380 
MR. E. H. GRIFFITHS ON THE VALUE OF 
necessary to produce a given alteration was approximately knovTi after a few 
observafions. 
[Note by E. H. G. Added April, 1893. Lord Rayleigh has discussed the prin¬ 
ciples which should be observed when using a motor of the fan type (‘ Proc. Soc. Roy.,’ 
1881, p. 109), and has shown that when the motor is run at high speed the curves of 
work and of resistance cut each other at a greater angle than is possible when the 
rate of revolution is small, and thus a more uniform motion is obtained in the former, 
than in the latter, case. Our speed wheels were so arranged that the motor under 
ordinary circumstances ran at about 16 revolutions per second, which appeared to be 
as high a sjieed as was necessary.] 
Section V.— The Pressure in the Surrounding Space. 
As already mentioned, the tubes M and N (see Plate 3) led to the pumps and 
McLeod gauge. The circuit embraced several yards of glass tubing, a large number 
of junctions and four taps, of which three had to be absolutely air-tight. In addi¬ 
tion there were the four insulated junctions in the steel and the two in the calori¬ 
meter lid, and also the six openings (three in the steel and three in the calorimeter), 
through which the glass tubes which supported the calorimeter passed. All who 
have worked with low pressures will appreciate the difficulty of making such a series 
of joints sufficiently air-tight. 
The manner in which the difficulty was overcome in the openings through the steel 
and calorimeter is described in Section VII. In what we may term the external 
circuit the greatest difficulty was experienced in the joints between the glass and the 
steel tubes M and N. Throughout 1891 we used telescopic joints. The glass tubes, 
wliich closely fitted into the steel ones, were coated with hard shellac, and, wheu 
both glass and steel had been warmed, were pressed into position. There is no doubt 
that such joints can be made absolutely air-tight; but they cannot be trusted to last 
for any length of time. It is possible that shellac is sufficiently viscous to yield to 
long continued pressure, or, on the other hand, it may crack when exposed to sudden 
changes of temperature. Whatever the cause may be, we have found such joints 
most treacherous. Their very perfection when first made renders them the more 
dangerous, for they inspire the observer with a false confidence. 
Our experiences during 1891 led us to reject the use of shellac in any form."'' In 
1892 the connection between the glass and steel was made by platinum tubes of 
small bore, soldered into the steel at one end and fused into the glass at the other. 
However, the latter form of junction could not be relied on unless the diameter of 
the platinum was small, and this constriction of both the passages leading to the 
pumps greatly increased the labour of exhausting, for the process of diffusion through 
* From the commencement Ave avoided the use of india-rubber tubing. 
