Sir Frank Dyson 91 



that the free pendulum acts in such a way that should 

 the clock itself err even to the two-hundredth of a second 

 it is instantly corrected by the pendulum of which it 

 is the slave. This marvellous clock is of English make, 

 and has been installed during the office of the present 

 Astronomer Royal, Sir Frank Dyson. 



A second clock in the same room automatically sends 

 time signals to the great radio station at Rugby, whence 

 they are wirelessed to all parts of the Empire. Navigators 

 on all the seas receive these time signals by wireless, so 

 that these clocks may justly be said to be the most 

 important in the world. 



The work done at Greenwich is still largely that of 

 taking regular observations, such as observing the occul- 

 tation or hiding of stars by the moon, the exact time and 

 place of their disappearance and reappearance. You 

 might suppose that this sort of thing was no longer 

 necessary and that the moon's orbit was now perfectly 

 known. But this is not so. If the earth and the moon 

 were the only two bodies in the universe the problem 

 would be simple. But the earth, the sun, and the moon 

 are members of a triple system which is complicated by 

 the faint pulls exercised by the planets, and the result is a 

 problem of amazing intricacy. Calculations of the moon's 

 movements need, therefore, to be compared with observa- 

 tions, and the task is endless. 



One of the great triumphs of systematized observation 

 was the discovery of the planet Neptune. The observed 

 movements of Uranus were found to be out of accord 

 with its computed movements, and simply from this fact 

 Adams and Leverrier were able to state that there must 

 be another planet outside the orbit of Uranus. It was in 

 1845 that Adams sent his calculations to Airy, showing 

 that a new planet should be searched for, and in Septem- 

 ber 1846 Neptune was discovered by Dr Galle, of Berlin 

 Observatory. Airy has been blamed for failing to search 



