for Extracting and Condensing Air. 135 



pistons have a common piston rod, which is moved by two 

 belts or cords attached at one end to the cross bar a a, in the 

 middle of the rod. From this point of attachment the belts 

 pass in opposite directions to the bottom of the vertical cylin- 

 ders, where each passing over a pulley, ascends to be buckled 

 to the moving wheel. The two buckles are placed one on 

 each side, about an inch from the lower end of the toothed 

 quadrant. The length of the belt a b c is equal to G b d, 

 that is, to the length of the vertical piston rod, plus the dis- 

 tance between the bottom of the vertical cylinder, and d the 

 point of nearest approach of the central bar a a, which 

 point it occupies when the horizontal piston is depressed on 

 that side. In the same manner the length of the belt a e k 

 is equal to H ef. Knowing the length and attachment of the 

 belts, it is easy to understand how the motion of the horizontal 

 pistons is effected. In the position of the instrument repre- 

 sented in the figure, the wheel is turning in the direction G 

 ckH, the motion of the vertical pistons is just finished, the 

 belt a b c is relaxed, while the belt a e k is newly put upon the 

 stretch. By turning the wheel a little farther in the same 

 direction till the point k comes to H 9 the belt aek will assume 

 the position /£ k, and the piston will be depressed in the hori- 

 zontal cylinder on that side, while it is simultaneously raised 

 in the opposite one. The wheel is now turned in the opposite 

 direction ; the vertical pistons move first, and when their mo- 

 tion is completed, the belt abc being put upon the stretch, 

 the horizontal pistons are moved back into the position repre- 

 sented in the figure. Thus are the two sets of pistons moved 

 successively by turning the wheel a little more than a quarter 

 round alternately, in opposite directions. 



If it be wished to use the instrument as a condenser, the 

 only change to be made consists in lengthening the belts, and 

 shifting them each to the buckle on the opposite side, which 

 will plainly have the effect of reversing the order of succession 

 in the movement of the pistons. 



As the ascending vertical piston has to contend with the 

 whole atmospheric pressure, and does not, as in the common 

 air-pump, receive any assistance from the descending piston, 

 which is pressed equally in both directions, the force required 



