106 J. L. Smith—New form of Compensating Pendulum. 
Art. XIL—New form of Compensating Pendulum; by J. Law- 
RENCE SmiTH, Louisville, Ky. 
In the construction of this new form of compensating pen- 
dulum, I have taken advantage of the very great expansibilit 
of that combination of sulphur and caoutchouc known as Vul- 
canite or ebonite. Its coefficient of expansion is known to ap- 
Nea that of mercury in the ranges of temperature from 0° to 
The mechanical arrangement adopted is a very simple one. 
The rod of the pendulum is of round steel, with an adjusting 
screw at the lower end: a round rod of vulcanite with a hole 
in the center is passed on to the steel rod, fitting it loosely, and 
being held in place by the adjusting screw. ‘The bob of the 
pendulum consists of a heavy mass of brass with a hole 
through the center large enough to admit the vulcanite over 
which it passes, and by a properly arranged stop, rests on the 
end of the vulcanite farthest from the lower end of the pendu- 
lum, so that any expansion of the vulcanite elevates the brass 
bob, thus compensating for the downward expansion of the 
steel rod and brass bob. 
There is a simple mechanical arrangement for adjusting the 
proper difference between the length of the vuleanite and the 
other parts of the pendulum. : 
or a second pendulum to an astronomical clock, I have 
used the following dimensions: diameter of the steel rod 6 
mm. ; diameter of vulcanite, 25 mm.; length of same, 165 mm.; 
diameter of brass bob, 63mm; length of the same, 156 mm. 
These dimensions are in no way insisted on as being the best. 
For a half second pendulum I have used a steel rod 8 mm. 12 
diameter ; vulcanite 11 mm. in diameter and 63 mm. long; brass 
bob 38 mm. in diameter and 57 mm. long. 
_ Ihave had one of these pendulums attached to an astronom- 
ical clock, and after adjustment it has been running four 
months with very satisfactory results. Should this form of 
endulum prove itself constant and correct, it would certainly 
€ a convenient one for transportation, and very much 
costly than the ordinary form. And as for the half-second 
pendulum, in such constant use in mantle clocks, it will be o 
the greatest service and not add more than 20 cents cost to the 
commonest form of pendulum that can be used. 
As regards the uniformity of the coefficient of expansion of 
all vulcanites, of course it is not to be supposed that it can be 
relied upon, but a very simple method is used to ascertain it for 
nid oh gs specimen, or for a number made of the same lot of 
material, : 
