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sti'ument was improved, the periods at which those improvements 

 were made, and the parties by whom they were effected, are all more 

 or less confounded with each other, or mis-stated. 



It is in connection with the double-barrelled air-pump, that the 

 accepted history of the instrument is chiefly erroneous, but the mis- 

 takes made in reference to the more complex engine, have ultimately 

 involved in confusion even the authentic records of the steps by which 

 the earlier single-barrelled air-pump was improved, so that the ac- 

 count of its successive alterations must commence with its earliest 

 and simplest construction. 



The history of the English air-pump may be divided into four 

 stages, three of which belong to the seventeeth century, and the 

 fourth to the eighteenth. They are as follow, the dates, as given in 

 the original authorities, being according to the Old Style : — 



1659. The construction of a Pneumatical Engine consisting of 

 a single-barrelled pump with a solid piston, moved by a rack and 

 pinion, and a globular glass receiver directly communicating with the 

 cylinder, which had an aperture closed and opened by a plug moved 

 by the hand, and playing the part of a valve. 



1667. The separation of the glass receiver from the cylinder, 

 and introduction of the air-pump plate, on which bell-jars could be 

 placed and used as receivers. The pump, still single-barrelled, 

 and wrought by a rack and pinion, but with an aperture in the 

 piston instead of in the cylinder, furnished with a moveable stopper. 



1676. The introduction of a double-barrelled air-pump, with 

 self-acting valves in the cylinders and pistons, and with piston-rods 

 suspended at opposite ends of a cord passing over a pulley. 



1704. The combination of the rack and pinion of the first and 

 second air-pumps, with the two barrels, twin pistons, and self-acting 

 valves of the third. The following are the more important details 

 concerning those instruments. 



Sometime before 1658, Boyle having heard, as he informs us 

 (BircTis Boyle, Ed. 1772, vol. i., p. 6), of Guericke's air-pump and 

 pneumatic experiments, had an exhausting engine of some kind con- 

 structed for him by Gratorix, a London instrument-maker of the 

 time. No drawing or description of Gratorix's aii'-pump is extant, 

 but it was so ineffective a machine that it was set aside as useless 

 almost as soon as finished. Boyle had then recourse to Robert 



