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Volume 20, 1887
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Papers.—1. “Probable Discovery of the Physical Causation of Gravitation,” by T. Wakelin, B.A.


The following is a summary of the conclusions arrived at in the paper:—


That physical-science philosophers believe that gravitation must be caused by the action of some physical agent.


That no material agent can act directly on every particle of a very-large body.


That probably some physical agent acts on the outside of the Sun, and of Jupiter, producing the motion of certain spots.


That the physical agent would act more strongly on the side of the earth furthest from the Sun, to deflect the earth in its course, than it does on the nearer side.


That this force acts more strongly at or near the centre of the earth's shadow; the part of the earth under a vertical sun having the force of gravity weaker than the average.


That the clocks at the surface of the earth at the places indicated should be affected accordingly.


That this variation in the force of gravity is about coincident with the average day and night and seasonal temperature, taking the variation in the force of gravity due to latitude into account.


That the temperature of the clocks generally is the same as the day and night and seasonal temperature.


That mercurial or other compensation is such as to make the clock go faster in summer and slower in winter than the proper rate.


That when regulating a clock for variations of temperature, the compensation may really compensate for day and night and seasonal variations in the force of gravity, coincident with the changes in position of the

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centre of the earth's shadow, as well as for day and night and seasonal variations in temperature.


That if compensation is made for a variation in the force of gravity, as well as for a variation in temperature, the quantity of mercury actually found necessary should be greater than the amount found sufficient by calculation to compensate for variation of temperature alone.


It is pointed out that the valuation of evidence is sometimes a matter of great difficulty; but it is hoped that the evidence afforded by the facts brought out by the regulation of the Melbourne Observatory clock will be considered clear.


The Astronomer Royal of Victoria, and Assistant Astronomer, kindly furnished the following information. The temperature of the clock is obtained by maximum and minimum thermometers in or on the clock.:—

Weight of steel rod and fittings = 1.92 lbs.
" cast-iron jar = 4.38 "
" mercury = 16.00 "
Diameter of jar (internal) = 2 ¼ inches.
Length of jar 8 ½ "
Half-swing of pendulum 1 deg. 58 minutes.
A model of this clock, exhibited by the maker, Chas. Frodsham, was exhibited at the Paris Exhibition, 1867, and found by the Horological Jury to be one of the best in existence.


The best authority on clocks then, Sir E. Beckett, (Encyclo. Brit., Art. “Clocks,”) says that “a jar 2 inches in diameter requires the jar to be filled with mercury to a height of 6.8 inches.” The Director of the Observatory found that 8½ inches was not sufficient to compensate for variation of temperature. The rate, however—a slightly losing one in summer, and a gaining one in winter-seems practically perfect.


In removing from the latitude of London to that of Melbourne less mercury should be required.

The paper concludes as follows:—

“The foregoing shows that the astronomical clock at the Melbourne Observatory has a quantity of mercury in excess of what is required to compensate for changes of length of pendulum due to changes of temperature, and that such excess probably compensates for variations in the force of gravity. If the jar had been filled to a height of 6.8 inches with mercury—the proper quantity to compensate for variations of temperature—then the Melbourne clock would have been too slow in summer and too fast in winter, this showing that the force acting on the pendulum—the force of gravity—would be stronger in winter than in summer.

“If the force of gravity is found to vary with the time of the year and the time of day, then it is shown that the force of gravity is the action of some physical agent.

“The facts and reasoning go to prove that such is the case. Thus is it shown that the physical causation of gravitation is probably, discovered.”