38 PROFESSOR C. V. BOYS ON THE 
importance, and the time required for carrying them out differs also very much. 
Some few need only to be performed on the first putting together of the apparatus. 
Operation 1. 
After placing the instrument in the selected spot, with its centre tube vertically 
below the edge of the central overhead pulley, it is to be levelled accurately by 
placing a spirit level on the lid, and adjusting the Jevelling screws, until the bubble 
occupies the same position in the tube when the lid, carrying with it the level, 
is slowly made to revolve. Fix also the scale and large telescope in position. 
Operation 2. 
Hang up the lead balls by their wires and upper supporting pieces, pinning the latter 
to the thickened lower ends of the two steel bands. These are carried over the 
flat-rimmed pullies, and carry at their other ends counter-weights, so that the balls will 
remain suspended at any level. If the central tube is not in position the three readings, 
a, b, and ¢, fig. A, are made with the cathetometer, and thus the true distance from 
a to the centre of the lead ball, when the wire is stretched by the weight of the latter, 
is known. If, after one experiment, it is desired to make this measure, but not to 
move the central tube, since it is impossible to remove the lead balls completely, they 
Fig. A. 

must be left suspended at such a height that the 6 of each is visible, This can be 
accomplished without taking away the lid and lid pillars, since they can be left 
suspended by their counterweight at such a height that the a cf the (ultimately) 
upper ball is below the lid, while the @ of the (ultimately) lower ball is above its lid 
pillar. Since all hang freely without touching one another, the a’s and 6’s can 
be determined, and on adding the known radius of the lead balls, the distances ah 
are known. 
Operation 3. 
Hang the gold balls by their eye hooks upon a pin driven into any temporary stand. 
Measure with the cathetometer the three positions d, ce, and f of each, fig. B. From 
