On the stopping of Astronomical Clocks. Q77 
pump, these were connected by a brass rod, passing 
through a collar of leather, and being quickly turned by a 
cord, heat was readily excited; several trials were made, 
in all of which, except one, when the density of the air 
was diminished, less heat was excited. In that, about 
fourteen fifteenths of the air was exhausted—notwithstand- 
ing this, the heat was greater within the receiver than 
without. In this instance pressure was applied as far as 
could be, and allow the spindle to turn rapidly, the ex- 
periment was afterwards repeated with the same appara- 
tus and gentle pressure, and the heat was much greater 
without the receiver than within. 
Upon considering all the experiments there appear 
reason to believe that air assists in exciting heat by fric- 
tion, and the question arises, is it wholly by compression, 
or is there any other cause ? 
If the experiment should be repeated it would be well 
to ascertain that the spindles excited equal heat——that 
time be allowed for the air to leave the pores of the wood 
—and that the density of the air in the receiver during 
the experiment be noted. 
Respectfully, Yours, 
JAMES T. WATSON. 
New-York, May 8, 1824. 
Art. XVIII.—On the stopping of Astronomical Clocks ; by 
Wm. Howarp, M. D., Professor of Natural Philosophy, 
University of Maryland. 
One cause of the stopping of delicate astronomical clocks 
has been supposed to be the attraction exerted by the weight 
on the bob of the pendulum when the two become oppo- 
site to each other. The very minute quality of this at- 
tractive force renders it difficult to conceive that it can so 
disturb the motion of a heavy pendulum as finally to stop 
it; and the following observations induce me to believe 
that in the cases which have been thus explained, the true 
cause is very different. 
I have a French clock with a very heavy half-second 
compensation pendulum, fitted with Lepaute’s dead beat 
escapement. A weight of one and a half pounds in de- 
