188 Dr Searle, A Simple Method of determining 
20 to 25 cm. of mercury above the atmospheric pressure. The 
apparatus is then allowed to stand for some minutes in order that 
the rise of temperature due to the compression of the air may die 
away. A screen should be placed between the can and the observer 
to prevent the transfer of heat from his body to the can. 
When the gauge readings have become steady, they are recorded. 
The tap is then opened for a time ¢ seconds—one minute or more 
—and is then closed, and the new gauge readings are taken and 
recorded*. This process is repeated for various initial pressures. 
If the barometric pressure and the temperature remain constant, 
we see, by (15), that the value of t/X should have the same value 
for each of the experiments with a given tube. 
The radius of the flow tube is found from the mass of mercury 
required to fill the tube. It is here assumed that the bore of the 
tube is uniform; the method of obtaining the small calibration 
correction is explained in § 6. 
The volume of the can is best found from the mass of water 
required to fill it; small corrections are required for the various 
tubes connected to the can. 
§6. Calibration correction. We shall now investigate the 
correction required when the flow tube is not of uniform radius. 
By (8) we see that we have to replace //a,‘ in (10) by 
[Ss 
Onan 
and thus formula (15) becomes 
asl ie ne, 
I= ae ee Ct tittttieteeeeees (16) 
The value of [dx/a‘ is found as follows. Suppose that n —1 marks 
are made on the tube, dividing it into n parts, each J/n cm. in 
length. Let a thread of mercury of mass m grms., whose length 
is approximately //n cm., be introduced into the tube. Let the 
thread be moved along the tube so that its centre approximately 
coincides with the centre of each of the m parts in turn, and let 
Gi, Yo, --- be its length in the n positions. Then, if the tube be 
treated as uniform over each of the n parts, we may put 
Tpaz=m/q, WpaZ2=m/GQ2., &c., 
where p is the density of mercury. 
* After the tap has been closed, the pressure in the vessel sometimes rises 
gradually by a few tenths of a millimetre. This indicates that, although the 
thermal conduction from the walls of the vessel has not kept the temperature of 
the expanding air quite constant, yet the fall of temperature has been very slight. 
