HOROLOGY. 
brations of the balance, by means of the 
spiral spring, was, if not newly discovered, 
at least perfected and brought into general 
notice ; and that principle added a great 
value to the detached escapement, while 
this mechanism secured the utility of the 
principle, by offering the species of insulated 
balance which it required. From some 
theories and experiments long known to 
the world, it would appear that the vibra- 
tion of a spring might be always supposed of 
equal duration ; and that advantage Dr. 
Hooke asserted himself to have attained 
with his invention in watches, which had 
been shown to several persons. The prin- 
ciple, however, could not be generally 
trusted, according to Dr. Hooke himself, 
who, in the postscript to his description of 
Helioscopes (p. 29,) declares that he had 
explained how the vibrations might be so 
regulated, as to make their durations either 
all equal, or the greater, slower, or quicker, 
than the less, and that in any proportion as- 
signed. We must suspect that these ideas 
were not properly digested, or regret that 
their communication by the author, in his 
lectures in Gresham College, was not suf- 
ficiently explicit to give precise rules for 
practice, and fix the attention of watch- 
makers upon the subject. After those 
hints, the principle seems to have been 
very little attended to for many years, and 
the isochronism was frequently attempted to 
be effected by means of mechanical con- 
trivances in the escapement. Harrison en- 
deavoured to accomplish that important 
object by the form of the back of the pal- 
lets ; and on the return of the voyage to Ja- 
maica, added for the same purpose the cy- 
cloidal pin, to regulate the balance spring ; 
but this method of adjustment never ap- 
peared satisfactory or certain. P. le Roy, 
in his “ Memoire sur la meilleure Maniere 
dc mesnrer le Temps en Mer,” rewarded in 
1766, first announced distinctly the disco- 
very of a general principle, proper to pro- 
duce the isochronism, by means of the 
balance spring, which is expressed as fol- 
lows : “ that in every spring sufficiently long, 
a certain portion of it will be isochronal, 
whether long or short ; that the length of 
this portion being found, if it be lessened, 
the long vibrations will be quicker than the 
short ones; and that on the contrary, if the 
length be increased, the small arcs will be 
performed in less time than the great arcs.” 
This important property of the spring, 
enabled P. le Roy to bring to a happy issue 
his labours for the improvement of chrono- 
metry ; and the art is indebted to him for 
the practical utility of that discovery, as 
much as for the invention of the detached 
escapement. 
Berthoud found that the spiral spring, In 
order to be isochronal, must have an ascend- 
ing force in arithmetical progression, and 
that this property may be effected, not only 
by the length of the spring, but by the 
number of coils, and the tapering or de- 
creasing thickness from the centre to the 
extremity, &c. He adds, besides the pro- 
portions of the tapering in many springs, 
which he had actually tried, and gives mi- 
nute accounts of the experiments made 
with them in several timekeepers. 
The late Mr. Arnold applied to the 
balance the cylindrical or helical spring, 
which had been employed long before to a 
variety of purposes instead of the spiral, 
which had been constantly used in watches 
since the time of Dr. Hooke and M. 
Huygens. This is one of the articles of his 
patent of 1782, whence it would appear, that 
provided the spring be made of that form, 
the vibrations cannot fail to be isochronal 
but experience is contrary to that notion, 
and artists are obliged to attend to a va- 
riety of circumstances in the application of 
the helical, as well as that of the spiral 
spring. At present, some watchmakers 
think that the helical spring does not pos- 
sess any advantage with regard to that pro- 
perty ; but as the opinion of other persons 
is in the affirmative, while all the manufac- 
turers, as far as our knowledge goes, agree 
in considering the cylindrical form as more 
easily managed than the other, its applica- 
tion seems entitled to the merit of a practi- 
cal improvement. 
Mr. Earnshaw, in the explanation of his 
timekeepers, presented to the board of 
longitude, after noticing the insufficiency of 
the cylindrical spring, states, that he had, 
by long perseverance, found how to make 
springs increasing in thickness to the outer 
end, in order to effect the isochronism of 
the vibrations. This method of obtaining 
isochronal vibrations had been long before 
explained by Berthoud, with rega'rd to the 
spiral spring, in that part of his treatise on 
marine time-pieces which we have already 
quoted. 
This artist states, as a considerable disco- 
very, that the balance spring falls off or 
tires in its strength, and he gives an allow- 
ance for it ; but the fact is neither so con- 
stant nor so equable as to admit of his ge- 
neral remedy. 
