-16 PROFESSOR C. V. BOYS ON THE 
grooved wheels. The central wheel of the set near the west wall is round grooved, 
and the other two, which can be set either 6 or 4 inches apart, have flat-bottomed 
grooves. The purposes which these wheels serve are numerous and important. In 
the first place the middle ones are employed to reduce the friction of the lid, as has 
already been explained. In one of the cathetometer operations the lead balls and the 
tops of their supporting pieces have to be observed in order to find the levels of their 
centres when they are hanging out of sight inside the apparatus. At the same time 
the lid must be raised, and held out of the way; but it cannot conveniently be 
removed altogether. To accomplish this, steel bands are passed over the flat-grooved 
pullies, and are each of them pinned to the ball holder at one end and hooked to an 
exactly equal counter-weight at the other. The balls can then be raised, and will 
remain hanging at any level at which they may be left. Two cords are hooked on to 
the eyes of the pillars R R of the lid, and after passing round the outermost pullies 
above, converge, and then, becoming single, pass over the central pulley next the 
west wali. There, a weight exactly equal to the lid, serves to counterbalance it, so 
that it will remain suspended in a horizontal position at any level. The height is se 
chosen that one of the ball holders is just above the pillar on its side, while the other 
is just below the lid on the other. The balls are then at the same level, and their 
upper portions can be seen just above the edge of the casting C. The balls under 
these conditions hang quite freely, neither touching the instrument nor being deflected 
hy contact between their wires or steel bands with the lid. The steel is necessary to 
give detiniteness to the positions of the lead balls during the cathetometer measures 
as if they were to hang from cord the twisting and uncertain and variable stretching 
would make accurate measurement impossible. The central overhead wheel alone is 
employed in placing the small balls in position. I used at first, after fixing them to 
their own fibres and hooks, and measuring the distances when hanging from the point 
of the hooks to the tops and bottoms of the balls, to get them in through the window, 
supporting the hook by a bent pin held in one hand and passing the fibre over a bent 
pin held in the other. The process was one of great delicacy and difficulty, but it 
answered with gold balls ‘2 mch in diameter. It was, however, next to impossible 
with balls of double the weight, as the fibre would not, under such a strain, bend 
round a pin, a polished steel rod, or anything that [ could think of. I had therefore 
to adopt the plan with the overhead wheel, which has never failed. A pin, with the 
point bent at right angles to form a horizontal hook, is tied to a piece of sewing silk, 
and allowed to hang from the central pulley. A weight equal to the ball is tied to 
the other end. The pin-hook is inserted in the eye of one of the hooks and eyes 
from which the gold ball is suspended, and pulled up till the ball is over the tube. 
It is then let down until the eye is opposite the window, when its hook is made to 
rest upon the point of a large pin held in one hand ; by this means it is transferred 
to the side hook where it is left hanging by its eye, and ready to be placed upon the 
arm of the mirror when that is in position. 
