S52 



ttS»Ctk€»Q. 



CHEAP AND SIMPLE AIR PUMP. 



by which the barrel communicates with the receiver. This 

 ]>timp differs from the former in the following particulars: 

 first, in coram unicating with the receiver through the top 

 of the barrel: secondly, in having the valve, in the piston, 

 fastened to the end of the piston-rod: and thirdly, in the 

 bottom of the piston being convex, and the bottom of the 

 bjirrel concave. After the valve c and d are finished, and 

 fixed in their places; the convex and concave surfaces of 

 the piston and of the barrel are to be well fitted by grind- 

 ing; so that, when they meet, all air may be excluded from 

 between them. The reason for mating the bottom of the 

 piston convex, is, that the air may be prevented from lodg^ 

 iDg between the piston, and the sides of the barrel; as the 

 leathers, by this construction, will reach the bottom of the 

 ban-el; for the bottom of the piston being convex, it will 

 have no assignable thickness where it comes into contact with 

 the sides of the barrel ; as is shown at k k, PI. VIII, fig. 3, 

 the dark part representing the leathers. This pump will 

 work as follows. — 



When the piston-rod (fig. 3) is pushed down, the valve «l 

 will shut by the pressure ; and the air below it, having no 

 other exit, will be forced out at the valve c, which will shut, 

 by the reaction of the brass spring //, as soon as all the air 

 is expelled; and when the piston is again drawn upwards, 

 it will continue shut, for the same reason, and also by the 

 pressure of the atmosphere. — As the valve d is fixed to the 

 end of the piston-rod, it will be impossible to draw up the 

 piston, without first opening this valve; and by this means 

 establishing a communication with the receiver, as repre- 

 sented fig. 4, by the same means also, the oscillations of the 

 mercury in the gage will be prevented : during the ascent 

 oS the piston, therefore, the air will pass freely, through the 

 passages g- g-, g g^ into the lower part of the barrel; and 

 when the piston is again pressed downwards, the first effect 

 of the force applied will be to shut the valve c?, and the air 

 below it' will be expelled at C as before, 



I have had a pump made of this latter construction, and 

 find it to answer my expectations, in every respect, as far 

 as. the principle is concerned ; but I have never been able, 

 entirely to prevent the different juncture of the pump 



(whi^h 



