98 Mr. Kemp's Description of a New Mercurial Air-Pump. 



is a tube proceeding from the upper part of the barrel A, and ter- 

 minating in the other exhausting vessel G, as is seen in Fig. I. 

 where it joins the upright tube that connects the under part of the 

 barrel B with the vessel G, Fig. 1. N, Fig. 2. is also a tube pro- 

 ceeding from the upper part of the barrel B, Fig. 1. which joins 

 the tube EE, Fig. 2. and terminates in the vessel F. 



The under part of the barrel A, along with the upper part of 

 the barrel B, communicate with the exhausting vessel P, and the 

 under part of the barrel B, along with the upper part of the barrel 

 A, communicate with the exhausting vessel G. 



The capacity of the barrel A^ when its piston is at the top, and 

 that of the barrel B when its piston is at the bottom, must be pre- 

 cisely equal to the capacity of the exhausting vessel F, and a por- 

 tion of the valve cup G ; and in like manner with the other ex- 

 hausting vessel and barrels. 



Suppose now, the barrel A, having its piston at the top, along 

 with its tube EE, to be both filled with mercury up to its entrance 

 into the exhausting vessel F ; and that the barrel B, Fig. 1. hav- 

 ing its piston at the bottom, and its tube N, coming from the 

 top of the barrel, both filled with mercury up to the entrance of 

 the tube into the exhausting vessel F. When the piston R de- 

 scends in the barrel A, Fig. 2. and the piston of the barrel B con- 

 sequently ascends, the mercury is forced through the tubes EE and 

 N, into the exhausting vessel F, and shuts the float valve M, at 

 the termination of the tube HH, which leads to the receiver, and 

 effectually prevents any air from returning by that passage. As 

 the mercury rises, it completely expels the air through the valve G, 

 which is provided with a cup, to admit of a portion of the mercury 

 flowing in and floating the valve. The under part of the barrel B, 

 and the upper part of the barrel A, are now in like manner to be 

 filled. 



Upon the next stroke, the piston R of the barrel A ascends, and 

 a vacuum being produced in the under part of the barrel A, and 

 upper part of the barrel B, a portion of the mercury contained in 

 the cup G, passes through it into the exhausting vessel F, until 

 the float valve regains its place. A suflicient portion of mercury, 

 hoAvever, remains in the cup to seal it efi'ectually against any re- 

 turn of air. As the mercury descends, a perfect Torricellian va- 

 cuum is formed in the upper part of the vessel F, and the air in 

 the receiver, by its elasticity, forces open the valve M, against the 

 tendency of the mercury to retain it in its place, which rushes up 

 to supply the vacuum. When the mercury has descended to the 

 bottom of the vessel F, the float-ball, carrying the valve M, falls 

 about the 1-1 6th of an inch, and allows the air from the receiver 

 to flow freely into the exhausting vessel. In like manner, at every 

 succeeding stroke, the mercury rises in the exhausting vessel, and 

 forces out the air by the valve G, and as the elasticity of the air 

 in the receiver becomes gradually weaker as the exhaustion pro- 

 ceeds, none of it can pass from the receiver until the mercury has 



