318 Proceedings of the Royal Society of Edinburgh. [Sess. 
a straight line, but after some adjustments of the damping and earthing 
arrangements, this difficulty was overcome. 
The position of the water-dropper is on the south wall of the labora- 
tory, the tube projecting a distance of 6 feet from the wall at a height 
of 34 feet above the ground-level. As the laboratory itself is 245 feet 
above sea-level, the height of the collector is 279 feet measured from 
sea-level. 
From October to the end of December 1911 the water-dropper was in 
continuous operation, but it has been decided not to discuss any of the 
observations obtained during that period as the details of the apparatus 
were frequently altered in order to overcome various difficulties. At first 
it was found difficult to get a point image of light on the drum sufficiently 
intense to give a good trace and yet not so large as to produce a broad line. 
An incandescent electric lamp was placed outside the box containing the 
clockwork and electrometer, and the light from this passed down a tube 
through a lens and was reflected from the mirror of the electrometer on to 
the drum. The lamp and lens were adjusted until the image of one of the 
filaments appeared sharp on the drum, and then this line was cut down to 
a point by placing immediately in front of the drum a narrow slit. This 
has been found to work quite satisfactorily since its installation. 
A Dolezalek paper needle was found unsuitable for the measurement of 
high atmospheric potentials. When the potential of the needle reaches a 
certain limit, the needle moves towards the quadrants and gets discharged. 
This has already been noticed by G. W. Walker,* who overcame the 
difficulty by prolonging the vertical axis of the needle and loading it with 
a small nut. A specially heavy needle, made of aluminium, was used in 
this work. A platinum wire attached to the vertical axis of the needle 
dipped in an insulated dashpot containing sulphuric acid, which acted as 
an efficient means of damping. 
Since the beginning of the year 1912 the whole apparatus has worked 
satisfactorily. There have been occasional breakdowns of insulation, and 
on several days the water in the tube leading from the tank became frozen. 
It is comparatively easy to decide by inspection of the record whether the 
instrument and insulation are working properly or not, and the records of 
any days on which there was ground for suspecting that something was 
wrong have been neglected. 
§ 2. The electrograph, of course, records merely the actual electric 
potential at the point where the water breaks up into drops. For purposes 
of comparison it is desirable that the results should be reduced to ground- 
* Proc. Roy. Soc ., lxxxiv. p. 585, 1910-1911. 
