162 Proceedings of the Asiatic Society. [ Ave. 
To compare, therefore, time at one period of the earth’s existence 
with time at another period we require, in addition to the sideral 
and solar second, a mechanical second which would be defined as 
the period, or a definite portion of the period of vibration of a 
body practically quite unacted upon by any force. Such a time- 
keeper has been made at the University of Glasgow and consists 
of aspring pendulum truly balanced about its centre of inertia 
and hermetically sealed in an exhausted glass tube. The vibra- 
tions of such a pendulum are of course not in the least affected by 
the earth, and could only be influenced by the little air that has 
unavoidably been left in the tube producing some change in the 
metal of which the pendulum is composed. The number of vibra- 
tions of the pendulum per second are carefully counted now, and 
will be counted again at some future period, when the number 
will be apparently greater per second than it is now, since a 
sidereal second then will really be a longer time than a sidereal 
second now. In this way the actual loss of speed of the earth’s 
diurnal rotation can be practically measured. 
Col. Tennant said :— 
He did not see how our idea of inertia involved an idea of 
time. Inertia was the passive power by which change of state 
was resisted, and when a body was at rest there was no ques- 
tion of time involved; but he had not come prepared to discuss 
this point in detail. The paper seemed to be mainly leading 
to the question which had lately been raised as to the effect of the 
tides in retarding the Tarth’s rotation on its axis, and he 
thought some account of this might be interesting. 
He (Col. T.) would remark on Mr. Ayrton’s statement that an 
increase in the duration of a revolution of 0.44 of a minute ina 
century has been found by calculation. The whole tidal problem is 
of extreme complication, and in its generality cannot be touched by 
analysis. The motion of the water has been deduced on certain 
hypotheses which are very far indeed from representing existent 
facts. Thus we have some knowledge of what the motion would be 
in a canal surrounding the earth equatorially and of uniform, or great, 
depth and section, or in similar canals passing through the poles ; 
