March 22, 1907] 



SCIENCE 



449 



the double three-leg one for the great clock 

 at the Houses of Parliament about fifty 

 years ago. Incidentally, it might be men- 

 tioned that this "Westminster clock has 

 turned out to be the finest timekeeper of 

 any public clock in the world. The orig- 

 inal specifications required that the clock 

 should be guaranteed to perform within a 

 margin of a minute a week, which caused 

 the leading clock-makers of England to de- 

 cline to bid for the work. However, under 

 Mr. Denison's supervision the clock was 

 built by Mr. Dent, and from reports of the 

 Astronomer Royal, who receives at Green- 

 wich two signals a day from this clock, sent 

 automatically, its error is rarely over a 

 second a week. 



The Denison gravity escapement is 

 shown in Fig. 6. This escapement con- 



FlG. 6. Denison Gravity Escapement. 



sists of two gravity-impulse pallets pivoted 

 as nearly as possible in a line with the 

 bending point of the pendulum spring and 

 touching the pendulum near the bottom of 

 the figure. The locking wheel is made up 

 of two thin plates having three long teeth 

 or 'legs' each. These two plates are 

 squared on the arbor a little distance apart, 

 one on each side of the pallets. Between 

 them are three pins which lift the pallets. 

 In the figure, one of the front legs is rest- 

 ing on a block screwed to the front of the 

 right-hand pallet. There is a similar block 

 screwed to the back of the left-hand pallet 

 for the legs of the back-plate, which is 



shaded in the figure, to lock upon. Pro- 

 jecting inward from each of the pallets is 

 an arm. The tip of the one on the right- 

 hand pallet is just in contact with one of 

 the pins which has lifted the pallet to the 

 position shown. The pendulum is travel- 

 ing in the direction indicated by the arrow, 

 and the left-hand pallet has just given im- 

 pulse. The pendulum rod in its swing will 

 push the right-hand pallet far enough for 

 the leg of the front locking plate, which is 

 now resting on the block, to escape, Di- 

 rectly it escapes, the left-hand pallet is 

 lifted free of the pendulum rod by the 

 lowest of the three pins. After the locking 

 wheel has passed through 60°, a leg of the 

 back locking plate is caught by the locking 

 block on the left-hand pallet. As the three- 

 leaved pinion always lifts the pallets the 

 same distance, the pallets in returning give 

 a constant impulse to the pendulum. 



About fifteen or twenty years ago the 

 Riefiers, elockmakers of Munich, intro- 

 duced into their clocks an escapement in 

 which the impulse is communicated to the 

 pendulum through the suspension spring. 

 The pendulum is supported by a rocking 

 frame to which is attached the anchor 

 carrying the pallets which are acted upon 

 by the escapement wheel. Just after the 

 pendulum has passed through its vertical 

 position, the escapement wheel, when re- 

 leased, gives to the supporting frame of the 

 pendulum suspension spring a slight tilt in 

 the opposite direction from that in which 

 the pendulum is moving, thus increasing 

 the tension of the spring due to the swing 

 of the pendulum to one side. 



In the next few minutes I wish to con- 

 sider the accuracy with which our astro- 

 nomical clocks perform their function. 



The earliest star catalogue of precision 

 is that of Bradley and the observations 

 upon which it is based were made about a 

 third of a century after the introduction 

 of the compensated pendulum. In discussi 



