HOROLOGY. 



133 



in the last. The face of the pallet being undercut, had 

 been found requisite from experience, as is fait], in or- 

 der to prevent cutting or wearing. In Berthoud's box- 

 chronometers, or time- keepers, the face of the pallet is 

 . made straight, or in a line to the centre. One of these, 

 alter twenty-eight years going, the greater part of 

 which was from Europe to India, and in the Chinese 

 seas, was put into our hands, and neither the face of 

 the steel pallet, or that of the detent, had the least ap- 

 pearance of being any way marked. This was the more 

 remarkable, M the wheel was uncommonly thin. It 

 wat have been made of very fine brass. The wheel 

 bad ten teeth ; the ratio of the diameter to that of the 

 roller, or pallet, WM .530 to .S4O ; the balance weighed 

 174 grains, and made two vibrations in a second. The 

 balance WM suspended by a short and weak spring, 

 which had been broken by some accident before we got 

 it. The length of this suspension spring required to 

 be .9 of an inch, and of so delicate a nature that many 

 were made for it before the chronometer could be 

 broagbt any way near to time. It seemed, indeed, to 

 have more influence on the timing than any spiral 

 spring could possibly have. Each of the balance pi- 

 vots turned between the rollers, which were more than 

 ope inch in diameter ; and from them and the suspen- 

 sion spring, perhaps, arose that ease and freedom in 

 the motion of the balance, in consequence of which the 

 balance wheel teeth had little to do when impelling the 

 roller or pallet; and this may have been one cause 

 why the pallet face WM sot cat or marked. It may be 

 observed, that it had the eommoo spiral balance spring, 

 and a rum|)iasati ja nmiisf ing of two lamina?, or blades 

 of brass and steel, pinned together and ri vetted ; and 



in the moreable cod WM a screw, which, by its con- 

 nection with an arm in which the curb pins were pla- 

 ced, served to regulate for mean time. Three screws 

 in the balance were also need for this puipuse. 



nth. diameter of the paUat rotier has /consi- 

 derable pwsjortiuu to that of the wheel, the angle of 

 beless.andtheholdonthefaceoftbe 

 l will also be less; but the isapulst given will be 

 direct, and the chanw of stpmg, from any coan- 

 will also be greatly F 



i by external motion, ' 



ed. In thuaiigle more mutt beiMTluded than that which 

 M made from the drop of the tooth to it* asoaniii* the 

 |MlM. The angle of 'leap san tat it included between 

 the potet where thetao* escapes from the main pallet 



te pont were tke tooth escapes from 1 

 or roller, and the point to which the lifts 

 in returning, after having passsjd the 



It HI 



I !, r,- 



lodk'the wheel JtTli iii il li, that 'the ualocki 

 the wheel should be made with the least possible i 

 nee to the vibrations of the balance ; which k attained, 

 by having the end of the lifting-pallct M near to the 

 centre of the balance M is consistent with its getting 

 each a hold of the tiffing-apring, that, in it. coirse, it 

 can readily and easily bring out the detent from lock- 

 ing the wheel. The bold of the tooth oa the detent 

 eaoald not be more than the hundredth part of an inch. 

 P*. fa dofag thia. tto supplementary angle becomes 



th.rc., r- ,t ir.iy :e proj,rr sTCstJ tl,.",rt ""..",!.,'l!i -I ''* 



little longer. A little additional length wOl greatl/re- 

 duce the angle of Vspament, and not much tnrraan 

 the evil of a greater resistance to the vibrations) of the 



in a box chronometer, where the balance wheel has 

 twelve teeth, and the whole angle of scapement is to be 



60~, it is required to find the ratio of the diameter of the 

 wheel to that of the roller. The supplementary angle 

 being taken at 15, the angle of impulse must then be O 



i hic-h is rather wide as otherwise ; but it w ill be less detached 

 than this, when the thickness of the points of the teeth, 'icapement. 

 and the spaces for drop and escape, are not taken into 

 the computation. Now 3' .0 being divided by twelve, the 

 number of the wheel teeth, gives 30 for the quotient; 

 and again divided by 45, the number of degrees for the 

 angle of impulse, the quotient will be b. The diame- 

 ter of the wheel is supposed to be .6 of an inch. To find 

 that of the roller, say, as 1 2 : 6 : : 8 : 4. Four-tenths of 

 an inch is the diameter required for the roller, which 

 will give somewhat less than 45 for the angle of im- 

 pulse. The diameter of the roller may be found in ano- 

 ther way, sufficiently near for practice. The diameter 

 of the wheel is .6 of an inch, or .600 ; then say, as 

 1 IS : 355 : : .600 : 1.885 ; this last being divided by 12, 

 the number of the wheel teeth, gives for the quotient 

 .157, the distance between the teeth. This distance ta- 

 ken as a radius for the roller, would give 60 for the 

 an^'le of impulse. About one-fourth more of this added, 

 will give .800 for the radius, so that the angle may be 

 about 45. 



Nothing should be overlooked, which can contribute 

 to make the balance unlock the wheel with the least 

 possible resistance. When the wheel is locked by 

 the extremity of the teeth, it must be easier unlocked 

 than when the locking is at a less distance from the 

 centre. The unlocking cannot be done easier than 

 with such a wheel for a detached 'ncapement M was 

 contrived about fifteen years ago by Owen Robinson, Owm Ro _ 

 (ate Fig. 4.) a very judicious Wapement maker, who bingo's 

 wrought long with the late If r Arnold. This wheel is 'icapcnienc. 

 like that for Tyrer's scapement The long teeth of a r. Pl AT E 

 rttt rest on the detent, and the upright teeth give im- car in. 

 poise. It is evident that the unlocking with such teeth Fig- * 

 aiast be very easy, when compared with the teeth of 

 those wheels which are made after the ordinary way. 



Lest what has been said concerning the principle of 

 a detached 'acaptmant may not be sufficient, we shall rn- 

 deavour to describe the scapement itself, such as it is 

 at present commonly made, so M to give an idea of it, 

 and of the manner by which it acts. The balance-wheel 

 of a pocket chronometer h~ fifteen teeth not very deep 

 cut, and a little under cut on the face. A notch cut in- 

 to a round piece of steel or roller, which is thicker than 

 the wheel, forms the face of the pallet. .Sometimes a 

 small piece of ruby or sapphire is inserted into the notch 

 at the face of the pallet, for the wheel teeth to act upon, 

 so that no wearing may ensue. The ratio of the cli. 

 ameter of the wheel to that of the roller, is that of .485 

 U hen the wheel and roller are in their places, 

 the wheel supposed to be locked, the roller must turn 

 freely between two teeth, having only freedom, and 

 not much more. From the centre of the roller to 

 the point of one of the teeth, that next the last es- 

 caped, let a line be drawn at a tangent to this tooth. 

 On this line b placed the detent and lifting springs. 

 The detent piece, on which the wheel is locked, is 

 a small bit of fine stone, either ruby or sapphire, 

 set into steel, formed into a delicate spring, of such a 

 length as to be equal to that of the distance of two or 

 three spaces between the teeth, with a sole and steady 

 pin at one end, which must be fixed to the potence plate 

 by a screw. This is what is called the detent tpring, the 

 end of which goes within a little distance of the circle 

 described by the extremity of the lifting pallet. On the 

 left hand side of the detent spring is attached another 



