532 
Proceedings of the Royal Society 
of the telegraph instrument is not less than 200 ohms. The resist- 
ance of the galvanometer was given at 2 2 ‘9 ohms, so that about 
of the whole current may he taken as the fraction passing through 
G. The galvanometer was powerfully controlled by external magnets, 
the time of vibration of the mirror being extremely short. The 
direction of the currents was determined directly by comparing the 
deflections due to them with those given by a Daniell cell. 
We can by means of this arrangement ascertain at any time the 
direction of the currents passing through the line, and observe the 
variations in intensity from time to time. These currents I have 
assumed to be due to a difference of potential between the earth at 
Fort-William and the summit, and I think we are justified in 
treating the sheath of the cable as equivalent to an earth-plate at 
the Observatory, but it would be advisable in further investigations 
to have an actual earth-plate similar to that at the other end. 
The observations extend from 20th September to 4th October 
1885, and are hourly, the readings being taken just before the usual 
hourly meteorological observations. The curve of hourly values 
obtained by taking the mean deflection for each hour during the 
successive days shows a well-marked daily variation. During the 
early part of the night we have a current passing through the cable 
wp the hill, which appears to reach a maximum at 21 hours, 
and again at 2 hours. After 2 hours a rapid diminution takes 
place, and at 5 hours we have the current reversed. Immediately 
after this the potential at the summit again falls relatively to that 
at Fort- William, and we have a current coming up, increasing in 
strength to a well-marked maximum at 10 hours. This is suc- 
ceeded by another equalisation, and the current is again reversed 
about 13 hours, after which it passes down the line till about 18 
hours. This part of the curve, however, is much more irregular, 
the current being very unsteady. After 18 hours the current again 
sets in an upward direction through the line, increasing pretty 
regularly till the maximum at 21 hours. 
During the time over which these observations extend, the 
summit of Ben Nevis was almost continuously enveloped in fog 
or mist, and a succession of h£avy storms passed over the Observa- 
tory about the end of September. The daily curve shown may 
therefore be taken to hold only for conditions under which the air 
