198 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1949 
counters record the integrated time difference between each pair, 
A-B, B-C, C-A, the third comparison providing a check on the 
other two. 
A further convenience of the quartz clocks is that it is not necessary 
to maintain separate mean-time and sidereal-time clocks as it is 
with pendulum clocks. By means of suitable gearing, it is possible 
to take sidereal seconds direct from the phonic motor which gives 
also mean time seconds. The ratio of the mean time second to the 
sidereal time second is 1.002 737 909 293. This ratio can be closely 
represented by a gearing of 119/114 multipled by 317/330, which is 
only 4 parts in 10° small. These sidereal second signals are used for 
recording on the chronograph during the time determinations. When 
the rate of the clock relative to these signals has been derived it is a 
simple matter to infer its rate relative to true mean time seconds. 
The small error in the conversion from mean time to sidereal time is, 
of course, eliminated. 
For short-period prediction quartz clocks leave little to be desired. 
They have not as yet, however, reached the stage at which long-period 
prediction has the accuracy that is desirable. The difficulty arises 
from a slow drift in frequency to which they are all liable. The crystal, 
after cutting, appears to go through a slow ageing process; the drift 
in frequency is rather rapid at first, but progressively diminishes 
though it sems never to cease altogether. Iffor any reason the crystal 
should stop, through a tube or resistor giving out, it will not, when 
restarted, follow along its previous ageing curve; a new ageing cycle 
sets in. Any small disturbance, such as a slight temperature change, 
can alter the frequency drift somewhat. The effect of the frequency 
drift on the error of the clock increases with the square of the time 
so that, even though the drift may be quite small, its effects will 
become important with lapse of time. With the present scatter in 
the actual time determinations, several months’ observations are 
needed to give a sufficiently accurate derivation of the frequency 
drift, but there is always the uncertainty whether during this period 
some small disturbance may not have caused the rate of drift to change 
slightly. 
Moreover there are extraneous effects which can complicate the 
determination. During a period of several months, there will be a 
wide range in the right ascensions of the stars which are used for the 
time determinations. If there are periodic errors in the fundamental 
system of star places, a spurious factor will have entered into the 
determination of the frequency drift. The motions of the earth’s 
poles cause further complications. The poles have an irregular mo- 
tion, which is roughly circular, but with a variable radius. The 
extreme departures of the true poles from their mean positions are 
about 30 feet. ‘The movement of the pole along the meridian causes 
