228 Proceedings of the Royal Society of Victoria. 



solidity, evenness of texture and a good surface. The form is 

 simple, and, with the exception of the cross-heads, are figures of 

 revolution. It has been usual to have the knife-edge on which 

 the pendulum oscillates on the pendulum itself, but it is almost 

 impossible in that case to secure invariability of length, owing to 

 gradual blunting of the knife-edges by wear ; they are therefore 

 made part of the stand, while the planes are on the pendulum 

 cross-head ; by this means, wearing of the knife-edges or sharp- 

 ening them brings about no variation in the lengths of the 

 pendulums. 



Of the pendulums themselves, two are made from phosphor 

 bronze, and one from ordinary gun-metal, and care was taken 

 that the metal is solid and homogeneous throughout. In shape 

 they are all alike, the " bob " or " weight " of the pendulum is in 

 the form of two low truncated cones, base to base in one solid 

 casting 98mms. diameter at base of cones, and 36mm. thick ; the 

 rod is truly cylindrical, 1cm. diameter, the length of pendulum 

 over all 303mm., and from planes to bottom of bobs 235mms. 

 The rod is fixed to the bob by being turned down at one end to a 

 very long cone, which nearly fitted a hole in the bob of a similar 

 conicity ; the rod was then ground into the hole until it fitted 

 nearly up to a small shoulder at the top of the conical part of the 

 rod. The bob was now immersed in boiling water to expand it, 

 when the rod was inserted and driven up to the shoulder; in 

 cooling, the bob and rod became to all intents and purposes 

 solidly connected. The lower end of the rod projected slightly 

 • through the bob, while the bottom of the hole through the bob 

 was slightly countersunk ; the projecting rod was here carefully 

 rivetted, and the bottom of the bob then finished off. 



The suspensions are agate planes attached to metal cross-heads, 

 which are made to fit accurately and symetrically on the rods, 

 great care being taken to secure as perfect rectangularity of the 

 agate planes with the pendulum rod as possible. The suspension 

 cross-heads consist of cubes of gun-metal truly bored to fit on the 

 pendulum rods. The lower part of the cubes are widened out on 

 two sides to give a bearing for the planes as well as to form two 

 cylindrical arms, by which the pendulums are lifted and lowered 

 on to the knife edges of the stand. On the two faces of the cube 

 that are not widened out are fixed two small mirrors of parallel 



