New Mercurial Air Pump. 265 



drawn up, the mercury will fubfide from the bottom of the valve, and niake a vacuum in 

 the hemifphere. No air can enter by the valve, as it will be always furrounded by and ' 

 covered with mercury. , 



When the mercury (inks below the bottom of the tube b b, the air will enter the vacant 

 hemifphere from the gauge or receiver. When the pifton is deprefled, the mercury will 

 again rife in the hemifphere, and clofing the orifice of the tube b b, will expel the air 

 through the valve at E. The mercury will (as the exhauftion goes on) rife in the tubes 

 b b and d, and thereby indicate the degree of rarefaftion, but it can never rife fo high as 

 to run over the arch of the tube b b, becaufe the column of mercury in, that tube will be 

 long enough to balance the weight of the atmofphere. As the ofcillation of the mercury in 

 the tube b b\% apt to be very violent if not reftrained, a fmall cork ball, with a fliort piece of 

 iron wire ftruck into one fide of it, may be placed under the orifice of that tube, the iron, 

 wire rifing within the tube as reprefented at fig. 2. This ball will float on the mercury, 

 and being thereby prefled tight againft the -orifice of the tubes, will prevent the mer- 

 cury from rifing too faft, or producing any fudden ofcillation therein. The drawings 

 and defcription of an air pump on this principle, reduced to a very portable fize, were fent 

 to Mr. Adams, mathematical inftrumcnt maker, of Charing-crofs, London, in the be- 

 ginning of the year 1 796, who deemed the execution of it impra£licable. Being informed 

 of this, I defired the drawings, &c. might be laid before Mr. Nairne; who, though he 

 offered to make the inftrumcnt, gave it as his candid opinion that it would not anfwer my 

 purpofe to the full extent of my wiflies. I, therefore, dropt the idea of getting aa 

 inftrumcnt of this kind made in England, on account of the great diftance and length of 

 time ncceflary to carry on a correfpondence on fuch a fubjeft. 



Mr. Nairne having given no reafon, however, for his opinion, except that it would coft 

 a great deal of money, I was rcfolved to attempt the fabrication of one myfelf. As I had 

 no means of getting iron pipes made to the fize I wiflied, I was obliged to make (hift with 

 what I could get. The tube A A was of caft iron (and had formerly belonged to a water 

 pump) cut to the proper length. The tube B B was a gun barrel, and the reft of the 

 inftrumcnt was made up in the fame way, of any thing I could pick up. Being but x 

 bungling workman, however, having but few tools, and no advice, or affiftance, I never 

 could make all the joints perfedtly air tight, though I laboured hard to effeft it. 



1 am fully convinced, however, from the experience I have gained with this clumff 

 and imperfect inftrumcnt, that it would anfwer the purpofe if it were well conftrufted. 



The greatcft inconvenience attending the ufe of it was occafioned by the ofcillation of 

 the mercury, in confequenee of the great difference in the diameters of the tubes through 

 which it moves. 



Vol. IV.— September, 1800, M m De/crlpioK 



