a morning fog, dissipates 

 it. Clouds are seen to waste in a sunshiny day. But cold condenses and renders 

 visible the vapour. A tankard, or decanter, filled with cold water, will condense 

 the moisture of warm clear air, on its outside, where it becomes visible as dew, 

 coalesces into drops, descends in little streams. The sun heats the air of our at- 

 mosphere most near the surface of the earth; for there, besides the direct rays, 

 there are many reflections. Moreover, the earth itself being heated, commu- 

 nicates of its heat to the neighbouring air. The higher regions having only the 

 direct rays of the sun passing through them, are comparatively very cold. Hence 

 the cold air on the tops of mountains, and snow on some of them all the year, 

 even in the torrid zone. Hence hail in summer. If the atmosphere were, all 

 of it, both above and below, always of the same temper, as to cold or heat: 

 then the upper air would always be rarer than the lower, because the pressure on 

 it is less, consequently lighter, and therefore would keep its place. But the 

 upper air may be more condensed by cold, than the lower air by pressure. The 

 lower more expanded by heat, than the upper for want of pressure; in such cases 

 the upper air will become the heavier, the lower the lighter. The lower region 

 of air being heated and expanded, heaves up and supports, for some time, the 

 colder heavier air above, and will continue to support it while the equilibrium is 

 kept. Thus water is supported in an inverted open glass, while the equilibrura 

 is maintained by the equal pressure upwards of the air below; but the equilibrium 

 by any means breaking, the water descends on the heavier side, and the air rises 

 into its place. The lifted cold heavy air over a heated country, becoming by any 

 means unequally supported, or unequal in its weights, the heaviest part descends 

 first, and the rest follows impetuously. Hence gusts after heats, and hurricanes 

 in hot climates. Hence the air of gusts and hurricanes, cold, though in hot 

 climes and seasons; it coming from above. 



' The cold air descending from above, as it penetrates our warm region full of 

 watry particles, condenses them, renders them visible, forms a cloud, thick and 

 dark, overcasting sometimes at once, large and extensive, sometimes, when seen 

 at a distance, small at first, gradually increasing; the cold edge, or surface of the 

 cloud, condensing the vapours next it, which form smaller clouds, that join it, 

 increase its bulk, it descends with the wind and its acquired weight, draws nearer 

 the earth, becomes denser with continual additions of water, and discharges 

 heavy showers. Small black clouds thus appearing in a clear sky, in hot cli- 

 mates, portend storms, and warn seamen to hand their sails. 



