HKTWKKN ONK ATMOSPHERE AND HALF AN ATM< isrill.UE OF PRESSUBE. 353 



much extrapolation. The comparison of volumes at pressures atmospheric and half 

 atmospheric seems to meet these requirements, though we must not forget that 

 (apart from theory) the result is still of the nature of an extrapolation. On this 

 subject reference may be made to a paper by Sir W. RAMSAY and Dr. B. STEELE.* 



The guiding idea in the present apparatus, as in that of 1902, is the use of two 

 manometric gauges combined in a special manner. " The object is to test whether 

 when the volume of a gas is halved its pressure is doubled, and its attainment 

 requires two gauges indicating pressures which are in the ratio of 2 : 1. To this end we 

 may employ a pair of independent gauges as nearly as possible similar to one another, 

 the similarity being tested by combination in parallel, to borrow an electrical term. 

 When connected below with one reservoir of air and above with another reservoir, or 

 with a vacuum, the two gauges should reach their settings simultaneously, or at least 

 so nearly that a suitable connection can readily be applied. For brevity we may for 

 the present assume precise similarity. If now the two gauges be combined in series, 

 so that the low-pressure chamber of the first communicates with the high-pressure 

 chamber of the second, the combination constitutes a gauge suitable for measuring a 

 doubled pressure." 



Tin' Manometers. 



The construction of the gauges is modelled upon that used extensively in my 

 researches upon the density of gases, t An iron measuring rod, AB, is actually 

 applied to the two mercury surfaces, arranged so as to be vertically superposed. 

 This rod is of about 7 millims. diameter and is pointed below, A. At the upper 

 end, B, it divides at the level of the mercury into a sort of fork, and terminates in a 

 point similar to that at A, and, like it, directed downwards. The coincidence of 

 these points with their images reflected at the mercury surfaces is observed with 

 the aid of lenses of 20 millims. focus suitably held in position. It is, of course, 

 independent of any irregular refractions which the walls of the tube may exercise. 

 In each manometer the distance between the points is 15 inches or 381 millims. 



The internal diameter of the tubes, constituting the upper and lower chambers of 

 the manometers, is 22 millims. This is the diameter at the level of the " points " to 

 which the mercury surfaces are set. At the places where the iron rod emerges above 

 and below into the open, the glass is contracted until it Incomes an approximate tit, 

 and air-tightness is secured with the aid of cement. 



',. .,'.// .ll-l-<l,n/i'lii> i-t "/ .)/</'/"/">. 



Witli one important difference to be explained presently the general scheme is the 

 same as in 1902 and is sketched in fig. 1. The left-hand manometer can be 



* " On the Vapour-Densities of some Carbon Compounds ; an attempt to determine their Molecular 

 WeighU," ' Phil. Mag.,' Oct., 1893. 



t ' Roy. Soc. Proc.,' vol. 53, pp. 134-149, 1S93 ; ' Scientific Papers,' vol. 4, p. 41. 

 VOL. OC1V. A. 2 Z 



