HOROLOGY. 
steel in the compound, or gridiron pendu- 
lum, which has been almost universally 
adopted ever since. This contrivance could 
not be easily applied to balances ; but Har- 
rison, following still the principle of the 
different expansibility of metals, applied it 
in a manner which had not been thought of 
before, and made it act on the spiral spring, 
in order to produce the desired compensa- 
tion in the regulator. This method is 
described as follows : (Principles of Mr. 
Harrison's Time-keeper, p. xii., notes.) “ The 
thermometer kirb is composed of two thin 
plates of brass and steel rivet ted together in 
several places, which, by the greater ex- 
pansion of brass than steel by heat, and 
contraction by cold, becomes convex on 
the brass side in hot weather, and convex on 
the steel side in cold weather ; whence, one 
end being fixed, the other end obtains a 
motion corresponding with the changes of 
heat and cold, and the two pins at the end, 
between which the balance spring passes, 
and which it touches alternately as the 
spring bends and unbends itself, will shorten 
or lengthen the spring, as the change of 
heat and cold would otherwise require to be 
done by the hand in the manner used for 
regulating a common watch.” 
This kind of compensation has been since 
applied in other ways ; but the method, in 
genera!,, is liable to some material objec- 
tions, on account of its altering the length 
of the balance spring, and the difficulty, 
perhaps impossibility, of effecting with it an 
accurate adjustment. Pierre le Roy, eldest 
son and successor to Julienle Roy, the com- 
panion and friend of H. Sully, had the me- 
rit of accomplishing the great desideratum 
of making the compensation on the balance 
itself. In the chronometer, which was pre- 
sented to the King of France the 5th August, 
1766, and obtained the prize of the Acade- 
my of Sciences of Paris the 31st of the same 
month, that author executed a compensation 
in the balance, which he has fully explained 
in his description of that machine. (“ M6- 
moire sur la meilleure Maniere de mesurer 
le Temps en Mer, qui a remportc le Prix 
double au Jugement de 1’ Academie Royale 
des Sciences. Contenant la Description de 
la Montre a Longitudes, presentee a sa 
Majesty le 5 Aout, 1766.” Par M. le Roy, 
Horloger du Roi. pp. 41 to 44. This Me- 
moir accompanies the account of Cassini’s 
voyage in 1768, published in 1770.) This 
compensation is composed (fig, 4.) of two 
thermometers, t K t A K, of mercury and 
spirits of wine, made each in the form 
of a parallelogram, except in the upper 
branch, which bears the ball containing the 
spirits of wine, and is a little bent down- 
wards ; the mercury is in the lower part, and 
the vertical branch of the tube, A K, is open 
at the upper end. These two thermome- 
ters are placed opposite one another, the 
axis of the balance being in the same plane 
with the centrical lines of the tubes ; and 
the thermometers and balance are solidly 
attached together, and form a well poised 
and steady regulator. At the middle tem- 
perature of the atmosphere, the quicksilver 
stands at K A 1 1\ ; but, when an increase of 
heat occurs, the alcohol, by its expansion, 
forces the mercury from the exterior branch, 
t K t, towards A K, and a portion of the 
mass of the regulator contracts by approach- 
ing the centre. On the contrary, if the va- 
riation consist of an additional degree of 
cold, the mercury moves towards the exte- 
rior branch, and the weight towards the 
circumference of the balance becomes 
greater. Thus, if the thermometers are well 
adjusted, the effects of all the changes of 
temper ature in the balance will be compen- 
sated, and the regulator will act with the 
same uniformity as if its figure were not 
liable to such alterations. 
Peter le Roy mentions his knowledge of 
Harrison’s expansion curb acquired soon after 
he made his thermometrical balance ; and 
he constructed a balance accordingly on the 
principle of Harrison’s curb ; the arms or 
arcs of which act by flexure, and are ad- 
justed by moveable weights. He proved 
the effect of such arms by experiment; but 
gave the preference to his own mercurial 
compensation. Peter le Roy’s second inven- 
tion is the same as is now used : but though 
so publicly declared in the face of the French 
government and academy in 1766, Arnold 
took an English patent for it in 1782. 
We have not yet taken any notice of 
improvements made in the escapement; be- 
cause after all the plans proposed for this 
most essential part of chronometers, the 
principle of what is called the detached es- 
capement is the only one now used ; and, 
being established upon long experience, 
seems to merit the preference given to it 
over all the constructions proposed tili now. 
We shall content ourselves with stating in a 
general manner the beginning and progress 
of that escapement. 
In all the escapements known till the 
middle of the last century, the escape wheel 
was in continual contact with the jfallets be- 
longing to the axis of the balance wheel; 
