TIME-KEEPING IN LONDON. 



161 



(k) slides freely on the crutch-rod shown back of it in the figure, but 

 is held by the screw on the end of the spindle (I) which bangs from 

 the nut (m) at the crutch-axis. By turning the nut (m) the weight 

 (k) can be lowered or raised, and this makes the clock gain or lose. 



But the nicety of the correction of variations due to changes of 

 temperature has brought to light variations due to another cause 

 commonly quite overlooked ; it has been found that the pendulum is 

 affected by changes of barometric pressure. A change in the ba- 

 rometer of an inch and a half will sensibly alter the rate of the pen- 

 dulum. The difficulty might be avoided by placing the clock in a 

 vacuum, but this is evidently impracticable. In the Greenwich clock 

 the method shown in Fig. 4 has been adopted to counteract the effects 



Fig. 4. Greenwich Clock : Arrangement for Compensation for Barometric Pressure. 



of barometric changes. To the pendulum-bob are attached two verti- 

 cal bar-magnets, one in front (a) with the north pole down, the other 

 at the back (and therefore not shown in the figure), with the south pole 

 down. Below these and normally at a distance of 3f inches from 

 them is a horseshoe magnet (b) which hangs on one end of a lever (c) 

 nicely balanced on knife-edges at A ; the other end of the lever (c) 

 rests by means of a rod (d) on a float (e) in the shorter leg of a siphon 



VOL. XXII. 11 



