548 11 O R O 
makers of that nation ; and, to do them juftice, we muft 
allow that Peter, the eldeft fon of Julien le Roy, who was 
himfelf an eminent watch-maker, had the honour to be 
the firft who accomplifhed the fuggefted defideratum, by 
means of two thermometers, one of mercury, and the 
other of alcohol, attached to and carried by the balance it- 
felf, which contrivance effected the compenlation, by 
bringing a portion of the mercury nearer to, or by re¬ 
moving it farther from, the centre of the balance, accord¬ 
ing to the different ftates of the atmofphere. A chrono¬ 
meter on this conltruition was prefented by Peter le Roy 
to. the king of France, on the 5th of Auguft, 1766, for 
which the prize of the Academy of Sciences was awarded 
4 rim on the laft day of the fame month; he alfo publifned 
an account to accompany the piece, entitled, Memoire 
fur la meiileure maniere de mefurer le temps en mer, See. 
in which memoir he afferts, a circumftance veiy worthy 
of notice, that he made another compenfation-baiance en¬ 
tirely of pieces of different metals; viz. of brafs and fteel 
riveted together, like Harrifon’s compenf:;tion-curb, but 
bent into two feparate femicircles in fuch a way, that 
each, carrying a metallic weight near its extremity, 
brought it alternately nearer to, or removed it farther 
from, the centre of the balance thus formed, agreeably to 
the variations of the atmofpheric temperature; and, 
though the inventor preferred at the time the thermome- 
trical compsnfation, yet the metallic one., fpoken of ki 
the memoir in queition, was, no doubt, the archetype of 
all our prefent compenfation-balances. 
It might now have been expected that a time-piece, 
with a movement aided by friftion-rollers, or by jewels 
in the pivot-holes, and with a compenfation-baiance re¬ 
gulated by a fpiral l'pring, would have performed alike 
under all circumftances ; but ftill it was found that, how¬ 
ever well the fufee was fhaped, and adjuiled to the dif¬ 
ferent intenfities of the main-fpring, yet fuch an unequal 
tranfmiffion of that power took place, even in the belt 
movements, in confequence of there being alternately fa¬ 
vourable and unfavourable pofitions of the afting teeth 
of the wheels and pinions, and in confequence of the im¬ 
pediments to free motion occafioned by the variable den- 
lity of the oil uied, and by the acceffion of particles of 
extraneous matter, that the aftion of the pallet-wheel 
upon the pallets then in ufe was found fufficiently irre¬ 
gular to occafion an inequality in the impulfes given to 
the balance, and a confequent inequality in the magni¬ 
tude of the arcs of vibration. This inequality in the 
magnitude of the arcs of vibration would not, indeed, 
have aftefted the rate of the going of the chronometer, if 
all lengths of the regulating-fpring had been found oil 
.trial to be equally ifochronal; but the fame Peter le Roy 
difeovered what Dr. Hooke knew long before, (fee his 
Poftcript to a Defcription of Heliofcopes, See.) that there 
is a certain length in each good uniform l'pring which only 
is ifochronal, or, in other words, which has the property 
of regulating the balance fo, that all the arcs of vibration, 
long or fhort, lhall be performed in the fame time. 
This difcovery, or rather re-difeovery, of Peter le Roy, 
■ at a time when horometry had made confiderable ad¬ 
vances towards perfection, was calculated to do away the 
fanguine hopes that had been entertained of the good per¬ 
formance of time-pieces on Harrifon’s conftruftion, in 
which the effective length of the regulating-fpring was 
, conftantly altering with the variations of temperature; 
and to this circumftance principally may be imputed, per¬ 
haps, the ftnall number of time-pieces that were manu¬ 
factured after Harrifon’s model, notwithftanding the large 
premium which was awarded him. A remedy, for the 
unequal tranfmiffion of the maintaining-power had been, 
however, adopted by Ilarrifon, when he introduced the 
remontoir to produce equable action at the contrate- 
wheel of his pieces, a contrivance worthy of his genius, 
whether.the idea was original, or borrowed by him from 
Huygens ; bur this was a remedy for only one of the two 
fiauies of irregularity in the momentum of his balance ; 
LOGY. 
it might, and probably did, equalize the maintaining- 
power nearly, but would not counteract the ifochronal 
defeCt produced in the regulating-fpring by the compen- 
fation-curb, in all the various arcs of vibration which 
every piece is liable to experience in their different ftates 
of foulnefs. 
