196 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1949 
%4o second. As the natural losing rate of the slave clock, 6 seconds 
a day, is equivalent to go second per minute, the synchronizer, which 
is actuated each half minute, should hit and miss alternately. For 
this reason it is called the ‘‘hit-and-miss” synchronizer. 
The first experimental Shortt free-pendulum clock was installed at 
the Edinburgh Observatory in 1921. It at once proved to be such an 
improvement upon previous pendulum clocks that two were installed 
at the Greenwich Observatory in 1923 and others in subsequent years. 
It was the excellent performance given by these clocks that made it 
necessary for astronomers for the first time to introduce the conception 
of mean sidereal time. Previously true sidereal time had been uni- 
versally used, as clocks were not good enough to be able to show up the 
small effects due to the short-period terms in nutation. The free- 
pendulum type of clock is capable of an accuracy of about one- 
hundredth of a second a day. Detailed investigation of their 
performance has shown, however, that such clocks are liable to 
frequent small erratic changes of rate, of the order of about 3 milli- 
seconds a day. Small though such changes are, they cause, by 
integration, an irregular wandering of the clock. For sending out 
time signals, it is always necessary to extrapolate beyond the latest 
time determination; these erratic changes of rate restrict the accuracy 
with which the error of the clock can be extrapolated. It can, on 
occasion, happen that 2 weeks or more may elapse without any check 
on the performance of the clock being possible and the transmitted 
time signals may consequently be appreciably in error. Moreover, 
because of the errors of observation, there is a natural scatter in the 
derived errors of the clock. In interpolating between the observed 
errors there is no means of distinguishing between scatter due to errors 
of observation and scatter due to the irregular wandering of the clock. 
It is possible, of course, to attempt to reduce the effects of the 
wandering by using the mean of several clocks. Nevertheless, very 
high accuracy cannot be obtained, because residual effects due to the 
irregularities are always present. 
A new standard of accuracy has been provided in recent years by 
the use of an oscillating quartz crystal, developed originally to serve 
as a precision standard of frequency. The quartz clock is based upon 
the piezoelectric property of quartz. If a plate of quartz is com- 
pressed, the two opposite faces become electrically charged, one 
positively and the other negatively. Conversely, if two opposite 
faces are given positive and negative charges respectively, the piece 
of quartz experiences a mechanical contraction or expansion. By 
rapidly alternating the electric charges, the quartz can be maintained 
in mechanical vibration. In the quartz clock an oscillating electrical 
circuit is used, the dimensions of the crystal being adjusted so that its 
