4 1 6* Prepare of the Air upon Fluids. [Book VI L 



bung-holes of cafks. It is a tube with a fmall aper- 

 ture at the bottom and the top. When full, if the 

 hole at the top is flopped with the thumb or finger, fo 

 as to prevent the preffure of the air at the top, the 

 liquor will not run out of the hole at the bottom, being 

 kept in by the force of the external air. 



It is proper to obferve, that all the effects which 

 depend upon the preffure of air, take place in a room 

 where the column of air is terminated by the ceiling, 

 as well as without doors where the column of air has 

 the whole height of the atmofphere ; and the reafcn 

 is, becaufe the air in the room has a communication 

 with that on the outfide, fuppofmg it to be only by 

 means of the key-hole. Thus a barometer placed in 

 a hall, will have its mercury as high as if it was placed 

 in an open field. 



The curious effects produced by fiphons, all depend 

 upon the preffure of the air. 



A fiphon is a bent tube ABC (fig. 3), made of 

 glafs, of metal, or of wood. One branch of which 

 A B, is fhorter than the other B C. In order to 

 make vie of this inilrument, place the extremity of 

 the fhort branch A B in the veffel E E, (fig. 4.) which 

 may be fuppofed to contain any fluid matter, as water 

 for inftance. If the air then is drawn by faction* 

 out of the fiphon at the extremity C of the long branch 

 BC, the liquor will begin to flow, and will not ceafe 

 while the fhort branch A B remains immerfed in the 

 fluid. It is eafy to fee that the preffure of the air 

 upon the furface of the fluid in the veffel, is the caufe 



* Suftion is here evidently ufed in the popular fenfe of the 

 word, to imply that aflion of the mouth which extrafts the air 

 from a given fpace : for in ilriift philosophical language there is 

 no fuch thing. 



of 



