330 SECTIONAL TRANSACTIONS.—A. 
point is determined by the angle of intersection of the K.P. and E.Q. lines that pass 
through it and by the line intervals inits neighbourhood. For most practical purposes 
it will be possible, after a little experience, to appreciate the potential and phase 
anomalies and their practical significance by a simple inspection of the linear diagram 
that is provided by the two series of lines. It is confidently believed that this new 
method of representing the rather complex surface potential conditions that are 
met in practice will prove more satisfactory to the electrical prospector than any 
hitherto developed. 
Dr. J. H. Jonzs. 
Several types of seismograph have been designed for recording artificial earth- 
quakes in connection with geophysical prospecting. 
These instruments have to be robust, portable and highly sensitive, and the 
pendulum system must have a suitable periodic time and a convenient damping 
arrangement. 
It is important to design the pendulum to give a maximum magnification of the 
ground movement at the end of the helm. 
The motion of the helm is further magnified by some mechanical or electrical 
device. 
There is a serious disadvantage common to all the mechanical methods. The 
coupling of the magnifying device to the pendulum introduces a constraint on the 
motion and reduces the periodic time of the system. A simple device for magnifying 
the motion of the pendulum has been used by the writer. 
In this method a phosphor bronze strip, carrying a soft iron element and a small 
mirror, is attached to the end of the helm. The soft iron element is situated in the 
magnetic field of two small magnets, and is set in rotation by the motion of the 
pendulum. . 
The periodic time of the pendulum system is increased by the action of the magnetic 
field on the element and by simple adjustments of the magnets; both the period and 
magnification can be varied over wide limits. 
Mr. LANCASTER JONES. 
The gravity gradiometer was evolved to meet the special conditions obtaining in 
an area, suspected of bearing iron ore, which was covered by the tide for the greater 
part of each day. As a very close network of stations had to be observed, an 
extremely portable and quick-acting gravity balance was necessary. This rapidity 
of action is obtained in the gradiometer by reducing the horizontal separation of 
the effective masses of the suspended system, and thereby its moment of inertia and 
period of oscillation. Asa result the time necessary for a single observation is reduced 
by about one half, from fifty to twenty-five minutes. By a suitable rearrangement 
of the masses the system is made insensitive to curvature quantities, with the con- 
sequence that only three observations are necessary per station with a single-beam 
instrument, or two observations with a double-beam balance, as compared with the 
five and three observations respectively in the case of the ordinary balance. The 
total time necessary at each station is therefore further reduced proportionately. 
Dr. Ezer Grirrirus, F.R.S., and Mr. J. H. Awpery.—The Thermo- 
physical Properties of Refrigerants. 
For the construction of entropy-temperature charts for various fluids of interest 
in refrigeration it is necessary to have data concerning their physical constants over 
a wide range of temperature. 
Under the auspices of the Engineering Committee of the Food Investigation 
Board we have measured the latent heat and specific volume (i.e. the volume per 
gm. of vapour, under saturation vapour pressure) of dichloroethylene and trichloroethy- 
lene, ethyl chloride and methyl chloride, sulphur dioxide, pentane and ether over a 
range of temperature. 
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