TIT 



HOROLOGY. 



HOROLOGY. 



718 



being found, it is not desirable to alter it in bringing the machine to 

 time. To effect an adjustment, the two screws c c have been introduced, 

 the drawing out of which from the centre causes the machine to lose, 

 and the screwing them in to gain. 



Many important matters relating to chronometers apply in a smaller 

 degree to spring clocks and to pocket-watches, and will receive notice 

 as we go on. But it is necessary here to say a few words concerning the 

 eompetitire trials of chronometers, which have partly resulted from, 

 and have partly suggested the beautiful inventions perfected within 

 the last few years by Eiffe, Frodsham, Loseby, Dent, and others. 

 Between the years 1822 and 1835, the Admiralty gave rewards as 

 prizes for the good performance of individual chronometers : two or 

 three sums being given annually to the makers of those which showed 

 the smallest amount of error. Nothing was given for any new principle ; 

 but the general construction was improved by this competition. The 

 relative order of merit was arrived at by taking the number of seconds 

 in the greatest difference between one week's indications and the next, 

 and adding to it twice the preatest difference between one week's rate 

 and the next ; the sum was the trial number ; and that chronometer 

 gained the prize which exhibited the smallest trial number. The trials 

 were conducted at Greenwich Observatory, under the Astronomer 

 Boyal. After the year 1835 the prizes were withdrawn ; but the 

 makers were equally ready to compete, on the score of reputation and 

 commercial advantage. The test was rendered more severe ; for every 

 chronometer was exposed in the open air to the fiercest cold of winter ; 

 and at other times to the air of a chamber heated to 100 Fahr. In 

 successive years different makers obtained the post of honour ; for instance 

 Poole in 1845, Button in 1846, Frodsham in 1847, Hewettin 1848, Eiffe 

 in 1849. In the five years here indicated, 219 chronometers were tested 

 at Greenwich ; of which 79 were bought for public use, al a maximum 

 price of 62/. During the course of these refined experiments, a fact 

 was observed which amounts almost to a new discovery. However 

 perfect may be the compensation of a chronometer for certain tem- 

 peratures, it was found that it did not remain constant for all 

 temperatures. If a chronometer be planned to resist great cold 

 and great heat, it gains at medium temperatures ; but if specially 

 adapted for the latter, it errs slightly in very cold and very hot 

 weather. The late Mr. Dent explained this fact by supposing that 

 the inertia of a spring balance, as usually constructed, cannot be 

 made to vary uniformly according to temperature, but will vary more 

 rapidly in cold weather than in hot. Chronometer-makers at once 

 set about devising a remedy ; and this remedy, however produced, is 

 called the secondary compensation. Loseby, Dent, Eiffe, Molyneux, 

 and other makers, have invented highly-curious arrangements of 

 spring balance, all of which have been severely tested at Greenwich, 

 and some of them rewarded by government grants. Loseby's 

 arrangement, which received high commendation from the Astro- 

 nomer Royal, may be thus briefly described. Attached to the 

 balance is a curved tube containing mercury. The mercury, on 

 expanding with an increase of temperature, arrives at certain parts of 

 the tube inclined in different degrees to the radii of the balance ; and 

 therefore its successive expansions produce successive effects of different 

 magnitudes on the momentum of the balance. By giving different 

 forms to the tube, the law of the successive alteration of the momen- 

 tum may be made to adapt itself to the law of alteration of the 

 elasticity of the spring, whatever that law may be. 



tig Cl'fks. In describing spring-clocks, we shall at the same 

 time treat of many parts of the mechanism which are applicable like- 

 wise to chronometers, but not described in the foregoing paragraphs. 



In jig. 4, A B c D may be taken as representing the front plate of 

 an eight-day spring clock (which is supposed to be transparent), and is 

 attached to another plate of similar form by five strong pillars between 

 which the wheels here shown are placed. E are two barrels con- 

 taining springs ; the one on the right gives motion to the train of 

 wheels called the going or natch train ; the other to the tritiny train 

 of wheels e, f, y, h , and fly i. In producing the series of movements, a 

 in the main wheel of 96 teeth, acting in the centre wheel-pinion k of 

 eight leaves, to which is attached the centre wheel 6 ; this revolves in 

 an hour, and acts in the third wheel-pinion I, on which is fixed the 

 third wheel r, acting in the swing or scape wheel-pinion m (not seen in 

 the cut) ; to this pinion is iixed the swing- wheel d, whose teeth act alter- 

 nately on the two pallets H o, and thereby give motion to the pendulum 

 by means of a piece attached to the arbor of the pallets, one end of which 

 enters a slit made in the pendulum for its reception. P r are the two 

 fusees, the use of which has already been described in connection with 

 chronometer*. The method in which the fusees are attached to their 

 respective wheels a and e is shown iajiy. 6, where is the main wheel of 

 96 teeth hollowed out to receive the click 6 and its spring c which are 

 attacked to the wheel, the ratchet d being attached to the under side 

 of the fusee by two screws. In Jiy. 4, e it the striking main wheel, 

 having 84 teeth ; this drives the pinion p of eight leaves, on which in 

 the pin-wheel /of 64 teeth, into the rim of which are put eight pins to 

 lift the hammer , by acting upon its tail t. The pin-wheel /driviM 

 the pallet-pinion q of eight leaves, on which is fixed the pallet-wheel y 

 of 66 teeth ; this pallet-wheel acts in the warning wheel-pinion r of 

 leven leaves, on which i the warning-wheel k of 48 or 50 teeth, acting 

 in the fly pinion i. When in action a pin in the pin-wheel catches the 

 tail of the hammer *, and raising it, the hammer-head > recedes from 



the bell ; and as soon as the pin leaves the tail of the hammer, the 

 force of the spring acting on the lower part of the hammer produces 



Fig. 4. 





a blow on the bell. The number of strokes on the bell is regulated by 

 mechanism placed on the outside of the front plate of the clock, but is 

 removed from the figure just described, to prevent confusion. (See 

 fy, 5.) On the centre wheel-pinion Is (fy. 4), whose arbor comes 



Striking Mechanism. 



through the front plate about one and a half inches, is placed the 

 minute wheel o (./?</. 5), which revolves with the centre wheel in an 

 hour, and carries the minute-hand of the clock. This wheel has a pipe 

 nearly as long as the centre-wheel arbor, the upper end of which is 

 squared to receive the minute hand ; and by means of a small spring 

 beneath the wheel, which rests upon a shoulder just above the upper 

 surface of the front plate, and acts against the upper surface of the 

 wheel, the wheel, together with the hand, is forced against a pin over 



