[ 546 ] 



air's paHage, the fame as that v/hich would be In the cylinder, 

 when the pifton is elevated from the bottom to the top, fup- 

 pofing there was no communication between the rec'' and cyl' 

 during fuch elevation; fo ;hat if there were a perfedl vacuum 

 made in the cyT ptr Jl\ by raifing the pifton, the rarefadion 



in 



tlie hole under the valve ; open the ciimmunication between the circulating pipe and 

 the barrel, then tuvn the key back again, and dcprefs the pifton to the bottom ; ob- 

 ferve where the mere'' (lands ; turn the key, to join the circulating pipe with the 

 barrel ; raife the pifton half way and keep it fo a confiderable time ; then flowly open 

 the reef duct, and put down the pifton ; if the mere' rifes higher than the mark, air 

 has got in either by the pifton rod, its collar of leathers not having been clofe enough 

 prefled to the bottom of the box ; or by the top of the pump not clofed ; or by the 

 joint of the circulating pipe with the duft in the box ; each of which muft be exa- 

 mined. If the valve admits air, it will be known by being raifcd during the eleva- 

 tion of the pifton before it gets to the top, when the air has been greatly rarefied in 

 the recr, or when both the duds are clofed by the key. 



If it be chofen to let the air gradually into the rec"- without loofening the pin, it 

 may be done with fafety to the tube of the gage-veftel, by taking off the valve ; and 

 raifing the pifton half way, 'till the barrel under it is filled with air of the natural 

 denfity ; then let the cock be half turned, and the pifton raifed to the top, to rarefy 

 the air under it ; the cock may then be cautioufly turned 'till the air gett into the 

 rec% the pifton being afterwards put down to force into it what air remains under it : 

 this may be repeated. 



If the pifton rod moves ftiffly through its collar of leathers it will be eafed by 

 fmearing the rod all aroand with fome oil. The joints, &c. may be made air-faft, by 

 meltint' the cement on them witli a blow-pipe, or by a knob of metal, fixed on the 

 end of a wire, heated by a blow-pipe. 



The cock of the gage-veflel (and all others ufed) fliould have the ley one inch and 

 I long, and its mean diameter | of an inch at the leaft. It cannot be too carefully 

 formed and poliQied. 



