432 



NATURE 



{April I, 1875 



duluni, now detached, passes on lo the left ; in returning 

 to the right, the light "passing spiing" before spoken 

 of allows the pendulum to pass without disturbing the 

 detent ; on going again to ttie left, the pendulum again 

 receives impulse as already described. The right-hand 

 pallet forms no essential part of the escapement, but is 

 simply a safety pallet designed to catch the wheel in case 

 of accident to the locking-stone during the time that the 

 left-hand palUt is beyond the range of the wheel. The 

 escape wheel carrying the seconds hand thus moves 

 once only in each complete or double vibration of the 

 pendulum, or every two seconds. 



An ordmary mercurial seconds pendulum was first con- 

 structed, with jar of larger diameter than is usually made, 

 but this did not give satisfactory resuhs. Notably it was 

 found, whilst still on trial in the workshop, that when the 

 temperature of the apartment was raised, the clock in- 

 creased considerably its losing rate, which only slowly 

 returned towards its previous value, showing quick action 

 on the rod and slow action on the quicksilver. This 

 pendulum was finally discarded and another made em- 

 ploying entirely a metallic compensation. A central steel 

 rod is encircled by a zinc tube resting on the rating nut 

 on the steel rod ; the zmc tube is in its turn encircled by 

 a steel tube which rests at its upper end on the zinc tube, 

 and carries at its lower end the cylindrical leaden pen- 

 dulum bob attached at its centre to the steel tube. The 

 weight oi the bob is about twenty-six pounds. Slots are 

 cut in the outer steel tube, and holes are made in the inter- 

 mediate zinc tube, so as better to expose the inner parts 

 of the compound pendulum rod to the action ol tempera- 

 ture. For final adjustment of the compensation two 

 straight compensated brass and steel bars (// and / in the 

 figure) are carried- by a collar, holding by Iriction on the 

 crutch axis, but capable of being easily turned on the axis. 

 The bars carry small weights at their extremities, as shown. 

 Increase of temperature should accelerate or retard the 

 clock according as the brass or steel lamina is respec- 

 tively uppermost. The bars were at first placed in the 

 upright (neutral) position, and it is anticipated that, by 

 turning them into an inclined position as respects the 

 pendulum rod, power will be given within a certain limit 

 (reached when the bars stand horizontal) of correcting 

 any defect in the primary compensation, but, on account 

 of the uniform temperature of the Magnetic Basement, no 

 opportunity has yet arisen for testing the efficiency of the 

 apparatus. A contrivance is also added with the object 

 01 making very small changes of rate without stopping the 

 pendulum. A weight /■ slides freely on the crutch rod, 

 but is tapped to receive the screw cut on the lower portion 

 of the spindle /, the upper end of which terminates in a 

 nut -III at the crutch axis. By turning this nut the position 

 of the small weight on the crutch rod is altered, and the 

 clock rate correspondingly changed. To make the clock 

 lose, the weight must be raised.'* 



In the arrangement of the going power the ratchet is 

 so constructed that it does not touch the great wheel on 

 its flat part, with the object of avoiding unnecessary fric- 

 tion when the maintaining spring alone is acting. The 

 driving weight of the clock is about ^\ lb., and in order 

 to avoid sympathetic vibration, it is made to descend in 

 a compartment of the clock-case separate from that con- 

 taining the pendulum ; it also bears shghtly against the 

 side oi the compartment. 



