a TRANSACTIONS OF SECTION A, 409 
changes of the attraction of the mass of the earth, denoted by kF. The 
results obtained by Darwin and Hecker, and confirmed by Schweydar, show 
that the two numbers h and k are connected by the equation h—k=1}. From 
this result alone we cannot determine either h or k, and so cannot determine 
the actual height of the corporeal tide without having recourse either to 
hypothesis or to new observations. If we adopt Kelvin’s hypothesis, we 
find k=%h, and thence h=§8, k=i, and the corresponding estimated 
height of the corporeal lunar tide is about 46 cm. If, however, we bring 
in the fact of observation, discovered by Dr. S. C. Chandler, viz., that 
the period of variations of latitude (about ten months if the earth were 
absolutely rigid) is actually about fourteen months, we can determine k in 
terms of known quantities. Variations of latitude imply an adjustment of 
the earth’s figure to rotation about an instantaneous axis which does not 
quite coincide with a principal axis. The corresponding inequality of 
‘centrifugal force’ has the same effect as a certain external force pro- 
ducing a deformation of the earth and a genuine disturbance of gravity. 
If the force in question is denoted by F, the genuine disturbance of gravity 
may be denoted by kF, where the coefficient k is necessarily the same as in 
the tidal problem. It has been proved independently by Sir J. Larmor and 
the author that k& is about 74. It thence appears that h=# approxi- 
mately, and that the height of the corporeal lunar tide is about 33 cm. The 
earth would accordingly appear to be more rigid than Lord Kelvin estimated 
it to he, a result confirmed by the interpretation of seismographic records. 
DEPARTMENT OF GENERAL Puysics, 
The following Papers were read :— 
1. The Lengthening of Loaded Wires when Twisted, 
By Professor J. H. Poyntine, F.R.S. 
An analysis of a pure shear, taking into account quantities of the order e?, 
where ¢ is the angle of shear, shows that there may be a pressure perpendicular to 
the planes of shear proportional to «?. If this applies to a twisted wire and the 
pressure is not applied the wire should lengthen on twisting. The author described 
experiments on steel, brass, and copper wires, all of which when loaded sufficiently 
to take out kinks showed a lengthening d/ =, sa°6°/7, where a is the radius, 
J the length, 6 the angle of twist, and s is for steel about 1, for copper and brass 
about 1°6,! 
2. The Angular Momentum in a Beam of Circularly Polarised Light. 
Ly Professor J. H. Poyntine, FBS, 
From the analogy of a shaft revolving uniformly and transmitting strain and 
energy with the ‘natural’ velocity ./ rigidity + density the suggestion is 
obtained’ that the angular momentum given per second to a normal receiving 
surface by a beam of circularly polarised light of wave-length A and energy 
E per c.c, is EA/2e, 
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1 See Proc. Roy. Sac., A, vol. 1xxxii. p. 546. * Ibid. A, vol, lxxxii, p. 560. 
