MATHEMATICAL SCIENCES-SIXTEENTH CENT. 173 



soned in the cylinder to escape as the piston rises, the apparatus is,, 

 after the opening at B is closed again, in a position again to remove 

 from the globe A two-thirds of its remaining air by the descent of the 

 piston. Each time that the piston is drawn, two-thirds of whatever air 

 may be in the globe being thus removed, we should have remaining 



FIG. 74- 



FIG. 75- 



after the successive strokes i, |-, ^L, etc., of the original quantity. 

 The mode in which Guericke cut-off the communication with the 

 globe was by a stop-cock as shown in Fig. 75, and the air expelled 

 from the cylinder on the rise of the piston escaped through a valve at 

 the top. The stop-cock and valve were surrounded by oil, which filled 

 the conical vessel above the barrel, in order to prevent access of air, 

 which was liable to find its way between the parts of the apparatus. In 

 Fig. 75 the globe is separated and raised a little above the position it 

 would occupy when in connection with the cylinder. Fig. 74 shows 

 another arrangement of the barrel of the air-pump, dispensing with the 

 oil-vessel, and replacing the valve by a small lateral opening, which is 

 closed by hand each time the piston is drawn out. The air-pump 

 subsequently received many improvements, and has been constructed 

 in many different forms. Fig. 76 is a form of air-pump now often 



