TIME-KEEPING IN LONDON. 



159 



an appreciable interval of time was required for perfect compensation ; 

 a modification of the gridiron form, shown in Fig. 2, was therefore 

 adopted. The pendulum was designed by Messrs. E. Dent & Co., of 

 London, for the Transit of Venus Expedition (1874), but has since been 

 used for the primary standard time-keeper of the United Kingdom. Its 

 construction will be best understood by reference to the section shown 

 in Fig. 3. To the lower end of a steel rod, suspended in the ordinary 

 manner, is attached the screw for rating the pendulum. On this screw 

 and surrounding the rod rests a zinc tube, extending upward ; inclos- 

 ing the zinc tube and attached to its top is a steel tube extending down- 

 ward ; on a collar, at the lower end of the steel tube, hangs the cylin- 

 drical leaden bob, attached at its center. Slots and holes are cut in the 

 tubes in order to equally expose all parts. The following table, taken 

 from the official records of the Royal Observatory, is published by 

 Messrs. E. Dent & Co., for the purpose of showing the performance 

 of a clock with steel and zinc pendulum : 



CLOCK DENT 1914. 



DATE. 



Days. Hours 



1871 September 3 21.. 



17 21.. 



24 21.. 

 October 1 22. . 



8 21.. 



15 21.. 



22 21.. 

 29 21.. 



November 5 22. . 



12 22.. 



19 21.. 



26 22. . 



December 3 21.. 



10 22.. 



17 21.. 

 26 0.. 

 31 22.. 



1872 January 7 22.. 



14 21.. 



21 21.. 



23 22.. 

 February 4 22. . 



11 22.. 



18 21.. 



25 22.. 

 March 3 21. . 



10 22.. 



17 21.. 



Clock slow of Green- 

 wich sidereal time. 



Minutes. Seconds. 



Mean daily- 

 losing rate 

 during each 

 interval. 



Seconds. 



Average tem- 

 perature of 

 external 



During the whole time of rating, the clock was situated in a small 

 hut erected for observing the Transit of Venus. No record of the 

 temperature of the hut was kept, but the variations would be rery 



