CHRONOMETER. 
general fact it follows that, allowing 24 
hours for the whole rotation of the earth, 
and proportionally for every smaller part 
of tlie rotation, we may determine (pro- 
vided the apparent time at two places be 
known) what is the difference of longitude 
between them. Thus, if a chronometer set 
to the time at Greenwich, were to be car- 
ried to Petersbiirgh, in Russia, it would in- 
dicate time two hours later than tlie clocks 
at Greenwich ; that is to say, it would 
shew when it was noon at Greenwich, in- 
stead of when at Petersburgh. The ob- 
vious conclusion would be, that the sun ar- 
rives at the meridian of Petersburgh ear- 
lier, and consequently that this town lies 
mere easterly than Greenwich ; and as 
2 hours are in proportion to 24 hours, so is 
360°, the earth’s circumference, to 30°, the 
longitude of St. Petersburgh, reckoned 
from Greenwich. Upon the same princi- 
ple it is that the clocks in a large town 
ought not to indicate the same time. Thus 
tlie clacks at St. Paul’s, St. Clement’s, St. 
Martin’s, and St. George’s, Hanover Square, 
in London, ought to strike each four seconds 
after the other j and this difference, it may 
be added, would nearly vanish, if heard 
from any of the westerly stations j on ac- 
count of the time employed for the passage 
of sound ; and for the same reason it would 
be nearly doubled in the opposite direc- 
tion. 
From the intimate relation which sub- 
sists between the construction of watches 
and clocks, the similitude of the escape- 
ments, and the common principles upon 
which the compensations for heat and cold 
are effected in each, we shall explain the 
principles of each under the general article 
Horology ; and at present we shall only 
give ail account of the nature of the expe- 
dients adopted to produce superior accu- 
racy in these portable machines. 
The train of wheels, which constitutes so 
large a part of every time-piece, must ne- 
cessarily transmit the force of the first 
mover with periodical irregularities arising 
from oblique actions of their teeth upon 
each other ; and these irregularities will be 
subject to Other variations arising from the 
greater or less degree of fluidity in the oil 
applied to the pivots and elsewhere. The 
first mover also in a portable machine being 
a spring will be more rigid and conse- 
quently act with greater power when cold 
than when hot. ‘ The balance, or vibrating 
measurer of the time, is a wheel, or equiva- 
lent piece, fixed on an axis, upon which it 
could freely turnj but this liberty is re- 
strained by a fine spring called the pendu- 
lum spring, which is fastened to the axis, 
and after taking several turns round with- 
out touching it, the other end of the spring 
is fixed to the frame of the machine. By 
this contrivance the balance will, if not 
prevented, come to rest in one particular 
position 5 and if, at any time disturbed, it 
will only vibrate each way from the line of 
quiescence, performing larger or smaller 
arcs according to the disturbing force. This 
force in a watch or time-keeper is commu- 
nicated from the train ; most commonly 
during the time of each vibration : and the 
machinery or contrivance by or through 
which the successive impulses, or actions, 
arc made, is called a scapement or Escape- 
ment, several of which are described at 
the articles referred to. 
According to the nature of the escape- 
ment and tlie part of the vibrating arc at 
which the impulses are applied, the vibra- 
tions of the balance may be made to em- 
ploy a longer or shorter time than they 
would have employed if the balance had 
been separate from the works. Thus, in 
the common watch, tliese impulses quicken 
the vibrations ; and consequently an in- 
crease in the maintaining force will make 
tlie watch go faster ; as may be easily tried 
by gently forcing the key in the opposite 
direction to winding up. 
If the balance and its spring were to 
continue unchanged in all temperatures and 
under all circumstances, and if its long and 
short vibrations measured equal times 
when separate from the machine, it would 
only be required that the escapement 
should be so constructed as neither to acce- 
lerate nor retard them. But none of these 
conditions can be had in the ordinary struc- 
ture of watches, and in the superior time- 
pieces considerable difficulties are found in 
the attempt to obtain tliem. 
By the natural conti'action, to which all 
bodies are subject when cooled, the diame- 
ter of the balance will be less the lower 
the temperature ; it will therefore be more 
easily carried by the vibrating forces, and 
will then vibrate more quickly. 
The spring attached to tlie balance, which 
is called the pendulum spring, will like- 
wise act with greater force when cold, and 
on this account also the vibrations will be 
quicker. 
The remedies for these causes of imper- 
fection are the following : 
1. The Remontoire. As the irregularities 
