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place yourself on an eminence, nearly facing the sun. 

 Then, if there be a gentle wind, there will be a re- 

 flection of light from the body of the air in the vale 

 below. And you will see the undulations of waves of 

 air almost as perfectly as you may those of water, 

 agitated by a gentle wind. And yet in truth it is not 

 the air which you see, but the vapours that float therein. 



One property of air is its weight or gravity. This 

 you will immediately feel if you lay your hand on the 

 mouth of a vessel, which is emptied of air. If you Jay 

 a square piece of glass on the orifice of an air pump, 

 when the air is drawn out, it will be broke to shivers 

 with a great noise. Or extract the air from between two 

 smoothly polished marbles, and close the edges with 

 wax, they will then be so strongly prest together, as not 

 easily to be separated. But we need no other proof of 

 it than the barometer : a glass tube, close at one end, 

 and filled with mercury ; immerge the other end in a 

 bason of the same fluid, and when it is erected, the mer- 

 cury in the tube will rise thirty inches above the surface 

 of that in the bason. 



The changes then in the barometer are wholly owing 

 to the changes in the weight of the atmosphere. But 

 to what are these owing ? It seems chiefly to the 

 winds. For 1. These must alter the weight of the air 

 in any particular place, either by bringing together and 

 accumulating the air, which is the case when two winds 

 blow at the same time from opposite points ; or by 

 sweeping away part of the air, as when two winds blow 

 opposite ways from the same point ; or lastly, by cutting 

 tfrY the pressure of the atmosphere, which happens when 

 any wind blows briskly any way. 2. Cold nitrous par- 

 ticles load the atmosphere, and increase its weight.- 



3, So do heavy, dry exhalations from the earth. 



4. The air being- rendered heavier is more able to 

 support the vapours, which being intermixed with it, 

 make the weather fair and serene. When it is rendered 

 lighter by the contrary causes, it becomes unable to sup- 

 port the vapours, which then sink, gather into drops, 

 and fall in rain. 



