86 Description of an Air Pump. 



After testing the utility of these improvements for about five 

 years, being desirous that all using furnaces might avail them- 

 selves of them, I have sent this communication for insertion in 

 your interesting record of the daily improvements going on in the 

 arts and sciences. 



Art. VII. — Description of an Ai?^ Pump of a very simple con- 

 structioii, which acts both as an exhauster and condenser ; by 

 John Johnston, A. M., Professor of Natural Science in the 

 Wesleyan University, Middletown. 



The last No. of this Journal* contains a description of a very 

 ingenious air pump invented by Dr. Hare, Professor of Chemistry 

 in the University of Pennsylvania, which is capable of perform- 

 ing on a much larger scale precisely the same operations as the 

 one I am about to describe, but in quite a different manner. The 

 next day after I had contracted with Messrs. Brown & Francis, in 

 New York, for this air pump, which is now in possession of the 

 Wesleyan University, I had the pleasure of viewing Dr. Hare's 

 in his laboratory in Philadelphia. 



This pump, as will be seen by the figure, has two barrels, in 

 which the pistons are worked precisely as in those in common 

 use, and, in general, it is constructed in a similar manner. The 

 pistons, however, are solid, and at the base of each barrel are two 

 valves, one opening upward and the other downward. In the 

 center of the firm piece of mahogany, which forms the base of 

 the instrument, are two brass tubes, which are seen in the figure 

 at A and B, by the removal of the plate of brass D. Of these 

 tubes, A communicates with the valves — one in each barrel — that 

 opens upward, and B with the valves that open downward. Now 

 when either of the pistons descends, the air in the barrel below it 

 will of course pass out through the downward opening valve 

 and tube B connected with it ; and when it is again raised, the 

 air will pass in through the tube A and the upward opening valve. 

 At the center of the disc F, is an aperture, as in common air 

 pumps, into which a tube may be screwed, and directly beneath 



* Vol. xxxiii, page 237. 



