166 Proceedings of the Asiatic Society. [Ave. 
generally accepted,—seemed to be that the earth’s angular velocity 
of rotation, an ingredient in the calculation, was not constant as 
it had always been assumed, but was slowly diminishing. The 
tide-action on the surface of the globe is considered by many 
eminent physicists sufficient to bring about such a result. On the 
other hand, if the earth is contracting in radius by a process of 
cooling denudation or degradation an acceleration of rotation to 
some extent would be the consequence. Hence it becomes a 
matter of interest, if possible, to contrive an accurate time-keeper 
who should be independent of the earth’s rotation, and serve to 
make its deviation from constancy indisputable. He would be 
glad if Mr, Ayrton would kindly explain the particular mode in 
which this end was sought to be attained in-the case of the Glas- 
gow instrument. 
Mr. Ayrton said :— 
The pendulum at the Glasgow University to which I referred is 
made simply on the principle of the balance wheel of a watch, that 
is, a certain mass of metal is made to oscillate by the action of a 
spring, and independently of the action of gravity. The friction, 
however, of the pivot of the balance wheel is obviated by the wheel 
and spring being virtually in one. The arrangement can be best 
understood by imagining a straight flat piece of spring rigidly 
fixed at one end and having a mass of metal fixed at the other 
end which mass oscillates in a horizontal plane by the action of the 
spring. The actual arrangement of the pendulum is really more 
complicated than this, but the principal of action is as described. 
As regards the President’s remark that the earth by contracting 
may acquire sufficient acceleration to compensate for the retarda- 
tion produced by tidal friction, I would mention that Laplace proved 
solely from Fourier’s theory of the conduction of heat that the 
acceleration of the earth’s diurnal rotation produced by shrinking 
from cooling could not have amounted to z3;th of a second in the 
last 2,000 years. Sir William Thomson has also shown that the 
acceleration from this cause must be extremely small compared 
with the probable retardation produced by tidal friction. 
