ROBERT BOYLE. ii 



direction. Now Guericke, in his machine, had two of these 

 valves, one covering a hole in the piston, another covering the 

 mouth of the vessel where the barrel was inserted ; and both 

 opening outwards. In consequence of this arrangement, when 

 the piston, after having been drawn out, as we have already 

 described, was again pushed back, the air in the barrel was 

 prevented from getting back into the vessel by the farther 

 valve, now shut against it, while it was at the same time provided 

 with an easy means of escape by the other, through which, 

 accordingly, it passed away. Here, then, was one barrelful of 

 the air in the vessel dislodged ; and the same process had only 

 to be repeated a sufficient number of times in order to extract 

 as much more as was desired. The quantity, however, removed 

 every time was, of course, always becoming less ; for, although 

 it filled the same space, it was more attenuated. 



The principle, therefore, upon which the first air-pump was 

 constructed was the expansibiUty of the air, which the inventor 

 was enabled to take advantage of through means of the valve. 

 These (wo things, in fact, constitute the air-pump ; and what- 

 ever improvements have been since introduced in the con- 

 struction of the machine have gone only to make the working 

 of it more convenient and effective. In this latter respect the 

 defects of Guericke's apparatus, as might be expected, were 

 considerable. Among others, with which it was chargeable, it 

 required the continual labour of tAvo men for several hours at 

 the pump to exhaust the air from a vessel of only moderate 

 size; the precautions which Guericke used to prevent the 

 intrusion of air from without, between the piston and the sidefj 

 of the barrel, during the working of the machine, were both 

 imperfect for that purpose, and greatly added to the difiiculties 

 and incommodiousness ol the operation ; and, above all, from 



