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■pipe : the valve is aliroft cylindrical, tapering but a little from 

 the bafc upward, and being laid on the plate with a fmall drop 

 of oil interpofed, the hollow cylindrical cap E, very little wider 

 internaHy than the valve, is put over it, being faflened down 

 on the projefling circular part of the plate a a. To let the 

 air pafs through this cap, three little holes e, e are made in its 

 iides, the centers of which arc juft as high as the lower edge 

 -of the valve D, when it is raifed to touch the top of the cap ; 

 the holes being higher than the plate under the valve, that the 

 oil may not run out by them. When this valve is found to 

 admit air into the barrel, it is occafioned either by the oil's be- 

 ing blown away, or fome particle of duft, &c. getting between 

 it and the plate, (which would produce the fame cffed in any 

 valvej and is remedied by taking it off, wiping it clean and 

 applying another drop of oil : the greater the quantity of air is, 

 which paffes by the valve, the more is the oil diflipated, and 

 confequently this chiefly happens in the beginning of the ex- 

 hauftion, when much air is drawn from the receiver, and thrown 

 out of the barrel at each ftroke ; fo that whenever great rare- 

 faction is requifite, I often found it convenient towards the end 

 of the operation to apply a frelTi drop of oil to the valve, keeping 

 the piflon in the mean time elevated to the top of the barrel 

 whilft the valve is taken off, until it be replaced ; which is 

 done in an inftant : thus it will perform perfe£tly well, if quite 

 clean and free from duft ; but as the fmalleft particle of dirt 

 •or mucus (which is apt to be produced from the oil's corroding 



the 



