ConJtriiBlon of an Air Pump wlthoui the Lower Vahc, 125 



cock with the outward air. By repeating this till the gauge rifes no longer, we may know the 

 valves will open no more while the weight of the atmofphere lies on them ; and the rare- 

 fa£lion in the receiver can be carried no further. When the weight of the atmofphere is 

 to be removed, after conducing as in the former experiment, raife the open end of the tuiie 

 above the furface of the mercury, and then work the valve pump, and the air will be rarefied 

 over the valves and in the tube to the fame degree (we may fee when the valve of this pump 

 has done playing, by unfcrewiug the cap that covers it). The open end of the tube is then 

 to be immerfed into the mercury, and the great pump worked. The air which pafl'es 

 through the valves will then raife the gauge by condenfation ; and thus by alternately railing 

 and depreding the tube, and working the two pumps in their turns, we may carry the 

 rarefaftion of the air In the receiver as far as the power of the pump will go. If one of Mr. 

 Smeaton's pear gauges be ufed in the receiver, as he direfts, the difference of the rarefaftion 

 in the two experiments may be known. And as the air above the valves may be rarefied 10 

 different degrees, it may be known, by the two gauges, what proportion the rarefaftion above 

 the valves bears to the degree of excefs in the receiver. This condenfing gauge can be 

 taken off, and a button fcrewcd into the hole in its ftead, in any cafe wherein a greater degree 

 of condenfation is required than the glafs will bear. When a glafs receiver is ufed, this 

 gauge may be placed within it, where it will meafure any degree of condenfation the re- 

 ceiver will bear without danger to the gauge ; or the capacity of any receiver may be mea- 

 fured by this gauge before it is removed from its place, by fhewing how many ftrokes of the 

 winch will throw one atmofphere into the receiver ; then turning the cock to prevent any 

 air efcaping, change the gauge for the button. When this is done, the degree of condenfa- 

 tion may be further meafured by the number of ftrokes. 



As in cafes where great condenfation is required, there muft be a great deal of labour, 

 and a great ftrain on the teeth of the wheel and pitton rods, on account of the great diame- 

 ter of the piftons * -, to remedy this, Mr. Prince has fitted a condenfer of a fmaller bore 

 than the barrel of the great pump to the ciftern of the valve pump, to be fcrewed on occa- 

 fionally, by which the condenfation may be finiflied, inftead of the great pump. Or, to 

 fave the work and expence of this condenfer, the valve pump, if made a little larger, may 

 be eafily fitted for the fame purpofe, by having a plate made to fcrew into the bottom of 

 the cylinder occafionally, with a valve on it opening into the ciftern : a hole muft alfo be 

 made to be opened on the fame occafion near the topof the cylinder, to let air in below the 

 pifton when this is drawn up above it. 



The common gauge, which is generally placed under the receiver plate in this pump, is 

 placed in tlie front, that it may be feen by the perfon who is working the pump, and that 

 the place may be left free for other ufes. 



The plate is fo fixed to the pipe leading to the cock, that it may be taken off at pleafure, 

 and ufed as a transferer; or any tube or apparatus may be fixed to it to perform fome ex- 

 periments without removing it, wliich will fave trouble and make lefs apparatus neceffary. 



•In this pump the piflons arc two inches diameter; fo that ilicrc will be aljout forty-eight rounds added 

 to the refinance in opening the valves for every atmofphere thiown into the receiver. P. 



It is certainly an injury to an cxhaufting pump to ufc it for condenfing. This operation ought always to be 

 ptrformed with a longfyringc of fmall bore. Such an inflrument would coft much lefs than the condenfing 

 part of an air pump, and will at the fame time have much more power. N. 



The 



