ro6 Defcriptioii of a nnu Air-pump, 



one of them is connected at b, with a duft formed in the piece of metal k, attached to the 

 box of leathers C at the top of the barrel, which dud turning in an angkj without pene- 

 trating the box, opens into the barrel : this pipe and duft are joined by a fimple application 

 of their orifices, which are made in the contiguous fides of both, and the joint made air- 

 tight, by cement melted around with a blow-pipe ; and the fame might be eflefted by the 

 prefiure of a fcrew clofmg the furfaces with oiled leather Interpofcd. By this pipe when 

 ■ the valve is fhut, and tlie key of the cock fo placed that the communication of this pipe 

 with the bottom of the barrel is open, if the pifton be deprefled from the top, all the air 

 which is under it will be forced to pafs by this pipe into the barrel above the pifton ; and 

 •vice verfd if the pifton be elevated from the bottom ; fo that according to the motion of the 

 pifton, the air under, or over it, within the barrel, is made to change its place, and to cir- 

 culate from the fpace under or above the pifton to the contrary ; from which effect of this 

 pripe, I call it the circulating pipe. 



. The other pipe, or duft, is reprefented at c d, £g. 2. which exhibits a back view of the 

 pump : its extremity c enters the block of metal V to communicate with the dudl therein, 

 ^ad fo with that through the cock into the barrel : its end c has a wing foldered to it, by 

 which it is firmly fcrewed to the piece V; this being neceflary left the cement by which 

 alone the joint there is made air-faft, fliould be cracked by any (hock moving the barrel ; 

 or it might, perhaps, be fecured by the fcrew with only leather interpofed : this du£l: is a very 

 ■Vpide one, that the air may the fooner pafs through it ; it has a turn at right angles at d, 

 '^d another at e, from whence it extends under the pillar B, and pump-plate C D, through" 

 the middle of which it rifes at A ; it has a branch turning at right angles from A under the 

 pump-plate, the end of which terminates in a round orifice with an infide fcrew, that by 

 means of a pipe furnifhed with a ftop-cock ; or otherwife another pump-plate and receiver, 

 Qr feveral of them, may be connected with it, being laid on the fame table which fupports 

 tlic pump : when fuch are not ufed, the orifice is clofed with a ftop-fcrew as at D. From 

 tliis pipe rifes a branch E, inferted into the top of the brafs cap F of the glafs veflcl Y, and 

 opening into the veflel by a duft made in the thick plate of the cap. G is a reduced baro- 

 meter tube, to be filled in the moft perfeft manner with mercury, and immerfed into a pool 

 of mercury in the bottom of the glafs vefTel. The interval between the tube and the 

 neck of the veflel is clofed by cement, which, as the neck muft be a good deal wider 

 than the tube, is efi^e£ted by a deep ring (I) fitted to the tube, having a round plate at the 

 bottom of it ; which being let down on the tube after it has been immerfed in the ciftern,« 

 is joined with cement to the top of the cap. The tube was eafily immerged in the mercury 

 within the veflcl, by covering its open end, and fuftaining the mercury in it by a little plate, 

 or fcale of iron (P), fig. 7. having two threads tied to it, through two holes made near its 

 edges ; which threads were grafped together with the top of the tube between the fingers, 

 and the tube was inverted, and let down into the mercury in the veflel : the little plate was 

 then pulled up by one of the threads, it being made fo fmall as to pafs through the neck 

 of the glafs while the tube was within it, and remained immerfed in the mercury* : the 

 • I thought this the beft way of fixing; the reduced barometer, for if there were a feparate veffel as a 

 ciftern for the raercury, and a receiver placed over it and the tube, the eye when on a level with the furface 

 of the ttagnam mercury, could not fee the altitude of that in the tube with prccifion through two glaffes.— L. 



veflcl 



