THE MECHANICAL EQUIVALENT OF HEAT. 
381 
such narrow tubes is very slow when the difference of pressure at the ends falls below 
1 millirn. As we could find no way out of this difficulty we had to content ourselves 
with a higher pressure than we had proposed to work with. 
The taps used were of a kind that has a diagonal hole through the stopper and a 
mercury cup at each end ; they appear to be very perfect, and any leakage from either 
end can be detected by its effect on the mercury. One of these taps was in each 
glass tube leading from M and N, and, by their means, all the rest of the circuit 
could be cut off from the calorimeter. 
The tube connected with M led to a five-way Sprengel and a McLeod gauge (both 
by Hicks), The tube from N, after passing the first tap, branched into two arms, 
one of which led to a Geissler pump, while in the other was a three-way tap 
establishing connection with the water pump, or with a series of drying bottles, 
containing H 2 SO 4 ,. A tube containing Ba(HO )3 was fixed between the drying bottles 
and the calorimeter, as we thought it just possible that, otherwise, sufficient HoSO^ 
vapour might pass into the chamber to produce some action on the steel surfaces. 
When the apparatus was fixed together the temperature of the tank was raised to 
about 40° C. ; the air was then exhausted down to about 16 millims. by the water 
pump, dry air was passed in slowly, and this process was repeated several times. 
Finally the pressure was reduced as far as possible by the water pump (to about 12 
to 14 millims.), and the Sprengel and Geissler brought into play. 
It took about 100 double strokes with these pumps to bring the pressure down to 
1 millirn., so the labour involved was great, and want of time alone prevented our 
reducing the pressure further than we did. By increasing the number of the platinum 
tubes we hope to be able in future to work with much better vacua. 
The absence of leakage is indicated by the fact that the lapse of a week made no 
perceptible difference in the reading of the McLeod gauge, although a change of '01 
millirn. could have been easily detected. 
It was long before this degree of perfection was attained, for the difficulty of 
ascertaining the position of a slight leakage, in such a circuit, was very great, and 
many weary hours were expended in leak hunting and in consec|uent taking to pieces 
and putting together again the various portions of the apparatus. 
Section VI.— The Method of Maintaining a Constant E.M.F. 
General plan of the electrical connections* (fig. 2 , p. 382) ;— 
Leads passing through points numbered 1 and 3 belong to the Clark cell circuit. 
Leads passing through points numbered 2 and 4 belong to the storage cell circuit. 
The leads 1 and 2 , as also 3 and 4, unite at the lid of the calorimeter itself, as 
shown in the small sketch, 
* The actual posifciou of the galvanometers, &c., was different from the sketch, which is only intended 
to indicate the nature of the arrangement. 
