360 H O II O 
for the relaxation of the balance-fpring.'’ See Mr. Earn- 
fhaw’s Explanation, pages 8 and 9. 
Thefe fprings that have the invifible. properties de- 
fcribed “ are made of loft Heel, rolled hard, and not har¬ 
dened and tempered with heat and cold, that procefs not 
being at all necellary." Their length varies in box-chro¬ 
nometers from 12 to 20 inches, and in pocket ones from 
5 to 7, like Mr. Arnold’s, and the lhape is cylindrical, 
with the two extreme coils each about half the diameter 
of the other coils, agreeably to the fpecification in one of 
Mr. Arnold’s patents. The thickneis of the Heel in the 
expanfion-pieces of the balance is about •p^oo' °f an inch, 
and that of the brafs twice, or nearly three times, as 
thick; the diameter of the balance without the weights 
and fcrews i±, and with them if, of an inch; the total 
weight of it, with weights and fcrews, jdwts. iogr. The 
main-fpring barrel is i^- inch, and the depth | inch, and 
the fpring in it has from 44 to 5 turns. The weight of 
each oPthe two balance-icrews is from 5 to 6 grains; and 
the figure which the expanlion-pieces aflume when cut 
from the original compound ring is fuffered to remain un¬ 
altered, on a fuppofition that bending by any mechanical 
means would injure the regularity of their obedience to 
the changes of temperature ; the locking and unlocking 
fprings are made as weak at the bending parts as is 
practicable to afford the necelfary reliffance, as a detent, 
to the fcapement-wheel; they are brought to lhape by 
filing, then Imoothed with a piece of fteel and oil-ftone 
powder, and laftly hardened and tempered. The pallets, 
which are of fteel, have each a jewel let in them, and thofe 
on the verge of the balance are twilled round and fet to 
■their fituations refpeCtively by the friction of their cen¬ 
tral holes. 
Mr. Earnlhaw has particularized fix advantages which 
his fcapement has over Mr. Arnold’s, in p. 12 of his Ex¬ 
planations; but, as the reader will probably think we 
have detained him too long upon this one chronometer, 
we refer him to the original information, in reading which, 
he will now be able to form an opinion for himl'elf as to 
the real exiftence of thofe advantages; indeed, when all 
reafoning on the fubjeCt is at an end, that particular con- 
ltrudtiou of a chronometer will no doubt ultimately pre¬ 
vail in practice, which lhall be found to perform the belt, 
and at the fmallelt expence. 
Brockbank's Chronometer .—The Meffrs. Brockbank, feh. 
and jun. have produced chronometers, or time-keepers, 
which have performed with a degree of accuracy equal to 
that of any other maker, though they never applied for 
any parliamentary remuneration; the original firm of the 
houfe was John and Miles Brockbank, the former of whom 
died, and the latter quitted bufinefs, and is fucceeded by 
his nephews, John and William Brockbank. 
We need fav but little of fuch parts as are common in 
an ordinary watch, which may be feen more particularly 
in Plate XII. The fpring-barrel contains a well-tempered 
main-fpring, ftronger than the fpring of an ordinary watch, 
becaufe the fcapement is a detached one, requiring the 
momentary impulfe given to it to be pretty ftrong; there 
is nothing particular in the conftruCtion of this barrel, 
except its fize, which is proportioned to the fpring it con¬ 
tains ; one of which, for a large box-chronometer, has 
been found equal to fupport a weight of nibs, or upwards ; 
the chain is alfo of the ufual conftrudtion, except that 
it is ftronger than is necelfary in a common watch. The 
fufee is grooved in fuch a way, after being made of the 
lhape of the fruftum of a paraboloid, that the decreafe of 
the acting radius is always inverlely proportional to the 
intenfity of the main-fpring; by which admirable contri¬ 
vance the effective power of this fpring is at all times very 
nearly alike; the adjuftment of the varying levers, or points 
of aftion, of the fufee, is made very conveniently, by a 
long lever with a moveable weight, like a fteelyard, being 
inferted on the fquare of the fufee-arbor made for the key. 
