t 
266 H O R O 
ia the other wheels and pinions, as they are of little or 
no importance, and may be varied considerably.” 
Mr. Arnold has given four portions of his fcapement- 
wheel in the third plate of his defcription delivered to the 
board of longitude; his find figure (hows the wheel locked, 
and the balance returning in a detached Hate from its fe- 
cond excurfion ; the fecond Shows the fituation of the 
three pallets at the inftapt of unlocking; the third, which 
is our fig. 91, (hows the fituation of the different parts 
when the impulfe is half given, the jewelled face of 
the^large pallet being in the line that joins the centres 
of the fcapement-w.heel and pallet; and Mr. Arnold’s 
fourth figure exhibits the parts at the moment when the 
impulfe ceafes to be given by the afting tooth of the 
wheel. The fcapement-wheel of Mr. Arnold's marine or 
box chronometer, fiiown at A, is made of brafs, and has 
12 teeth, with their triangular ends projecting upwards, 
or rather downwards when the face is up, from the plane 
of the wheel; B is the large impelling pallet of Heel, at 
fir ft made circular, but having its notch terminated by 
two ftraight lines pointing to its arbor, in one of which 
its jewel c is fixed; C is the locking-fpring, fcrewed at 
its remote end to the under furface of the upper plate, 
and playing in a notch, or ftraight groove, made in the 
plane of the plate to receive it; its weakeft part is about 
C, or between C and the fixing-icrew e , about which point 
it may be faid to turn as on a centre; but, having no pi¬ 
vot, it requires no oil; about the middle of this fpring, 
C, is a fecond weaker fpring, D, attached to it; the end 
of this (lender fpring, which is called the unlocking-fpring, 
comes nearer towards the centre of the large pallet than 
the fame end of the fpring C. The faid two fprings, thus 
attached together, have a great refemblance, as they are 
feen in the figure, to a metallic pen in a pocket-caie of 
inftruments, when one of the nibs is longer than the 
other; nearly at one third of the fpring C, from this in¬ 
terior end, and on the fide next to the wheel at a, is the 
locking-pallet, the ailing portion of which is a jewel; 
this jewelled pallet refts againft the heel of the tooth, or 
neareft angular point towards the centre of the wheel, 
and in the act of unlocking is driven inwards, to allow 
the projecting portion to move behind it, when the wheel 
is unlocked; but, left the locking-fpring Should yield to 
the prelfure of the wheel when locked at any time, a 
ferew b, tappe<j into a ftud in the upper plate oppofite the 
pallet a, bears againft the exterior fide of the locking- 
fpring, to prevent its falling back beyond a certain- limit. 
In the drawing, the pallet appears to be a continuation of 
the ferew through the fpring, on account of being placed 
over, or very nearly over, it; the centre of motion of the 
unlocking-fpring, D, is near the ferew we have juft de- 
lcribed, from which it is free, by being narrower than the 
locking-fpring C, and consequently alfo weaker, when 
equally thin ; the unlocking-l’pring, D, therefore, is at 
liberty to move back towards the ferew-head without af¬ 
fecting the pofition of the locking-fpring C; but when, 
by, an impulfe received, it moves in a contrary direction, 
it muft neceflarily take the locking-fpring along with it, 
and confequently the pallet a, attached to this locking- 
fpring alfo. The lifting or unlocking pallet, in Mr. Ar¬ 
nold’s construction, is a ftraight piece of Steel, d, carrying 
a jewel, which in one vibration follows the face of the 
large pallet a very little, and in the other precedes it as 
much; if two lines were drawn along the afting faces of 
tiiefe two pallets, which are fixed by friftion on the verge 
of the balance, the angle contained would be very fmall 
in the figures given by Mr. Arnold, in confequence of 
his locking on the fecond tooth; but we have not exaft 
data whereby to calculate it, as will be feen more parti¬ 
cularly in bur account of Mr. Earnlhaw’s chronometer, 
which follows, becaufe We are not informed by Mr. Ar¬ 
nold where the unlocking pallet refts when the regulat- 
ing-fpring of his balance remains quiefeent. Unlike all 
others that we have feen, Mr. Arnold’s locking-fpring 
receives an impulfe inwards, to Strike the locking-pallet 
jfrpm the tooth of its wheel; wc pretend not to alfert, 
LOGY. 