The obftacles to an equal tranfmiffion of force in thefe 
inftruments, led to the invention of various fcapements, 
both on the continent andin England ; fome of which 
were intended to act ifochronally in concert with the re- 
guiating-fpring, and others were fo conftruCted as to give 
the impulfe almoft inih ntaneoully, and at the molt fa¬ 
vourable inftant of the vibration of the balance, fo that 
the force derived from the maintaining-power, to perpe¬ 
tuate the vibrations., might derange the natural ifochro¬ 
nal property of the balance, and its regulating fpring, as 
little as poilible. 
On confidering this fubject, it occurred to the moft 
fcientitic artifts; that the regulating-power of the balance 
and balaiice-l'pring, which was'found to be too much un¬ 
der the dominion of the maintaining-power with the com¬ 
mon fcapements, would be the lea ft deranged if the im¬ 
pulfe derived from the maintaining-power were momen¬ 
tary, particularly if it were applied at that point of the 
vibration where the momentum of the balance is a maxi¬ 
mum ; it alfo occurred, that the momentum of the balance 
itfelf ought to be as great as practicable, compared with 
the impulfe given to the pallet-wheel, and likewife that a 
momentum compofed more of velocity than of weight 
would be moft favourable for a balance with {lender pi¬ 
vots. Thefe, and fimilar confiderations, the refult of 
much thought and reafoning, fuggefted a great variety of 
defigns for new fcapements, many of which have been 
brought into practice with an advantage corrclponding to 
the importance 'of the object. Thefe fcapements have 
obtained the appellation o i f ree or detached, from the cir¬ 
cumftance of their being detached from the balance dur¬ 
ing the greateft part of its vibration. It has been matter 
of contention among horologidal writers, who was really 
the firft inventor of a detached fenpement; but it feems 
now to be prettv generally admitted, that Peter le Roy, 
whom we have mentioned as the inventor of the firft com¬ 
penfation-baiance, was alfo the inventor of the firft de¬ 
tached Tcapement. 
A method having been devifed of limiting the quantity 
of impulfe given to the balance, fo as to be juft fufficient 
to keep it vibrating when put in motion, but not luffici- 
ent to produce motion from a ftate of quiefcence, as will 
be feen hereafter, and a felf-compenfating mechanifm 
having been'adapted to the balance itfelf, it may now be 
fairly inferred, that time-keepers have arrived nearly at 
their ne plus ultra of perfection. We final 1 therefore con¬ 
clude our narrative of the improvements in horometry, 
by pointing out briefly fome of the diftinguifhing features 
of the different conftructions of cur contemporaries. With¬ 
out entering into any detail of the French time-meafurers, 
it may be fufficient to ftate generally, that their detached 
fcapements have detents, or pieces to fufpend the main¬ 
taining-power for a certain time, moveable on an arbor 
with pivots; whereas the Englifh detents aft by means of 
fprings without pivots, which confequently require no 
oil, and are alfo of a more fimple conftruftion. 
The late Mr. Mudge laboured to eifeft a:i equalization 
in the impulfe given to the balance by remontoirs or fpi¬ 
ral fprings, afting fo conftantiy at'each vibration, that 
the fcapement of his time-keepers could not properly be 
called a detached one ; though the one introduced into 
the queen’s watch, as deferibed p. 345, which gives an 
impulfe, at every vibration, fufficiently great to produce 
motion from a ftate of reft, may be clafled among the de¬ 
tached kind. 
The late Mr. Arnold took out two patents for his im¬ 
provements in time-keepers, or chronometers, (for by this 
name he firft called them,), the One in April 1776, and the 
Other in 1782. The former of thefe patents was for the 
1 invention 
