180 
LORD RAYLEIGH AND MRS. SIDGWICK 
cury, and filled it by suction. It was then held down in the trough with iron weights 
till it was presumably of the same temperature as the mercury in the trough, which 
was taken at three places. It was then held by the fingers (previously cooled in other 
mercury), pressed against its two ends, and taken out of the trough, the mercury 
adhering to the outside was brushed off - , and the contents of the tube were emptied 
into a small porcelain crucible and weighed. But there was no doubt that when the 
fingers holding the tube were bare they pressed a little way—how much it was 
difficult to determine—into the tube, and when they were covered with stiff leather, 
or other stiff material, it was difficult to get a sufficiently good hold. However, in 
one case (No. 5) r was calculated from the weight so obtained with leather on the 
fingers. 
Tiie method, followed by Siemens and Sabine, of screwing an iron plate up against 
the end of the tube, was attempted, but we did not succeed in closing the orifice suffi¬ 
ciently tightly in this way. Ultimately we came to the conclusion that the best results 
would be obtained by weighing a thread of mercury nearly as long as the tube, and of 
which we could ascertain the actual length by direct measurement. We thought, 
also, that there might be some advantage in ascertaining the volume of the mercury 
from the same filling as that of which the resistance had been taken, as we could not 
be sure that the closeness of contact between the mercury and the glass was always 
the same, so that the same volume of mercury would always be contained in the same 
length of tube, nor that the tube itself was in no way altered by the action of the 
caustic potash used to clean it. The plan adopted was, therefore, after measuring the 
resistance, to keep the tube horizontal so as to retain in it most of the mercury while 
the terminals were removed, and then with microscopes and divided rod to measure 
the thread of mercury in the same way as the tubes were measured. The length so 
obtained is called in the table l. The greatest difference between l and L (that in 
measurement 11) is scarcely over 1 per cent., and in most cases the difference is con¬ 
siderably less, so that, considering how nearly cylindrical the tubes were, the error in 
the mean section introduced by using a thread of length l instead of L is quite in¬ 
appreciable. It was another advantage of our method that it avoided the necessity 
of filling the tube under mercury, which it would have been difficult to do with a tube 
so long as IY. 
The only difficulty in measuring the thread of mercury arose from the convexity of 
its ends. This was overcome by pressing them flat with little flat-ended vulcanite 
pins made to fit into the tube. The curvature of the ends when free was not always 
the same; but it was found that the length of the mercury held with pins varied little 
from the number calculated on the assumption that the ends were hemispherical, namely, 
the length of the portion of the column of mercury which was in contact with the glass 
added to two-thirds of the difference between this length and that between the convex 
extremities. In some cases, where, owing to the pins not fitting very well or other 
causes, there was a difficulty in flattening the ends properly, the calculated value was 
