DETERMINATION OF PRECISE TIME—SPENCER JONES 199 
a variation in latitude, which can be observed with a zenith tele- 
scope. The movement in the perpendicular direction causes a dis- 
placement of the meridian. The motion of the pole has two main 
components, with periods of a year and of about 14 months respec- 
tively. As a consequence of this motion, it would be found that if we 
had a perfect clock, with no rate at all, and observations which were 
entirely free from error, the clock would appear to have a slightly 
variable rate. This apparent variation of rate will affect the deter- 
mination of the frequency drift and give a spurious value. 
It is not possible at an observatory to measure the component of 
the polar motion at right angles to the meridian. At Greenwich 
an approximate compensation for the motion is made through the 
cooperation of the Naval Observatory, Washington, which sends 
regularly to Greenwich the observed movement of the pole along the 
meridian of Washington. If Washington were 90° in longitude west 
of Greenwich, the displacement along the meridian of Washington 
would also be the displacement at right angles to the meridian of 
Greenwich. But the longitude of Washington is only 77° west of 
Greenwich. However, the use of the Washington latitude-variation 
data does enable the greater part of the polar-motion effect to be elim- 
inated from the Greenwich clock curves and it has been noticeable 
that the inferred performance of the clocks has thereby been improved. 
The development of an atomic or molecular clock, in which the 
frequency of some selected atomic or molecular vibration will be 
subdivided to give a frequency closely equal to that of an oscillating 
quartz crystal and used to lock the vibrations of the crystal, is 
already foreshadowed by the work in progress at the National Bureau 
of Standards, Washington, in the development of an ammonia clock, 
in which the frequency of one particular mode of vibration of the 
ammonia molecule is used as the control. This work is as yet in its 
early stages and has not gone beyond the point of showing that the 
control of a quartz crystal in the way suggested is practicable. When 
the clock has been developed to the stage at which the accurate con- 
trol of a precision quartz clock becomes possible, the crystal will be 
prevented from drifting in frequency. ‘The clock error curve over a 
long period of time should then be represented by a straight line. 
Departures from a straight line could be attributed to periodic errors 
in the star places, to the polar motion, or to irregularities in the 
rate of rotation of the earth itself. Much more accurate long-term 
prediction would become possible, with a considerable gain in the 
accuracy of timekeeping. 
It has been well established that the length of the day is subject 
to small fluctuations. It has long been known that there are discord- 
ances between the observed and the tabular positions of the moon 
which are not attributable to imperfections in the theory of the 
866591—50-——14 
