Chap. 2.] cf different Denfities, 415 



height of the water then will be thirteen times and a 

 half of that of the mercury ; becaufe the fpecific 

 gravity of mercury is about thirteen times and a half 

 as great as that of water. 



This obfervation will alfo apply to the reciprocal 

 action of air and water, or air and mercury upon each 

 other, as was evinced in a former book, when treating 

 of tht Torricellian experiment and the barometer *. 

 Many of the phenomena, indeed, of hydroftatics and 

 hydraulics are to be referred to the prefiiire of the at- 

 mofphere, and for this reafon, the prefent fubjed has 

 been in part anticipated, when it was neceflfary to treat 

 of air as a fluid. 



It is, however, proper on the prefent occafion, tQ 

 recal to the reader's attention fome of the properties 

 of this fluid, and he will eafily remember, that as a 

 fluid, air is poiTeflTed of gravity, and confequently 

 prefles upon all bodies which oppofe it; and it is 

 neceflary to add, that like water, it prefles in all 

 directions. Its perpendicular prefTure has been al- 

 ready fuflkiently proved ; and its lateral preilure may 

 alfo be eafily demonftrated. If a fmall hole is made 

 with a girriblet, either in the fide or bottom of a cafk 

 or veflel which is quite full of liquor, it will not run 

 out, becaufe the external air which prefles againft the 

 hole, fuftains the liquor, which has not a fufficient 

 height to overcome its prefllire. Hence the neceflity 

 of a vent peg, to enable liquor to be drawn out of a 

 full cafk. The elafticity of the fmall quantity of air 

 which is introduced at the vent prefles the fluid, and 

 overcomes the preflure of the air at the cock. There 

 is an inftrumcnt in common ufc, called a Valencia, 

 for extracting fmall quantities of liquor out of the 



* See Book v. Chap. 9. 



bung- 



