On Atmospheric Phcenomena. 1 19 



each other, and, with the temporary help of the sun, rapidly 

 increasing the quantity of the clouds and enlarging the bulk 

 of the aqueous part of the atmosphere. Though this in- 

 crease of the atmosphere, by the combination of electricity 

 and water, can make but little difference in the whole, yet 

 as long as it operates the atmosphere must be getting higher 

 in that place, and at any rate more dense, and so must press 

 more heavily on the surface of the earth than under different 

 circumstances: — hence the rise of the barometer. When 

 it rises rapidly, the evaporation by means of electricity may 

 be inferred to be going on very fast. 



Agreeably to this, after long-continued hot weather, people 

 in general expect lightning; but this would not follow if 

 there were not an extra quantity in the air. This expecta- 

 tion is founded on experience, but a natural cause may be 

 assigned for the effect ; for, when no discharge of electricity 

 has been effected by the descent of rain for a long time, the 

 accumulation by the constant daily ascent of vapour, loaded 

 with electricity, must at last become so great in the upper 

 regions that an effort of nature to restore the equilibrium 

 may with certainty be expected to follow. When a change 

 in the direction of the wind brings an atmosphere from a 

 place where the sun has raised more vapour than there is 

 t'lectricity to support, the air will become damp enough to 

 form a slow conductor; then the barometer will begin to 

 sink, and stormy weather will follow ; for the clouds, beinc 

 thus robbed of a portion of their electricity, will condense, 

 and fall into the lowest station where the atmosphere ba- 

 lances them, which is generally about the height of a thou- 

 sand feet. The reason why they do not fall lower is, that 

 at this degree of descent the particles arc sufficiently con- 

 densed to unite and form rain, which then falls to the 

 ground, so that we continue to see only the part of ihe 

 cloud that still remains uncondenscd. But this is not the 

 only cause that prevents thcni from descending lower; for 

 sometimes this fall of rain from the clouds does not yet take 

 place, owing to the highly electrified vapour beginning to 

 give off to the clouds as large a supply as can be carried off 

 by so imperfect a conductor. The vapour then descends 

 ii 1 into 



