272 THE AIR-PUMP. 



Several discs of well greased leather are placed one upon 

 another, between the flat side of the conical piece and 

 the nut ; by screwing the nut down upon them, they 

 are pressed firmly together, and at the same time they 

 are squeezed out sideways, so as to fit close against 

 the sides of the cylinder and prevent the passage of air. 

 The piston is moved by a handle g, by means of a rack z, 

 and a cogwheel r, while a small cylinder above the rack 

 keeps the teeth of the latter in contact with those of 

 the wheel. The barrel is closed at one end by a solid 

 piece which is attached to it by six screws; the joint 

 being made air-tight by a greased leather washer. 

 The solid piece contains a conical cavity into 

 which the end of the piston .exactly fits; close to the 

 pointed end of the cavity there is a stop-cock, which 

 is shown on a larger scale at I, II, and III. This stop- 

 cock is perforated by two channels, one of which passes 

 right through it as in a common stop-cock, the other is 

 perpendicular to the first and passes from the side to 

 the middle only, so that both together form the figure 

 of the letter T. Opposite to the pointed end of the 

 conical cavity is a small tube a which opens freely into 

 the air; another tube with stout walls rises up- 

 wards from the stop-cock, and to the end of this tube 

 a plate of metal, , is screwed, which is either carefully 

 oround flat or has a flat plate of glass attached to it. 

 The screw which bears the plate projects a little be- 

 yond it, and various pieces of apparatus may be l 

 attached to it, when required. The vessel from which 

 the air is to be exhausted is usually called the receiver. 

 It is either provided with a neck which corresponds to 



