CHRONOMETER. 



l fie time, unless the spring be made of 

 a certain definite length, or tapered in 

 its thickness according to the experi- 

 ence which many artists in this country 

 possess. 



The escapement generally used in our 

 best chronometers, as we shall hereafter 

 see, consists of a toothed wheel at the 

 end of the train, which is prevented from 

 running down by a detent or hook, and 

 of two pallets, a longer and a shorter, 

 fixed upon the verge or axis of the ba- 

 lance. These pallets are so placed, that 

 when the face of the longer pallet has 

 just arrived before one of the teeth of 

 the wheel, the shorter pallet strikes out 

 the hook, and allows the wheel to push 

 forward the longer pallet with its tooth, 

 during which action, the hook falls again 

 into its place, to catch the succeeding 

 tooth. The balance therefore proceeds 

 in its vibration, and returns again with- 

 out disturbing the train ; because the 

 short pallet does not strike out the hook 

 in its backward course, but only acts on 

 a slender spring, resembling those for- 

 merly used in the jacks of * harpsichords. 

 In this manner the vibrations are kept 

 up ; and so little do the variations in 

 the maintaining power affect the rate, 

 when all the adjustments are made, that 

 if the main spring be let down to only a 

 small part of its ordinary tension, these 

 time-pieces will keep the same rate for 

 many hours together. 



However perfect, practically speaking, 

 the application of the maintaining power 

 may be, yet if the balance and its spring 

 be subject to vicissitudes from heat and 

 cold, it will be in vain to expect accuracy. 

 There are two ways of correcting this 

 compound time-measurer. The first, 

 which was invented by Peter Leroy,con- 

 sists in causing the balance to enlarge it- 

 self, instead ot contracting by heat ; by 

 which means the spring, when in the 

 state of greater rigidity, has more work to 

 do ; and the other acts by lengthening 

 or shortening the spring, when cold or 

 heat may have given it more or less of 

 force. This was invented by Harrison, 

 and depends on the well-known fact, that 

 a short spring is stiffer than a longer; so 

 that by shortening his spring at the time 

 when it was weakened by heat, and the 

 balance enlarged by the same cause, he 

 gave it the stiffness requisite to compen- 

 sate for these alterations ; and the same 

 contrivance produced the contrary effect 

 in cold temperatures. As we shall more 

 fully exhibit these inventions under the 

 article HOROLOGY, it is only necessary 

 to. observe, tbat Peter Leroy constructed 



his first time-piece with fluid thermome- 

 ters on the balance, and that he also in- 

 vented our present expansion balance ot 

 brass and steel, soldered or fused togeth- 

 er in the rim, which was afterwards in- 

 troduced and brought to great perfection 

 by Arnold. 



Machines, made upon the principles 

 here cursorily pointed out, have measur- 

 ed time to a wonderful degree of perfec- 

 tion; and from the immense maritime 

 trade of the British empire, and the sci- 

 entific disposition of many wealthy indi- 

 viduals, the demand has been so great, as 

 to have produced a very great number 

 of able workmen, fully equal to their 

 construction, at the same time that the 

 prices have been considerably reduced. 

 Most sea commanders of any respecta- 

 bility are provided with two or more of 

 them. 



Amongthe other causes of irregularity 

 in time measurers, the resistance of the 

 air has been occasionally considered by 

 authors. But artists seem to suppose, ei- 

 ther that it is a constant quantity , or that 

 its variations are not considerable enough 

 to be brought into the account. The 

 very accurate performance of some chro- 

 nometers, and the steady going of astro - 

 nomical clocks, seem to give weight to 

 this supposition : but on the other hand 

 it may be remarked,that though the slow 

 motion of heavy pendulums vibrating 1 

 through 'small arcs in astronomical clocks 

 must be subject to very little resistance 

 indeed from the air, yet it does not fol- 

 low that the rapid vibrations of a balance 

 may not be affected by this cause; and 

 the extreme precision ofsome chronome 

 ters will not, perhaps, be admitted as a 

 very strong argument, when we consider 

 that the changes from barometrical causes 

 may have compensated each other, and 

 that the most perfect machines will vary 

 as much as one second per day, from 

 causes which have not been yet clearly 

 detected, though these are probably re- 

 solved into that before us. We are 

 more particularly led to these reflection? 

 by a communication from Mr. Manton, of 

 Davies-street, who found by experiment 

 that a chronometer, which was going up- 

 on a gaining rate of five seconds per day, 

 did increase its arc of vibration by an ad- 

 ditional 50 degrees immediately upon the 

 air being exhausted; and that being kept 

 in vacuo, its rate became 37 seconds per 

 day, the gain being 34 seconds upon the 

 former rate. He nee it follows, that as the 

 difference between the highest and the 

 lowest stations of the barometer indicate 

 a change of about one-fourteenth part in 



