362 Mr. C. Vernon Boys [June 8, 



telescope, is correct. The large scale S is 9 feet long, and is divided 

 into fiftieths of an inch. There are 4800 divisions. 



Two beams Z x / 2 are seen in Fig. 2. The upper surfaces of these 

 are straight, and are adjusted by screws until they arc truly level. 

 These are used when the true optical distance from the mirror to the 

 scale is being measured. A steel tape, on which I engraved a fine 

 line near each end, rests upon the beams. At one end a slider carry- 

 ing a microscope is placed so as to see a fine line at the centre of the 

 mirror accurately in focus, while at the other a corresponding slider 

 is placed so that a projecting brass rod rests against the scale. At 

 the same time cross lines engraved upon the plate-glass bases are 

 placed exactly over the lines engraved on the steel tape. When 

 afterwards the microscope is focussed upon the end of the brass rod, 

 the distance between the cross lines, as measured by a scale, is the 

 amount that has to be added to the distance between the engraved 

 lines upon the tape, in order to obtain the distance from the scale to 

 the mirror. 



Overhead wheels are shown in Fig. 2, fastened to the roof above 

 the apparatus, and again close to the end wall. These serve many 

 purposes, as will appear later. Among others, the middle one of each 

 carries a cord fastened at one end to a crossbar joined at its ends by 

 guys to the pillars R of the lid, Fig. 1, and at the other to heavy 

 balance weights to counterbalance the balls M M and part of the lid. 

 Thus the friction is greatly reduced, and the tremor set up by rotating 

 the lid is in a corresponding degree slight. 



All time observations are made chronographically upon a drum 

 by the Cambridge Scientific Instrument Company. This is placed in 

 the adjoining vault. Two time-markers record with their points less 

 than 1/100 inch apart, one of them marking every second of the clock 

 with special marks for minutes and half-minutes, and the other every 

 depression of the key at my right hand. The late Prof. Pritchard 

 kindly lent me an astronomical clock for the purpose, to which I 

 fitted time-marking contacts ; but into the details of these I must 

 not enter. He also allowed me to make use of one of his assistants 

 to keep me informed of the rate of the clock from time to time. 



I have up to the present spoken vaguely of the large lead balls 

 and of the small gold balls, but have given no indication as to how 

 they are made and how I can be sure of the truth of their form and 

 their homogeneity. Mr. Munro, whose capacity for turning accurate 

 spherical work is well known, made for me two moulds of hard cast 

 iron, which I have on the table. One of these is for a 4^-inch lead 

 ball, and one for a 2^-inch lead ball. Each mould is made in two 

 halves, so truly as to shape and size that the thin steel disc that was 

 used as a template would distinctly rattle when in its place, but when 

 a strip of cigarette-paper was inserted on one side it could not be got 

 in at all. The upper half of each of these moulds is provided with a 

 cylindrical steel plunger accurately fitting a central hole in the mould, 

 and with its encl turned to the same spherical surface when it is 



