of a new Pendulum. 15 
lum always of the same length, whatever be the heat ; and 
thus a pendulum may be formed, according to what has been 
demonstrated, always of one length. 
On cpnsidering the several different methods of finding a 
measure of lengths which could be always and universally as - 
certained, I am persuaded that the taking the difference of 
the length of two pendulums, vibrating different times, ap- 
pears not only to be the most perfect, but the easiest attain- 
able. Mr. Whitehurst contrived an apparatus for the pur- 
pose of ascertaining this difference, an account of which was 
read in the Royal Society, and afterwards withdrawn and 
published by the author himself. After his death, I purchased 
this apparatus. 
There was no means in it whatever of keeping the pendu- 
lum of the same length when the heat should vary ; conse- 
quently it was impossible that any accurate admeasurement 
of the different lengths of two pendulums keeping different 
times could be ascertained. Mr. Whitehurst, indeed, had en- 
deavoured to keep his pendulum of the same degree of heat ; 
but I know from many experiments, among which some were 
for hatching eggs, how extremely difficult it is to maintain 
the same heat in any considerable mass, and that the means 
which may be employed to keep it within four or five degrees 
are almost totally inapplicable to pendulums ; so that his ex- 
periments must have been defective. I therefore endeavoured 
to contrive a means of rendering the pendulum in his machine 
always of the same length, whatever the heat might be, by 
some addition to it. I thought of the principle, and formed 
the apparatus above described for this purpose. 
It would be improper for me to repeat what has already 