The brass vertical sliding piece shown at the lower left- 

 hand side in Fig. i carries at its upper end two brass 

 bars, each of which has at its right-hand extremity, 

 between the jaws, a slender steel spring for galvanic 

 contact ; the lower spring carries a semicircular piece 



* As regards the efficiency of the zinc and steel compensation, it may be 

 liere mentioned that the transit clocks made for the Transit ot Venus txpe- 

 ditions were provided with pendulums compensated in this way. Some of 

 these clocks underwent very severe trial at Greenwich before the various 

 expeditions set out, with most satisfactory results. They seemed, indeed, to 

 be superior to clocks fitted with the ordinary mercurial compensation. 



projecting downwards, which a pin (jewel) on the crutch 

 rod lifts in passing, bringing the springs in contact at 

 each vibration (these parts are concealed in the figure by 

 the crutch rod) ; the contact takes place when the pen- 

 dulum is vertical, and the acting surfaces of the springs 

 are, one platinum, the other gold, an arrangement that 

 has been supposed to be preferable to making both sur- 

 faces of platinum. By means of the screws n and 0, which 

 both act on sliders, the contact-springs can be adjusted 

 in the vertical and horizontal directions respectively. 

 Other contact-springs in connection with the brass bars 

 p q gvl the other side of the back plate are ordinarily in 

 contact, but the contact is broken at one second in each 

 minute by an arm on the escape-wheel spindle. The 

 combination of these contacts permits the clock to com- 

 plete a galvanic circuit at fifty-nine of the seconds in each 

 minute, and omit the sixtieth, for a purpose to be hereafter 

 mentioned. 



No contrivance was originally applied to the clock for 

 correction of the barometric inequality, but the clock had 

 not been in use many months before the extreme steadi- 

 ness of its rate otherwise brought out with marked distinct- 

 ness the existence of the inequality. It was easily seen 

 that for a decrease of one inch in the barometer reading, 

 the clock increased its daily gaining rate by about three- 

 tenths of a second. The Astronomer Royal eventually 

 arranged a plan for correction of the inequality, founded 

 on the magnetic principle long previously in use at the 

 Royal Observatory for daily adjustment of the mean solar 

 standard clock, and the apparatus has been applied to 

 the clock by Messrs. Dent. Two bar magnets, each 

 about six inches long, are fixed vertically to the bob of 

 the clock pendulum, one in front (shown at a, Fig. 2), the 

 other at the back. The lower pole of the front magnet 

 is a north pole ; the lower pole of the back magnet is a 

 south pole. Below these a horseshoe magnet, b, having its 

 poles precisely under those of the pendulum magnets, is 

 carried transversely at the end of the lever c, the extremity 

 of the opposite arm of the lever being attached by the 

 rod d to the float c in the lower leg of a syphon baro- 

 meter. The leverturns on knife edges. A plan of the lever 

 (on a smaller scale) is given, as well as a section through 

 the point A. Weights can be added at f to counterpoise 

 the horseshoe magnet. The rise or fall of the principal 

 barometric column correspondingly raises or depresses 

 the horseshoe magnet, and, increasing or decreasing the 

 magnetic action between its poles and those of the pen- 

 dulum magnets, compensates, by the change of rate pro- 

 duced, for that arising from variation in the pressure of 

 the atmosphere. As the clock gained with low barometer, 

 it was necessary to place the magnets so that there should 

 be attraction between the adjacent ends ; that is, that they 

 should be dissimilar poles. One other point may be 

 mentioned in connection with this apparatus. The cistern 

 in which the float rests is made with an area four times 

 as great as that of the upper tube ; so that for a change Oi 

 one inch of barometer reading, the horseshoe magnet is 

 shifted only two-tenths of an inch, whilst the average dis- 

 tance between its poles and those of the pendulum 

 magnets is about 3^ inches : that is to say, the extent of 

 variation of the position of the horseshoe magnet should 

 be a small fraction of the whole distance, because, with 

 this condition, the effect produced on the rate by equal 

 increments of distance is then practically uniform. The 

 action of the apparatus on the Greenwich clock has, as 

 regards correction of the inequahty of rate, been quite 

 successful ; and further, the extent of the pendulum arc, 

 which was before subject to a slight variation, is now very 

 constant, and amounts (the total arc) to about 2" 33' with 

 scarcely any change. 



This account of the clock will scarcely be complete 

 without some brief description of the use made of it. It 

 has been mentioned that the clock completes a galvanic 

 circuit fifty-nine times in each minute, but omits the 



