424 
LORD RAYLEIGH ON THE LAW OF THE PRESSURE OF GASES 
equality, to maintain the same sloped position during the subsequent use when the 
gauges must be combined in series. But in this case it would hardly be advisable to 
trust to wood-work in the mounting. At any rate in my experiments the gauges 
were erected with measuring-rods vertical, an arrangement which has at least the 
advantage that a displacement is of less importance as well as more easily detected 
At the close of the observations upon the various gases it became necessary to 
compare the gauges with full precision. 
For this purpose, they were connected (without india-rubber) in parallel, the upjoer 
chambers of both being in communication with the jDump, and the lower chambers of 
both in communication with the gas reservoirs GI. Had the lengths of the measuring- 
rods been absolutely equal, this equality would be very simply jjroved by the 
possibility of so adjusting the pressure of the gas and the supj)ly of mercury to the 
two manometers that all four mercury surfaces could be set simultaneously. It was 
very evident that no such simultaneous setting Avas possible, and the problem 
remained to evaluate the small outstanding difference. To pass from one manometer 
to the other, either the volume or the temperature had to be varied. 
In principle it would perhaps be simplest to keejD the volume constant and determine 
what difference of temperature (about half a degree) would be required to make the 
transition. But the temjDerature of the undrowned parts (now increased in volume) 
could not be ascertained with great precision, so that I preferred to vary the 
volume and to trust to alternations backwards and forwards for securing that the 
mean temperature in the two cases to be compared should not be different. Thus 
in one set, including seven observations following continuously, four alternate obseiwa- 
tions were settings with one manometer and three Avere settings AAuth the other. 
According to the thermometers, the mean temperature in the first case Avas for the 
droAvned volume 11°’38 and foi- the (much smaller) undroAAmed Amlume 12°76. In 
the second case the corresjjonding temperatures Avere 11°'39 and 12°'80, so that the 
differences could be neglected. The volume changes Avere efiected in the side-tube 
J K, and the mean difference in the tAvo cases Avas ‘411 cub. centim. It AAnll be 
understood that in order to define the volume both manometers Avere ahvays set below. 
The Avhole volume Avas reckoned at 294 cub. centims., of Avhich about 258 cub. centims. 
represents the capacity of the bulbs G I droAAuied in the Avater-bath. According to 
these data the proportional difference in the lengths of the measuring-rods, equal to 
the })roportional difference of the above determined volumes, is ‘00140. Taa’o other 
similar sets of observations gave ‘00136, ‘00137 ; so that the mean adopted A^alue is 
‘00138. The measuring-rod of the manometer on the right, fig. 2, is the longer. 
As in the case of the volumes, any error in the aboA^e comparison is hah^ed in the 
actual application. If be the length of the rod in the right-hand manometer, 
the length on the left, AA’e are concerned only Avith the ratio Hj + Ho : Ho. And 
from the value above determined we p’et 
o 
I^+ u. 
Ho 
1‘99862 . 
(5). 
