[ 33^ ] 



was taken that the metal of the cock lliould be free from pores, 

 by which if air were admitted in the working, it would be 

 very hard to difcover it, as this might take place only in 

 certain pofitions of the key ; and the ointment might fometimes 

 prevent and fometimes fufFer it, fo that it might elude trials ; 

 the cock was alfo fo truly and fmoothly ground as to produce 

 an intimate conta<5l of the key and fhell throughout, (for air 

 will penetrate where liquids would not) it was alfo made of a 

 true conical fliape, its fides being perfedly ftrait from end to 

 end. The cock mufl be lubricated and made air-tight by oint- 

 ment *, and as all oil or greafy ointment has an acid in it, 

 (of which perhaps it can never be diverted without deftroying 

 its lubricity) which corrodes the brafs, and ftifFens the oil or 

 ointment itfelf, by impregnating it with green ruft or verde- 

 gris, I found it necelTary, after the cock was finiflied, to have 

 the key and the infide of the fliell coated with tin f . 



The 



• This I have made of one part common rofin, one part oil, and one part and 

 a half frefh futt : the oil and rofin is firft melted, and when a little cooled, the 

 fuet is added, that the heat requifite to melt the rofin may not burn the fibrous 

 part of the fuet and deftoy its firmncfs : more oil is to be added in cold than in 

 warm weather. The quality of the ointment for the cock is of the utmoft con- 

 fequence ; for if ever the paits of the metal of the cock come into immediate 

 contact, it will not be air-fall: ; oil will not render it fo, (nor confequently would 

 a valve) but the ointment mufl be ftifF and adhefive ; yet not too ftifF, as then 

 it would be hard to turn the key, and it could not be diflinguithed whether its 

 lliffncfs was owing to the ointment, or to the fridion of the parts of the metal 

 getting into immediate contadl, which would be pernicious. 



f This was done by making the metal very clean and bright by rubbing with 

 whiting and water ; and having heated it as hot as boiling watef, and prefer\-ed it 



fo, 



