CHEAP AND SIMPLE AIR PUMP. S49 



tion of hitlrogen will be of intermediate specific gravity 

 between the alkaline metalloids and the common metals* 

 and that the earths themselves must have a degree of solubi- 

 lity in water, intermediate between the alkalis and the com- 

 mon metallic oxides. 



1 am, with much respect. 

 Yours &c. 



Edinhurgh^^lUtFeb. J. MURRAY. 



1811. 



II. 



Cheap and simple Construction of an Air Pump* In a letter 

 from a Correspondent. 



To Mr. NICHOLSON. 

 SIR, 



A 



S you were so good as to publish a paper, which I sent Auempfsto 

 to you some months a<'o, on a method of ioininp- long or btut consifucta 

 metallic tubes, &c., Ihave taken the liberty of troubling you pump, 

 with the following account of some attempts, which 1 have 

 made, to simplify the construction of the air pump. Not 

 being willing to go to the expense of one of the best air 

 pumps, and at the same time equally unwilling to spoil my 

 experiments by a bad one; no other alternative remained, 

 than either to improve the old construction, or to invent a 

 new one. How far this attempt may have succeeded, 1 

 must leave to the judgment of yourself, and of your readers, 



Tiie following plan was the first that struck me: Let 1st. consirut 

 A B. (Fl. VIj, fig. 1 and 2), represent the barrel ol" the ''°"- 

 pump ; C C the piston rod, which is to be made solid, either 

 of brass or steel, and accurately ground; and D the piston, 

 which is made to move air tight in the barrel, by means of 

 leathers as usual. The rod also moves in a collar of lea- 

 thers, a fl, and & 6, is a cup to hold oil — so far the construc- 

 tion is the same as in Smeaton's air pump. The alterati^u 

 which I haveipade is ip the valves^ which ^re of solid brass; * 



c ]« 



