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better than the brafs one here defcribed, until I added to the 

 latter the circulating-pipe, which gave it the fuperiority : but 

 pewter, thou2;h befl to refifl the adion of the oil, being too 

 loft ; the barrel, if made of brafs, fliould be tinned in the in- 

 fide, in the manner before defcribed, which will fomewhat im- 

 prove it (and perhaps fubftantial gilding would be ftill better :) 

 but if one were made of glafs, and enclofed in a metal cafe 

 with cement; I think it could be eafily ground and polifhed, 

 and a pifton previoufly formed to fit it, in a metal hollow cy- 

 linder or ring of equal bore : fuch a pifton, having its top and 

 bottom plates tinned in the common manner, would I believe 

 conftitute a better pump than any ever yet made, and procure 

 more credit to the theory here laid down. 



In that theory I have affumed, that the degree of rarefac- 

 tion, which can be effedled, will depend on that which the 

 pump is capable of making under the pifion, without regard to 

 the fize of the rec' ; and as this is not at all the cafe with 

 refpedl to air admitted in the working, from the pump's being 

 leaky, which it ever is in a greater or lefTer degree ; and an 

 objedion may therefore arife from the confideration that on 

 opening the communication between the cyl' and rec' at every 

 flroke, any unextraded air under the pifton, and any otherwife 

 intruded, will be together diffufed in the reC and cyl' ; from 

 whence it may feem, no difference of efted could arife from 



thefe 



