422 
Proceedings of the Royal Society 
100, was obtained by comparing a sovereign and a shilling; it 
agrees well with the ratio obtained afterwards. The surfaces of the 
coins were cleaned with hath-brick and wiped against flannel; in 
no case were the readings anomalous at the beginning. The com 
parison obtained for platinum is only an approximation. It was 
obtained by comparing a small platinum capsule, kindly lent me by 
Mr H. K. Mill, with a florin and a penny. The true value is pro- 
bably nearer to that for gold. 
In the third series of observations I made many comparisons with 
discs of copper, zinc, lead, iron, tin, and brass. The reason why so 
many comparisons were made was to clear up an anomaly which 
appeared when the surfaces were polished with emery paper. In 
the case of the observations of 6th December, one side of each disc 
was rubbed with emery paper, and then wiped with a dry flannel 
cloth. The copper disc was first tried ; it gave readings none of 
which differed greatly from the average, namely, 147. But the zinc 
disc when taken gave on the first whisk + 110,* on the second + 45 ; 
after a few whisks the deflection vanished, then became negative, 
and gave a series of ten readings agreeing closely with their average, 
95. The iron disc also gave a positive deflection at first -f 80; it 
likewise changed after a slight brushing, and gave ten readings 
agreeing pretty well with their average, 71. When a fresh part of 
either of these discs was rubbed, the deflection became positive. In 
the case of the lead disc the deflection was considerably smaller 
than the average at the beginning. In the ct "e of the copper and 
the tin discs there was no such marked irregularity ; but the average 
of a series for copper taken at the end, 187, was greater than the 
average of the series taken at the beginning of the observations, 
147. The values entered in the upper line for the 6th are obtained 
by giving copper its initial average, and those in the lower line by 
giving it the final average. 
Next morning, 7th December, no emery paper was used; the sur- 
faces were merely rubbed with a piece of white flannel which had 
been well dried. The copper disc gave regular negative deflections 
as before, but the zinc disc gave a positive deflection at first. After 
a few brushings across the middle zone of the disc, the deflection 
became negative, and remained about an average. The first read- 
'When no sign is put before a number, the sign - is to be understood. 
