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XVI. Apparatus for Measurement of Low Pressures of Gas. By 

 Professor M'Leod, Indian Civil-Engineering College, Cooper's 

 Hill*. 



THIS apparatus was devised for estimating the pressure of a 

 gas when its tension is so low that the 

 indications of the barometer cannot safely 

 be relied on, unless indeed a very wide 

 barometer and an accurate cathetometer 

 be employed. The method consists in con- 

 densing a known volume of the gas into 

 a smaller space and measuring its tension 

 under the new conditions. 



The form of the apparatus is the follow- 

 ing : — The tube a communicates with the 

 Sprengel, and with the apparatus to be ex- 

 hausted ; b is a siphon-barometer w ith a 

 tube about 5 millimetres in diameter; and 

 the principal parts of the measuring-appa- 

 ratus consist of c, a globe of about 48 cubic 

 centims. capacity with the volume-tube at 

 the top, and d the pressure-tube; these 

 two are exactly of the same diameter, to 

 avoid error from capillarity. The tube at 

 the bottom of the globe is ground into a 

 funnel-shaped portion at the top of the 

 wide tube e ; and to the side of the latter 

 the pressure-tube d is joined. The volume- 

 tube at the top of the globe is graduated 

 in millimetres from above downwards, the 

 lowest division in this particular apparatus 

 being 45 ; the pressure-tube d is also gra- 

 duated in millimetres, the being placed 

 at the level of the 45 th division on the vo- 

 lume-tube. A ball-and-socket joint con- 

 nects the bottom of e with a vertical tube 

 / about 800 millims. long, which is con- 

 nected at its lower extremity by means of 

 a flexible tube with the mercury-reservoir g; 

 a stopcock at h permits the regulation of 

 the flow of mercury into the apparatus : 

 this may be conveniently turned by a rod, 

 so that the operator may watch the rise of 



* Read before the Physical Society, June 13, 1874. Ccainimicated by 

 the Society. 



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