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acts by lengthening or shortening the spring 
wiien cold or heat may have given it more 
or less of force. This was invented by 
Harrison, and depends on the well-known 
fact that a short spring is stitFcr than a 
longer ; so that by shortening his spring at 
the time when it was weakened by heat and 
the balance enlarged by the same cause, he 
gave it the stiffness requisite to compensate 
for these alterations ; and the same con- 
trivance produced the contrary effect in 
cold temperatures. As we shall more fully 
exhibit these inventions under the article 
Horology , it is only necessary to observe 
that Peter Leroy constructed his first time 
piece with fluid thermometers on the ba- 
lance, and that he also invented our pre- 
sent expansion balance of brass and steel, 
soldered or fused together in the rim, which 
was afterwards introduced and brought to 
great perfection by Arnold. 
Machines, made upon the principles here 
cursorily pointed out, have measured 
time to a wmnderful degree of perfection ; 
and from the immense maritime trade of 
the British empire, and the scientific dispo- 
sition of many wealthy individuals, the de- 
mand has been so great as to have produced 
a very great number of able workmen 
fully equal to their constniction, at the 
same time that the prices have been consi- 
derably reduced. Most sea commanders of 
any respectability are provided with two or 
more of them. 
Among the other causes of irregularity in 
time measurers, the resistance of the air 
has been occasionally considered by au- 
thors. But artists seem to suppose, either 
that it is a constant quantity, or that its 
variations are not considerable enough to 
be brought into the account. The very 
accurate performance of some chronome- 
ters, and the steady going of astronomical 
clocks seem to give weight to this suppo- 
sition ; but on the other hand it may be re- 
marked that though the slow motion of 
heavy pendulums vibrating through small 
arcs in astronomical clocks, must be subject 
to very little resistance indeed from the air, 
yet it does not follow that the rapid vibra- 
tions of a balance may not be affected by 
this cause ; and the extreme precision of 
some chronometers will not, perhaps, be ad- 
mitted as a very strong argument, when we 
consider that the changes from barometrical 
causes may have compensated each other, 
and that the most perfect machines will vary 
as much as one second per day, from causes 
which have not been yet clearly detected, 
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though these are probably resolvable info 
that before ns. We are more particularly 
led to these reflections by a communication 
fi om Mr. Manton, of Davies-street, who 
found by expei'iment that a chronometer, 
which w'as going upon a gaining rate of 
five seconds per day, did increase its arc of 
vibration by an additional 50 degrees im- 
mediately upon the air being exhausted, and 
that being kept in vacuo, its rate became 37 
seconds per day, the gain being 34 seconds 
upon the former rate. Hence it follows, 
that as the difference between the highest 
and the lowest stations of the barometer 
indicate a change of about one-fourteenth 
part in the density of the air, the corres- 
pondent change per day, in the rate, may 
be two seconds and a half, or about one 
second per inch. Hence it may happen 
that a capital time-keeper shall indicate a 
more steady rate from week to week than 
from day to day. 
The causes of imperfection in chrono- 
meters, which still call for farther exertions 
of sagacity in our artists, are 1. The spring 
gradually tires or falls off from its strength, 
and neither the law of this variation nor its 
remedy are known. The effects of this 
change are, that all the adjustments are dis- 
turbed by it. 2. There is great reason to 
apprehend that the expansion-bars of brass 
and steel do change in their relative powers 
of flexture by their continued action on 
each other, though it is probable they settle 
at last. 3. The wear of tlie acting parts is 
uncertain, and will affect the time of strik- 
ing out the detent and the arc of impulse. 
4. No certain rules have been given, or 
are perhaps known, for making all the 
vibrations equal in time. If w'e suppose the 
long and short vibrations to be at first ad- 
justable, with certainty, to equal times, not 
only for the extremes but for all the means 
or intermediate arcs, it will not follow that 
the falling off from wear or from tiring, 
or from change in the balance, will continue 
to be accompanied by the same isochro- 
nism. 5. The best artists find very great 
difficulty in adjusting a pocket chronometer 
for all positions, preserving at the same 
time the other needful adjustments. See 
Escapement, Horology, Pendulum, 
Train, and the articles thence refer- 
red. 
CHRYSALIS, in natural history, a state 
of rest and seeming insensibility which but- 
terflies, moths, and several other kinds of 
insects, must pass through before they ar- 
rive at their winged op most perfect state. 
