THE AIR-PUMP. 



271 



is to form a smooth hollow cylinder of exact form, is 

 attached to two supports which are fixed upon a flat 

 ooard. In the cjdinder (often called the barrel of the 



A 



187 (A, an. proj., veal *ize ; 2?, | real she ; I. II.III. real size 



r-pump) the piston k works air-tight ; it consists of a 

 mical piece of metal from which a strong screw pro- 

 els, which fits into a nut, bored in a disc of metal. 



