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they were then, when cold, turned in a lathe on the pifton, 

 with a very Iharp tool *. 



The iron rod of the piflon is -A-f-ths of an inch in diameter, 

 exadtly cylindrical, and very fmoothly polifhed ; it moves 

 through a collar of leathers (the fame kind as thofe of the 

 pifton) enclofed in the brafs box C, the lower plate of which 

 is fixed on the top of the barrel by the four fcrews 4, 4, 

 pafled through the broad prominent rim of it ; and the plate 

 was fo well adapted to the top of the barrel, by grinding it 

 on it with a circular motion, as to require only the interpo- 

 fition of a little fti£F ointment to make the joining air-faft. 

 A cover is, in the fame manner, adapted by three fcrews to 

 the projedling rim of the box containing the collar of leathers, 

 in the middle of which cover is fixed the focket or collar E, 

 above an inch long, through which the pifton rod moves. 

 As it is altogether necefl!ary that this rod ftiould move moft 



S s 2 exaftly 



• If tc "d leather were ufed for the pifton, &c. it might be foaked in oil 

 alone ; ' .des that it is apt to grow too hard by compreffion, and alfo to cor- 



rode the ^lafs of the barrel more from the acid imparted by the baric with which 

 it is tanned, it alfo emits a greater quantity of elaftic fluid in vacuo than leather 

 which is not tanned. On thefe accounts I have ufed buck-fkin leather for the 

 pifton, but t!)e texture of this being very loofe, oil only would not fill its pores 

 fo as to make the pifton air-tight, even when the leathers were clofely prefTed ; 

 and it was neceflary to fill its grain with the above ftiffer compofition. Yet this 

 leather will alfo generate air, and is harder to be made air-faft ; it alfo moves fo 

 ftiffly in the barrel that perhaps good fhoe-leather is preferable to it. 



