A New Pendulum Apparatus. 229 



glass, silvered at the back. The agate planes are fitted into a 

 separate piece of gun-metal by means of a groove planed out on 

 one face with V's at the sides ; the agates are ground to a 

 lievelled edge on two sides to fit into this groove, and are driven 

 in tight in such a way as to be free from any strain that would 

 •crack or splinter them. The agate planes thus fitted are then 

 ground and polished as one plane. The agates and their matrix 

 of gun-metal fit precisely on planes at the bottoms of the cross- 

 heads, and are secured by four small steel screws. Now, as it is 

 absolutely necessary the agate planes should be accurately at 

 right angles to the pendulum rods, they had to be carefully tested, 

 for no matter how accurate may be the workmanship in fitting 

 the cross-heads, some small errors are sure to remain. To do this 

 I arranged a spectrometer with a piece of metal exactly the size 

 of the pendulum rod, fixed horizontally, on which to place the cross- 

 head and agates, then illuminating the slit of the collimator, read 

 the angle of reflection from one of the planes in both horizontal 

 and vertical direction with the telescope. The cross-head was 

 then reversed 180° till the other plane came under the collimator, 

 and the angles read again ; any difference of angles was got rid 

 of by lightly, scraping the bottom surface of the cross-head on 

 which the agate plate rested. By this means the agate planes 

 were brought practically at right angles with the rods in both 

 directions. Great care was also taken in securing the cross-heads 

 t) the rods. Tfiey fitted sufficiently tight to enable swings to be 

 taken for ascertaining their times of vibration, and when this was 

 satisfactory, a hole was bored through cube and rod and a conical 

 steel pin driven firmly through both. The top of the rod and 

 cross-head were tlien finished off" together, and the pendulums 

 were thus completed. Every part had been well smoothed and 

 Iiighly polished previously to the final fixing of the cross-head. 

 The weights of the pendulums are approximately as follows : — 

 No. I., 1814 grammes; No. II., 1787 grammes; No. III., 1811 

 grammes. Arrangements are made to avoid the necessity of 

 handling or touching the pendulums, except with a leather-lined 

 lifting handle and a leather strap, by which they can be lifted 

 from their chamois-lined couches in the packing case, and placed 

 on the lowering forks of the stan 1 without touching any part 

 with the fingero. To preserve the'r invariability, all touching 



