[ 337 .1 



fo perfect as it is commonly fuppofed, for there will perhaps 



always be fuch an air-bubble formed, be the tube ever fo care- 

 fully filled : in that mentioned in the fubfequent trials however, 

 the bubble was not bigger than a pin's point, and fcarce vi- 

 fible. 



If after exhauftion the air were let fuddenly into the rec' 

 and gage-vcfTel, the mere'' would be forced up into the tube 

 with fuch violence as certainly to knock the top off it, or to 

 fplit it ; to prevent which, the cock is placed at H, to flop the 

 communication of the rec"^ dud with the branch of it which 

 goes to the gage-glafs, whenever it is requifite to let air fud- 

 denly into the lea : when this cock is not fhut, the communi- 

 cation between the rec' and gage-veffel muft never be opened, 

 (if the latter has been exhaufted and the mere' has fubfided 

 in the tube) 'till the pifton is previoufly put down to the 

 bottom, and the rec' is exhaufted. It would be a great improve- 

 ment of this machine, if another fuch cock were placed between 

 this one and the rec', to flop occafionally the main dud between 

 the barrel and rec' ; by which the rec would be doubly forti- 

 tified againft the intrufion of air, and the cock attached to the 

 barrel might then have its key taken out, when it requires frefh 

 ointment to be applied ; which will often be neceffary, when it 

 muft be turned many times during the exhaufiion of a large 

 rec', or when one is to be kept exhaufted many days, and it 

 may be requifite frequently in that time to pump out air which 



Vol. VI. U u might 



