Mr. Kemp's Description of a New Mercurial Air-Pump. 97 



In every air-pump, however, where valves exist, from the im- 

 perfection of their construction and from their wear, they are always 

 admitting minute portions of air to return into the receiver. The 

 evaporation of the oil also tends to destroy the perfection of the va- 

 cuum in cases of high rarefaction. 



Various attempts have been made to obviate this ; among others, 

 that of employing mercury to form the vacuum, whereby every 

 particle of air would be expelled ; but the difficulty attending the 

 opening and shutting of the valves, has completely prevented any 

 form of mercurial air-pump from being used. 



In the plan of the mercurial air-pump which I now propose, the 

 action of the valves is entirely dependent upon the motion of the 

 pistons in the working of the pump, A Torricellian vacuum is 

 produced in the exhausting vessels at each stroke of the piston, 

 whereby a much greater degree of rarefaction takes place m the 

 receiver than by any other means. In this arrangement, the ex- 

 hausting vessels may be considered as occupying the place of the 

 barrels in other pumps. Plate II. Fig. 1. represents a front view 

 of the pump, where the barrels, exhausting vessels, piston rods 

 and racks, are seen in their situations. IWXK is a strong wooden 

 frame- work, on the under part of which, XK, rest the barrels of 

 the pump. RS is a strong transverse bar for fastening the barrels 

 in their situation. CD is another wooden bar, upon which the 

 two exhausting vessels, F and G, are placed. HH are two tubes 

 leading from the exhausting vessels F and G, which unite, and 

 then proceed to the pump plate, which is supported on the trans- 

 verse frame W. The tubes HH carry each a floating valve vi, 

 which both open into the exhausting vessels. 



VV are the piston-rods with their racks. Behind the piston- 

 rods, rises a tube from the under part of each of the barrels, A and 

 B, which enters the exhausting vessels F and G. One of these 

 tubes, E, is seen in section in Fig. 2. N and P are two tubes 

 connecting the upper parts of both barrels with the exhausting 

 vessels, N connecting the barrel B with the exhausting vessel F ; 

 the other, P, connecting the barrel A with the exhausting vessel G. 



Fig. 2. is a section of the pump, where one of the barrels, ex- 

 hausting vessel, and different tubes communicating with them 

 are more distinctly seen. A is one of the barrels, which may be 

 formed of cast iron or glass, so as not to be acted on by the mer- 

 cury. R is a solid piston, with its rod c c attached, passing up 

 through an air-tight stuffing of leathers, above which is placed a 

 small cup, d, containing mercury, to prevent the insinuation of 

 air through the leathers, at the top of the barrel A. D is the 

 rack joined to the piston-rod. EE is an iron tube leading from 

 the bottom of the barrel A, to the glass exhausting vessel F. In 

 the upper part of the exhausting vessel F, is placed a cup and 

 floating valve, G, opening outwards. HH is a tube leading from 

 the pump plate L into the exhausting vessel, having at its ex- 

 tremity a float-ball, and valve, M, opening into the vessel F. P 



VOL. II. N 



