HOROLOGY. 



the short one, to which the impulse is given : with 

 a spring of greater length this principle is reversed ; i 

 whence it was concluded by Le Roy and Berthoud, : 

 that equality of time, or isochronism, in unequal 

 vibrations, could be more easily obtained by 

 lengthening the spring than by tapering it. In 

 principle, too, the stronger and shorter the spring, j 

 the quicker will be its vibrations. Thus effects of 

 an extremely varied description can be produced 

 on the motions of a watch by the slightest differ- 

 ence of length and taper in a hair-spring. And it 

 is thus that the correctness of the time-keeping is 

 essentially dependent on the principle adopted in 

 the formation of this apparently most insignificant 

 little appendage. So much is this the case, that 

 if the hair-spring be isochronal in a free or detached 

 escapement, the time shewn will be the same, not- 

 withstanding changes in the motion of the wheels, 

 or even in the power of the mainspring. In 

 England, where time-keepers have been brought 

 to their greatest perfection, it is considered that 

 isochronism is most easily attainable by using the 

 cylindrical helical spring, which is applied to all 

 marine chronometers. 



Compensation. 



In watches, even more than in clocks, variations 

 of temperature, unless provided for, produce varia- 

 tions in the rate of going, the increase or diminu- 

 tion of the temperature affecting to some extent 

 the moment of inertia of the balance, and to a 

 great extent the elastic force of the balance-spring. 

 A rise in the temperature makes the balance 

 expand, and therefore augments its moment of 

 inertia ; it adds to the length of the spring, and 

 thereby diminishes its elasticity, the elastic force 

 of a spring varying inversely as the length ; and 

 the time of vibration of the balance, which depends 

 upon the moment of inertia directly, and upon the 

 elastic force of the spring inversely, is increased 

 the watch, that is, goes more slowly in conse- 

 quence both of the increase of the inertia and of 

 the diminution of the elastic force of the spring. 

 A fall in the temperature is attended by opposite 

 results, the watch going more rapidly than before. 

 A watch without a compensated balance would 

 vary very much more than a clock without a com- 

 pensation pendulum, but that being usually carried 

 in the waistcoat pocket, it is kept at a pretty 

 uniform temperature. To invent a satisfactory 

 compensation, was at one time the great problem 

 for watchmakers. The compensation can obvi- 

 ously be made in either of two ways by an 

 expedient for shortening the effective length of 

 the balance-spring as the temperature rises, so 

 as to increase the elastic force of the spring ; 

 or by an expedient for diminishing the moment 

 of inertia of the balance as the temperature 

 rises, so as to correspond to the diminution of 

 the force of the spring. The first method was 

 that made use of by John Harrison, who first 

 succeeded in making a chronometer capable of 

 measuring time accurately in different tempera- 

 tures ; but an adaptation of the other method, 

 invented by Earnshaw, is that which is always 

 employed now : / a t' is the main bar of the 

 balance ; and / b, t' b' are two compound bars, 

 of which the outer part is of brass, and the inner 

 part of steel, carrying weights, b, b', which may be 

 screwed on at different places. The brass bar 



expands more with heat, and contracts more with 

 cold than the steel 

 bar ; therefore, as 

 the temperature 

 rises, the bars, 

 with their weights, 

 bend inwards, and 

 so the moment of 

 inertia of the bal- 

 ance is diminished; 

 as it falls, they bend 

 outwards, and the 

 moment of inertia 

 is increased ; and 

 of course the dim- 

 inution or the increase must be made exactly to 

 correspond to the diminution or increase in the 

 force of the spring. 



CHRONOMETERS. 



The chronometer is just a large watch fitted 

 with all the contrivances which experience has 

 shewn to be conducive to accurate time-keeping 

 for example, the cylindrical balance-spring, the 

 detached escapement, and the compensation- 

 balance. As a watch which will keep time in 

 one position will often not do so equally well in 

 another, marine chronometers are always set 

 horizontally in a box in gimbals, an arrangement 

 which keeps the chronometer horizontal, whatever 

 the motion of the vessel. 



The great importance of an accurate portable 

 time-keeper at sea is for determining the longitude. 

 From what has been already said on the subject 

 of time, it will be readily understood that if the 

 captain of a ship, leaving a port in Great Britain 

 with a chronometer set to Greenwich time, shall, 

 after sailing westward for a time, observe when 

 the sun is at his highest point, or on the meridian, 

 and find that his chronometer at that instant 

 marks one o'clock, he must be 15 west of Green- 

 wich. This use was first distinctly pointed out by 

 Sir Isaac Newton. A committee of the House of 

 Commons, of whom this philosopher formed one, 

 having been appointed, on the nth June 1714, to 

 consider the question of encouragement for the 

 invention of means for finding the longitude, the 

 result of their meetings was a memorial containing 

 an explanation of the different means proper for 

 ascertaining the longitude, and recommending 

 encouragement for the construction of chronome- 

 ters as the best means of ascertaining it. An act 

 of parliament was then passed, offering a reward 

 for this purpose. 



The first chronometer used at sea was invented 

 by John Harrison. After many years of study, it 

 was completed in 1736. After several further 

 trials and improvements, and two trial voyages to 

 America, undertaken for the satisfaction of the 

 commissioners, the last of which was completed 

 on the 1 8th September 1764, the reward of ,20,000 

 was finally awarded to Harrison. 



Somewhat later than this, several excellent 

 chronometers were produced in France by Ber- 

 thoud and Le Roy, to the latter of whom was 

 awarded the prize by the Academic Royale des 

 Sciences. Progress was still made in England by 

 Arnold, Earnshaw (the inventor of the compen- 

 sation still in use), and Mudge, to whom prizes 

 were awarded by *he Board of Longitude, and 



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