from theory, that this kind of aftion is either more or 
lefs favourable to the l'capement than when the locking- 
pallet is driven outwards -, in either cafe, if the face of the_ 
t’ooth is fuch that the fcapement-wheel has no recoil dur¬ 
ing the disengagement, the total refiftance will be nearly 
fimilar; for, though that part of the-tooth which is neareft 
the arbor of the wheel prelfes on the pallet more than the 
extreme points would do of the fame wheel under the fame 
circumllances, in confequence o' which the,prelTure is 
here greater near the termination of the impulse given to 
the locking-pallet than at its beginning, yet we do not 
conceive this to be any difadvantage, becaufe the lock¬ 
ing-pallet may thus be fuppofed to be unlocked gradu¬ 
ally, rather than by a fudden jerk, which muft be partly 
the cafe when the preflure of the wheel’s tooth againft the 
pallet is ,a maximum at the commencement of the impulfe 
that detaches the locking-pallet. 
We have already faid, that, in our fig. 91, the wheel is 
at the middle point of its arc of aftion, confequently, the 
lifting-pallet d has let go the contiguous end of the un¬ 
locking-fpring D, and the pallet a has returned to the 
ferew-point, to be ready to receive the next following 
tooth 3 ; the direftion of motion being fuch, that the teeth, 
1, 2, 3, &c. follow one another in fucceftion. To put the 
chronometer into true beat, the pallet d muft be in the 
middle of the arc of fcapement, which we fuppofe to be 
nearly in the pofition of our figure, when the balance- 
fpring is quiefeent. Aliy further notice here on the mode 
of afting in this fcapement, we think unnecefiary. 
Fig. 92 exhibits Mr. Arnold’s balance. The circular 
bar of metal a b, carrying three weights of adjuftment 
for pofition, within the expanfion-pieces c, d, we under- 
itand, were added to the original balance by Mr. Arnold, 
jun. but the generality of his chronometers have not 
had fuch addition, being capable of the neceflary adjuft¬ 
ment without. The expanfion-rims, which are about the 
third portion of a circle each, were originally foldered to¬ 
gether by an intermediate mixture, and bent into the re- 
quifite Ihape by a pair of pliers (haped on purpofe to give 
the defired curve regularly; and, in fome of the beft time¬ 
keepers Mr. Arnold informs us himfelf, that he ftill con¬ 
tinues this praftice, which is greatly reprobated by Mr. 
Earnlhaw. We will not undertake to decide the praftical 
question, which, in theory, we Should have lefs difficulty 
to decide ; the regularity of weight and Ihape eniured by 
turning in the lathe, which was at firft the lenior Brock- 
bank’s praftice, and is fince that of Mr. Earnlhaw, who 
formerly worked under him, Seems to promife fairly for 
anlwering the purpofe beft, particularly when two metals 
only are united by fufion ; but, it is contended by Mr. 
Arnold, that the true figure given in the lathe no longer 
remains when the ring is cut into portions, fuch as halves 
or thirds ; for he fays, the feparate parts afl'ume, by their 
elafticity, an inftantaneous alteration in their figure, gene¬ 
rally becoming portions of a circle of fmaller radius than 
that of the original ring, and the feparate portions do not 
al waySrundergo a change exaftly fimilar, owing to circum- 
llances which cannot be eafily detefted. Mr. Pennington, 
who is allowed to be inferior to none of his contempora¬ 
ries in praftical fkill, prefers Mr. Arnold’s method. The 
ferews, d, d, are for rate ; the cylindrical pieces, c, c, tap¬ 
ped for the ferews at the ends of the expanfion pieces, and 
having little holes at their exterior ends for a fork Icrevv- 
driver, are for the adjuftment for temperature and the two 
additional ferews, e, e, are for the adjustments for pofition, 
particularly when the interior ring a b is not introduced; 
thefe laft ferews, e, e, when ufed for pofition, will alter the 
momentum of the balance, and confequently the fate of 
going, if one of the two is not juft as much fcrewed in as 
the other is fcrewed out, when the rate is previously ad¬ 
justed. 
Mr. Arnold ufes a cylindrical fpring with his balance, 
and is very particular in afeertaining the exaft effeftive 
length that-Shall produce the fame rate, whether the arc 
of vibration be long or Short; the trial of this adjuftment 
is made by ufing the main-fpring greatly relaxed, or let 
i down 
