244 OF TEMPER ATURE CHAP. 9. 



laws, they began to investigate their causes. 

 Without detailing the particulars of the ob- 

 servations made by philosophers, from time to 

 time, it will be sufficient to observe, that the 

 variations of pressure, as far, at least, as we can 

 discover, may be caused, first, by a variation in 

 the volume of atmosphere, the density remain- 

 ing the same; or, secondly, by a variation of 

 density, the volume remaining the same: or, 

 lastly, by a variation in both density and 

 volume. But though these circumstances may 

 be conceived capable of effecting barometrical 

 variations, yet it is probable there may be many 

 other causes yet unknown. Many hypotheses 

 have been assumed to account for variations in 

 the density and volume : but after all, these 

 do not seem capable of accounting for all the 

 phaenomena which attend alterations in the 

 atmospheric pressure. As it is not my intention 

 to detail former experiments and hypotheses, I 

 shall conclude this chapter with a few simple 

 observations on facts, which I have made my- 

 self, many whereof do not appear to have been 

 noticed before. 



It is a common observation, that the falling 

 of the mercury, when gradual, is followed by 

 long continued Rain; when it suddenly sinks, or 



