[ 345 ] 



I PROCEED now to explain the principles on which the pump 

 here defcribed was conflrudled, and they are thefe : 



I ft, That the rarefaction produced in the rec will be by 

 the final a(5lion of any pump, if there be no obftacle to the 



air's 



by turning the key fo as to reft at the middle between its limits on either fide by 

 which both its dufts will be ftopt, then elevate the pifton, and have it fixed fo, by 

 putting a bit of wood acrofs between two of the teeth in the rack, which will hinder 

 its defcent : after a long time, if the pifton when let down, can touch the bottom of 

 the barrel, no air is admitted'by the pifton. This being afcertained, each of the dufts 

 are next to be tried ; in order to which, ftop the hole of the rec pipe in the pump- 

 plate ; exhauft the gage-veflll, and ftop its cock, and the barrel cock : if then the 

 mere'' rifes in the tube, the glafs-veflel itfelf admits- air by its cap or duiS ; which 

 being redlified, open the communication of the gage with the rec pipe by turning 

 the gage-cock ; if the marc' fhould then rife after a good while-, the air has entered 

 at the joint of the rec^ pipe. Thefe parts being fecured ; then, to try the circulating 

 pipe; firft exhauft the gage-glafs; ftop the end of this pipe at the top of the barrel,- 

 by disjoining it from the box of leathers, and interpofing a thin plate of metal be- 

 tween its opening and the part t, of the box ; melt cement about the edges of this 

 little plate, to feal up the orifice of the pipe there, having prtvioujly put down, the 

 pifton to the bottom ; then after opening the communication of the barrel with the 

 rec, mark the height of the mere'' ; elevate the pifton a little, and keeping it fo for 

 a good while that whatever air may have entered by the circulating pipe may difFufe 

 itfelf into the barrel, (to allow which this pipe muft now communicate with the 

 barrel) turn the key very flowly and cautioufly, to let this air pafs into the gage-glafs 

 without endangering its tube, and deprefs the pifton to the bottom, when if any air 

 had intruded by the pipe, the mere' will rife above its altitude before marked, fliew- 

 fng that the joint of the pipe with the piece attached to the barrel cock, was leaky j 

 which being remedied, reftore and fecure the communication of the pipe with the 

 top of the barrel ; exhauft the gage-glafs, at the laft ftroke of which procefs, when 

 the pifton is at the top^ keep, it there, 'till you have fealed with a plate and cement 

 Vol. VI. X x the 



