1917-18.] Studies in Clocks and Time-keeping. 
75 
XI. — Studies in Clocks and Time-keeping: No. 1. Theory of the 
Maintenance of Motion. By Professor R. A. Sampson, F.R.S. 
(MS. received January 14, 1918. Read January 14, 1918.) 
The studies of which the present paper is the first are intended to 
accumulate observations and discussions on all necessary points connected 
with exact time-keeping, such as the maintenance of motion, air-resistance, 
barometric error, escapement error, temperature compensation, and so forth. 
It may be taken that no clock so far made behaves uniformly so as to keep 
regular time. The clock is one of the fundamental measuring instruments 
of astronomy, but astronomical practice has kept in advance of the precision 
reached by its clocks by determining the clock error in the midst of each 
set of observations and so dispensing with the need for carrying forward 
for any length of time a doubtful clock rate. But there are many reasons 
why we should not be content with this, which amounts to an elimination 
of the clock. In the first place, it is not always feasible. In the next, all 
established indispensable instruments ought to be studied by themselves as 
pieces of apparatus, and the character of their faults defined. Again, it is 
unsatisfactory to accept as final test of time the earth’s, rotation, because 
this is almost certainly not quite steady.* There are many other questions 
which a distinct improvement in time-keeping might elucidate, such as the 
constancy of gravity. It is true we are at present a long stage from this, 
but the position is not hopeless. For although many admirable papers 
have been written on individual points, by Airy, Bessel, Foerster, and 
others, some of them very thorough, none of them have been pushed far 
enough to separate the controllable and calculable reasons for variation 
from the anomalous ones ; nor, in my opinion, has the observation of what 
the clocks actually do been complete enough ; and in the end clock per- 
formance has remained a matter for mechanical skill rather than theory, 
subject to the guidance of certain well-established traditional rules. One 
should not underrate the degree of precision that has been reached by 
these means. Time is measured out second by second, the pendulum being 
disturbed each second, or, in some constructions, every alternate second, by 
the maintenance. There are 86,400 seconds in the day. The three chief 
clocks dealt with below reach a standard in which the question usually 
* Of. Glauert, Mon. Not. R. Ad. Soc ., Ixxv, p. 489 (1915). 
