H O E O L 
fiimeiifions; but the quantity of adjuflment mult be pre- 
cifely the fame in each of the three fcrews, fuppofing the 
balance previoufly in equilibrio in all politions. On the 
contrary, when there is a conliderable lofs in the rate, the 
three fcrews nrnft be alike fcrewed in a certain quantity, 
depending on the quantity of the daily error, which a 
little practice only will determine ; but when one of the 
three fcrews is fcrewed in for pofition, each of the other 
two mult be fcrewed half the fame quantity out, and vice 
verfa , the taps being the fame in all. The convex fide of 
the rim of the balance is brafs, and the concave fide fteel; 
the fuperior expanfibility of the former, therefore, will occa- 
iion the remote ends of each of the three expanfion-pieces, 
forming the circle, to approach the centre of the balance 
in high temperatures, by reafon of thofe ends being at 
liberty, and the oppofite ends being fixed to the radial 
bars; but in cold weather the contrary will be the cafe ; 
the brafs, being more liable to contraft with cold, as well 
as more liable to elongate with heat, than fteel, exerts a 
natural effort, in low temperatures, to ftraighten the ex- 
panfion-bar, and therefore makes the remote end, in each 
of the three, recede from the centre of the balance; again, 
the fame heat that makes the remote end of each expanfion- 
piece approach the centre of the balance, makes, at the 
fame time, the radial levers elongate, and confequently 
removes the fixed end of the fame expanfion-pieces from 
the centre a certain quantity; and the compenjation conlifts 
in a due balance of the two oppofite and contemporary 
motions of the oppofite ends of each of the three expanfion- 
bars. It is found from experience, that the momentum 
of the balance is belt preferved in an equable Hate by 
loading each expanfion-piece with a metallic weight of 
fimilar dimenfions, and fo contrived that they may Hide 
along the expanfion-pieces, by adjuflment, till the exadt 
point is found fuch in each, that the equipoile of the whole 
is not deltroyed, and yet that the momentum will remain 
the fame in the oppofite extremes of temperature, as well 
as at a medium. In the balance there is alfo a pin fcrewed 
into the remote end of each expanfion-piece, exaftly op¬ 
pofite to as many projefting pieces of metal borne by the 
radial arms; which additions might appear to have 
fomething to do with the compenfation, but they are only 
precautionary contrivances, called guard-pieces, to prevent 
the bending of the expanfion-rims too much inward or 
outward by any jerk or other accidental qiufe. 
Mr. J. Brockbank, fen. having obferved that the coils 
of his l'pring enlarge their diameters, particularly near 
the upper end, when wound up by one entire revolution 
of the balance, ingenioully hit upon the idea of making 
this protrufion of the coils of his fpring fubfervient to the 
purpofe of limiting the extent of the vibration of his ba¬ 
lance, or of conftituting the contrivance called the detached 
banking. In an ordinary watch a pin is put into the rim 
of the balance, which projefts fufficientfy to prevent its 
palling two little ftuds, placed at equal diftances from the 
point of quiefcence of the laid pin, which ftuds, therefore, 
limit the quantity of the whole arc of vibration: this 
quantity, confequently, when two ftuds are ufed, mult 
necelfarily be lets than a circle, even if the common fcape- 
ment would admit of fuch a long vibration; and, if one 
ftud only were placed juft a femic-ircle each way from the 
pin’s quiefeent point, the limit would then be an exaft 
circle, after deducting the thicknefs of the ftud ; but in a 
chronometer one entire vibration generally exceeds a cir¬ 
cle, when the piece is clean ; a circumftance which confti- 
tutes one of its excellencies, by augmenting its momentum ; 
hence the contrivance wanted was to allow the balance to 
vibrate more than an entire circle, and then to Hop it at 
a given part of the fecond revolution: to effeft this pur¬ 
pofe, the late Mr. J. Brockbank invented the mechanilin 
we have mentioned, and which, we underftand, anfwers 
the intended purpofe very well. 
Our notices of this chronometer muft be very brief, as 
we do not know tl at any drawings of the fcapement have 
been publilhed. The order of the adjuftments is this: 
Vol. X. No. 667. 
