500 Signs of Weather [Book V. 



fometimes from its elafticity. It has been proved, 

 that thefe two properties of air fometimes vary, and, 

 confequently, the preflure which they produce. When- 

 ever the air diflblves a great quantity of water, its 

 fpecific gravity is increafed ; the column of air which 

 refts upon the reftrvoir of the barometer becomes hea- 

 vier, and the mercury riles. If the folution is not per- 

 fect, the tranfparency of the air will be clifturbed j 

 hence a kind of milt will be produced, which will ge- 

 nerally caufe the mercury in the barometer to rife ; 

 but if the folution is perfect, the tranfparency of the 

 air will be complete, and fine weather return, as the 

 mercury in the barometer predicted by its afcent. 

 While certain caufes determine this water, which is 

 held in folution, to defcend into the lower region of 

 the atmofphere, before it is fufficiently condenfed to 

 be regularly formed into rain, there is another part 

 of it which will have previoufly arrived at the furface 

 of the earth. As a proof of this, it is obfervable, that 

 when the weather is about to change to rain, all bodies 

 which are impenetrable to water, fuch as bars of iron, 

 hard ftones, &c. are found to be moift or wet. The 

 column of air which prefles upon the refervoir of the 

 barometer, will become, therefore, lighter by the lofs 

 of that portion of water already arrived at the earth ; 

 and the barometer will defcend, and predict the rain, 

 which will come in a fliort time after, being formed by 

 the remainder of the water, which will then have had 

 time to be formed into regular drops *. 



It muft be confeffed, that there are fome appear- 

 ances which feem to contradict this explanation. It 

 fometimes happens, that the barometer riles even 

 during rain, while the air difcharges itfelf of the water 

 which it held in folution : it alfo happens frequently, 



Briflbn. Vol. i. 



cfpecially 



