38 SABINE’S EXPERIMENTS. 
ledge consists the greatest practical difficulty with which this method of ex- 
periment has to contend, and the source from whence error is most to be 
apprehended ; the mass of brass of which the pendulum consists, does not 
or may not conform to changes of temperature so rapidly as the thermome- 
ter suspended as an index by its side; and when it is considered that, agree- 
ably to the expansion of brass, a quarter of a degree of Fahrenheit’s scale 
is equivalent to about one tenth of a vibration in 24 hours, or to rotor th of 
an inch in the Jength of the pendulum, which is a very appreciable quantity, 
the necessity of extreme precaution will be evident ; and especially if redu- 
cing the variation in the general temperature of the apartment in the 24 
hours within as small limits as possible, by suspending screens of baize and 
matting before the windows, and by thus keeping the room constantly dark, 
except when entered for the purpose of observation, or of comparing the 
clock, (which were allowed to occupy no more time than they absolutely re- 
quired.) The variation from the mean temperature in the 24 hours rarely 
equalled a degree and a half, although the range of the mean temperature 
itself exceeded that amount in the eleven days through which the observa- 
tions of coincidences were carried. 
The rate of the clock, or, in other words, the number of its vibrations in 
each 24 hours, from the commencement to the close of the series of coinci- 
dences, is the foundation of the corresponding rate of the detached pendu- 
lum : not finding readily a suitable situation for a transit instrument, | de- 
pended principally for this determination upon zenith distances of the sun 
and stars out of the meridian, observed with a repeating circle of six inches 
diameter, in the sufficient accuracy of which I had confidence from expe- 
rience : the cupola afforded a convenient place for the use of the circle, having 
windows opening in four directions, the sills of which were enlarged for its 
support ; the adjustments were carefully ascertained, although, as will be 
seen, the results are arranged in such manner as to render them finally in- 
dependent of the principal sources of inaccuracy, arising from a want of 
adjustment: the times of observation were noted by a chronometer of 
Messrs. Parkinson and Frodsham, No. 423, in the steady and uniform going 
of which I had also had much experience, and which was compared periodi- 
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