O G Y. g(5 i 
firft the adjuflment for temperature is made in heat of from 
90 0 to 120 0 of Fahrenheit, and alfo in as great a degree 
of cold as can be obtained ; fecondly, the chronometer is 
cleaned anew, and has frefih oil put in; thirdly the ad juft- 
ment for rate and pofitions is made; and, laftly, the rate 
is taken. There are, befides the three pallet-faces of 
fapphire, eight jewelled holes in the bell chronometers; 
viz. two at the balance-verge pivots in the cock and po- 
tence, two at the pivots of th.e fcapement-wheel, and two 
for each of the two next wheel pivots, called the fourth and 
third wheels of the movement, beginning with the great 
wheel as firft, but the third and fecond of the train, if we 
count from the centre or hour wheel. The pivots are 
tapering, in the form of a cone, and bear on their ends in 
aftion, which fnape gives ftrength, without adding to the 
friflion. 
Some years ago Mr. Edward Troughton contrived a 
jambol or gimbol for preferving the horizontal pofition of 
a pocket-chronometer at fea, which is loaded with a weight, 
turning on the point of a pin, like the card of a compafs, 
and continues to be made by Melfrs. Brockbank and Co. 
with confiderable advantage to the going of the chrono¬ 
meter. The teftimonies, both public and private, of the 
accuracy of fome of Melfrs. Brockbanks’ chronometers in 
meafuring time, are fufficiently numerous to eftablifh their 
credit. Several letters from naval officers, and rates, were 
delivered to the board of longitude during the time of Mr. 
Earnffiaw’s application for remuneration, which cannot be 
copied without an order from the board, otherwife it would 
have been in our power to have laid fome of them before 
the public. Governor Hunter, in his publication, has 
given a teftimony fo extraordinary, that we cannot, in 
jultice, with-hold it ; he has alferted, that, from the time 
of his fetting out from Port Jackfon in New Holland, on a 
twelve months voyage round a great part of the globe, in 
the courfe of which he remained fome time at the Cape of 
Good Hope, to the time of his return to the fame port, a 
time-keeper or chronometer by the Brockbanks was found 
to have gone fo well, that the error, at the end of the 
voyage, did not exceed one fecond of time. This, however, 
will be confidered rather as an extraordinary coincidence 
of the Hate of the watch at the beginning and end of the 
voyage, than as a proof that its accuracy was thus great 
at all the intermediate parts of the voyage. Mr. Gavin 
Lowe, of Ifiington, has a pocket-chronometer, the rate of 
which was given to fir Jofepli Banks to be laid before the 
board of longitude, which, we have heard, exceeds in 
accuracy the rate of any other pocket-chronometer that 
has been made, inafmuch as that the rate in it is not fen- 
fibly aftefted by cleaning in the courfe of many years wear. 
After being apprifed of thefe and other teftimonies, it 
was natural to inquire why the Brockbanks never applied 
to parliament for a public trial of any of their chrono¬ 
meters, with a view of obtaining the premium under the 
aft of Geo. III. but we underftand, that, after having been 
refufed a private trial at the royal oblervatory, and finding 
that much trouble was likely to attend the application to 
the board and lubfequent public trials, the idea at one 
time entertained was given up. 
Wat kinds Time-keeper. —Thirty guineas were voted in the 
year 1804, by the Society of Arts, to Mr. J. Watkins, 
of Giltfpur-ftreet, Weft Smithfield, for an improvement 
in time-keepers; from whom the following communica¬ 
tions were received. The explanations are not very lu¬ 
cid, but we have no other means of letting the reader into 
Mr. Watkins’s ideas. 
“ The good performance of time-keepers for afeertain- 
ing the longitude, (he obferves,) entirely depends on the 
regular and uniform vibrations of the balance, which 
likewife depends on two particular parts of the machine, 
viz. that which is called the fcapement, or the parts im¬ 
mediately connected with the balance to give it motion ; 
and a proper compenfation-part to regulate itfelf in dif¬ 
ferent climates; which two are the only parts now worth 
attention; for, although it muft.be acknowledged, that 
4 X time- 
