258 Mr. It. Gill on an Air-pump without Clearance. 



exhausted ; to the valve v is attached the rod s z, which termU 

 nates in the leathern piston q, fitted to a cylindrical hole bored 

 out in the piston-rod; by means of this arrangement the valve v 

 is opened by the ascent of the principal piston, and closed again 

 when the piston descends. As will be seen further on, it is ne- 

 cessary that the valve v be kept closed until the piston have tra- 

 versed a certain part of its upward stroke ; this is accomplished 

 by means of the spring s, which, while compressed, overcomes 

 the friction of the small piston and keeps the valve from opening ; 

 but after the piston has ascended the given portion of the stroke, 

 it remains expanded and ceases to act upon the valve, which 

 therefore opens in obedience to the efforts of the piston q. The 

 piston pp is similar in shape to the bottom of the cylinder, and 

 its tubular portion works in the annular space left between the 

 cylinder and its bell-shaped bottom ; into the upper part of the 

 piston is screwed the rod a, which also serves to fix the inner 

 edge of the leather ring dd, covering a series of apertures bored 

 in the flat top of the piston. As mentioned above, the piston- 

 rod is bored to receive the small piston q : as this last is required 

 merely to open and close the lower valve, and as the compression 

 of the air in the upper part of the hole would interfere with its 

 proper action, there are channels cut upon its surface which 

 allow the air to pass freely from one side of the piston to the 

 other. The small piston-rod z is not fitted perfectly to the 

 hole in the large piston-rod ; so that a narrow passage is left be- 

 tween the space under the piston and the small cylinder in the 

 piston-rod, and by means of the apertures y a communication is 

 made between the said small cylinder and the space above the 

 principal piston : the effect of this arrangement is that a slight 

 leakage is always maintained downwards through the piston. 

 On the cover of the cylinder is fitted the self-acting valve x, of 

 common construction, opening outwards, and surrounded by a 

 vessel containing oil so as to ensure its perfect action. 



To prepare the machine for work, the cover of the cylinder is 

 removed, the piston is lowered to the bottom of its stroke, and 

 then, opening the cock i, oil or other inevaporable liquid is poured 

 through the funnel h into the cylinder, so as to cover the piston 

 completely ; the cover, being replaced and the cock shut, the ap- 

 paratus is ready for action. On moving the piston upwards, the 

 valve v, if free, would also move upwards and open ; but the 

 spring s, being in a state of tension, keeps it upon it3 seat : and 

 this is necessary, as, if the valve were free to rise, the apparatus 

 being full of oil, some of it would enter the tube g, and might 

 pass on to the receiver ; the valve is therefore kept closed by the 

 spring. But as the piston is raised still higher, its tubular por- 

 tion emerging further from the annular space b b, causes the 