The number of turns of the fufee depends on the number 
of hours it is intended to be actuated by the fpring at one 
LOGY. 
winding up, and this number again depends on the ratio 
between the great wheel and the centre-pinion, as has been 
mentioned in our defcriptions of Harrifon’s and Mudge’s 
time-keepers. The angular point of the cap, at thef'maller' 
end of the fufee, is to eatch the Ihoulder of the guard, 
when the chain has filled all the turns of the fpiral groove; 
otherwife the chain would wind back again a little way, 
and the power of the main-fpring would become too great, 
or perhaps the fpring might even break by being over- 
ftrained. Concentric with the fufee at the large end, con¬ 
tiguous to the pillar-plate, are the great wheel, two 
ratchets, and a fecondary fpring to keep the chronometer 
going while it is wound up; this idea, and alfo the me- 
.chanifm proper for effedting fuch purpofe, originated, as 
we have feen, with lames Harrifon; but the prefent ap¬ 
plication of his principle is much more limple than his 
was, inafmuch as the fpring is fo conftrudted as not to 
require a barrel or box to contain it. 
In the pocket-chronometers, the great wheel, which is 
moveable round the fufee-arbor in one direction only, like 
that of a common watch, has 60 teeth: the centre-wheel 
arbor carries a pinion of 12 leaves, revolving in an hour; 
this arbor is ftronger than any other arbor in the train 
above, not only becaufe more power is imprefled on its 
pinion than on any other pinion higher in the train, but alfo 
becaufe the minute-hand is borne by it, and alfo motion 
given to the hour-hand from it; the centre wheel fixed 
on this arbor has 64. teeth, which impel the pinion of eight 
leaves on the fecond arbor; the fecond w'heel of 60 teeth 
is fall alfo to this arbor, and impels the third pinion of 
eight leaves faff: on the third arbor, which is the arbor for 
the feconds-hand,. which hand moves in a circle of 60 
out of the centre of the face, therefore cannot be miftaken. 
for the minute-hand, which is a matter of fome import¬ 
ance in a chronometer. On this arbor of the feconds- 
hand, a contrate wheel is ufually placed in a common 
watch, for the foie purpofe of altering the direction of 
motion, that the balance or crown wheel may have an 
horizontal arbor; but here the cafe is different; the ba¬ 
lance-wheel, or more properly fpeaking the fcapement- 
wheel, has a vertical arbor like the reft of the train; the 
third wheel therefore is here like the reft in pofition, and 
has 80 teeth driving its pinion of eight on the fourth arbor, 
which alfo carries the fcapement-wheel with 15 teeth. From 
this account of the movement, we now know that the fufee 
revolves in £§ of an hour, or in five hours, and alfo that, 
as there are turns in it, the whole period of continued, 
motion, at one winding up, will be 32 hours; alfo by our 
former method ot eftimating the value of a train exem¬ 
plified in Harrifon’s and Mudge’s trains ; we know that 
64 X 60 X So X 15X2 921600 „ . 
.X.—-—--— J - — --— 18000 are the num- 
8X8X8 ~ 512 
ber of vibrations in an hour, or five in a fecond; but, as 
there are only half as many audible beats as vibrations in this 
detached fcapement in a fecond, this circumltance would 
induce any perfon, not previoully acquainted with it, to 
conclude that the train is a lower or flower one than is 
ufed in an ordinary watch, though the reverie, in point 
of fa 61 , is the truth. In feme of the heft box-chronome¬ 
ters, the pinions are 10, and the train = 14400, owing to 
the large fize of the balance. 
The balance is a circle at firft turned in a lathe out of 
folid compound plate of fteel and brafs, a circular plate of 
fteel of the required diameter being covered with brafs by 
immerfion in a crucible of this metal, melted previoully 
by heat, after which it is divided into three equal por¬ 
tions, which will, by thefe operations, be of fimilar dimen- 
fions and weight when nicely divided; this method of 
uniting the metals, and of forming the balance in a lathe, 
was the invention of Mr. J. Brockbank, fen. At the outer 
end of each of the three radial bars, which are of fteel, 
are three fcrews of adjuftment for time, and alfo for poli- 
tion. When the chronometer gains confiderably, each of 
thefe three fcrews is fcrew’ed back or outwards, to increafe 
the momentum of the balance by enlarging its effedlive 
• dimenfions; 
