a V nescenfratied Air-Pump. S09: 
dita E: If we want to. know when they ceafe opening, 
_ with the weight of | the atmofphere | on. them,- draw. the pitom 
_ of the valve-pump up into its barrel, to prevent any air efcap- 
ing through that valve ; in this fituation, work the great pump 
again, and. if any air pafles. through. the valves into thé pipe; 
_ the gage will rife by condenfauon. - "This condenfed a air muft. 
_ then be let out. by opening the communication, at. the cock, i 
- with: the-outward: air. i By repeating this till the gage rifes. no: 
longer, . we-may know the valves will open no more, while: the 
» weight of the atmofphere lies on them ; and the rarefaction in 
-the receiver can be carried no further..- When the weight of the 
-atmofphere is to: be removed, after conducting. ns in the former 
„apation nib thg open end of the tube above the farface, of 
z| the then work the he waive -puimp;- and: the air. will 
be rarefied evel: e the: valves, and in the tube, to: -the fame degree | 
-(we-may-fee-when.the valve of this. pump bas done playing by 
-unfcréwing the cap that covers it.) Theopen ¢ end of the tube is 
-:£hen:to be immer fad into the mercury, and the great pump. work» 
edo The airwhich pafies thro’ the valves will then raife the gage ° 
pean eal hani by M gem raifing a and. depreiling 
T2273 
SE DARRE iE p E 
TRV eer 
the: rareta of: the ain du receiver ds far asctlié aièr m ihe 
pump willgo:. If one of. Mr. Smeatan’s pear-gages be-ufed in;the 
receiver, ashe directs, the differenceof: the rarefaction, in the 
two experiments, may be known. - (Andas the air above the valves - 
may be rarefied to. different degrees, we may know, by the two 
^ ‘gages, what proportion the rarefaction above the valves bears-to 
the degree of excefs in the receiver. - This-condénfing gage;can 
Be-taken off, and a button fcrewed into the hole in its ftead, in 
any cafe wherein a greater degree of coridenfation is required than 
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