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ments and machines. Of thefc, none has opened a wider field 

 of enquiry in natural philofophy than the air-pump; and I 

 am inclined to think, that this inftrument has not yet afforded 

 all the fervices of which it is capable, when I confider that air 

 is fo large a component part of moft bodies, that fince many 

 kinds of air are compounded of other aeriform fluids, and alTume 

 different qualities according to the varieties of their combination, 

 thefe component fluids would in many experiments be with moft 

 advantage united in vacuo -, and alfo that fomc electrical expe- 

 riments not only require a vacuum, but alfo one of the moft 

 perfed kind. 



With a view to this latter circumftance folely, fo long ago 

 as the year 1776, I fet about contriving an air-pump which 

 fliould be Ample in its conftrudion and yet exhauft more per- 

 fedlly than thofe of the common form ; and in 1778 I made one 

 (the barrel of which was of pewter- metal) on the principle, 

 which, I was fatisfied from the following theory, was the moft 

 effential, of leaving as little air as pofllble under the pifton 

 when put down to the bottom of the barrel ; and the perform- 

 ance of this machine was fo excellent as to induce me to fabri- 

 cate a larger and more perfedt one of the fame conftrudion, and 

 made in brafs-metal, which I had executed (in the country) in 

 the year 1785; but not finding it to perform as well as I had 

 cxpeded, I devifed an alteration and improvement in it, which 

 I had almoft finilhed the workmanfhip of in 1788, being that , 



which 



( 



