238 Intelligence and Miscellaneous Articles. 



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 centre 

 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 centre large enough 

 to admit the vulcanite, over which it passes, and, by a properly 

 arranged stop, rests on the end of the vulcanite furthest from the 

 lower end of the pendulum ; so that any expansion of the vulcanite 

 elevates the brass bob, thus compensating for the downward ex- 

 pansion of the steel rod and brass bob. 



There is a simple mechanical arrangement for adjusting the 

 proper difference between the length of the vulcanite and the other 

 parts of the pendulum. 



For a second pendulum to an astronomical clock' I have used 

 the following dimensions — diameter of the steel rod 6 millims., 

 diameter of vulcanite 25 millims., length of same 165 millims., 

 diameter of brass bob 63 millims., length of the same 156 millims. 

 These dimensions are in no way insisted on as being the best. For 

 a half-second pendulum I have used a steel rod 3 millims. in dia- 

 meter, vulcanite 11 millims. in diameter and 63 millims. long, brass 

 bob 38 millims. in diameter and 57 millims. long. 



I have had one of these pendulums attached to an astronomical 

 clock ; and, after adjustment, it has been running four months with 

 very satisfactory results. Should this form of pendulum prove 

 itself constant and correct, it would certainly be a convenient one 

 for transportation, and very much less costly than the ordinary 

 form. And as for the half-second pendulum, in such constant use 

 in mantle-clocks, it will be of the greatest service and not add more 

 than twenty 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 any 

 single specimen, or for a number made of the same lot of material. 



I have made experiments on several different specimens ; and the 

 results vary little from each other. The range of temperature with 

 which the experiments were made was from zero to 43° C, on a bar 

 25 millims. in diameter and 304 millims. long, this expanding in 

 length 9-10 millims., making the entire expansion equal to T ^-g- of 

 the entire length of the rod for a temperature ranging from freezing- 

 to boiling-point, giving as coefficient for linear expansion for one 

 degree Centigrade 0*000079365. This coefficient is seen to be 

 lower than that of mercury ; but from the fact that mercury cor- 

 rects the pendulum by only one half its expansion, and the vul- 

 canite is made to correct it by its entire expansion, the length of 

 vulcanite required is even less than the column of mercury used in 

 the mercurial pendulum. This instrument is one whose use depends 

 on its accuracy of operation after careful trial for some time. — 

 Silliman's American Journal, August 1876. 



