[ 344 ] 



I iiAvF, been thus particular in defcribing not only the 

 ■flruiflure of this pump but al o the reafons of the form of 

 each of its parts, and the circumftances on which depends 

 their right performance : becaufe the flrudlure of the machine 

 is novel ; and alfo that any artift who might make fuch a 

 one, may be guarded againfl: any alteration not duly confidered ; 

 and as no perfon will ever make exquifite experiments with 

 an air-pump unlefs he is himfelf, qualified to underftand and 

 to rectify little accidents and derangements, to which every 

 machine muft be liable, which is to refift fo fubtil an intruder 

 as air ; fo I am fure neither artift nor experimenter (for whom 

 alone this is written) will think me tedious in guarding him 

 againft miftakes, which it would he more tedious to difcover, 

 and far more fo to corre6l : in truth to make this defcription 

 as ufeful as I wifh, the operator fliould underftand the parti- 

 culars which are mentioned underneath *. 



I PROCEED 



* Suppofing every part of the pump to have been made of metal without pores ; 

 and thiit every part and pipe has been feparately tried, by being filled with condenfed 

 air, and fo plunged under water, to fee if the air efcapcs through any fecret porej 

 the barrel alfo joined with its cocl<, and w ith the piece of metal attached to it, being 

 thus tried in itfelf, after clofiiig the mouths of the dudls in the latter, and that every 

 part in itfelf is found air-tight ; if then the machine when put together, is found to 

 to be leaky, it muft be fo either in the joints, which are clofcd only with cement, or 

 in the cocks, the valve, or pifton ; and to difcover where the air intrudes, (and diffi- 

 cult indeed will it be found to exclude it altogether) each part may be tried thus : 

 The v.Jvc being taken oil", put the pifton down to the bottom, and clofe the bottom 



by 



