46 PROFESSOR C. V. BOYS ON THE 
and counterweight so as to reduce the weight of the lids and balls and therefore their 
friction to a small amount. Place the first of the train of wheels W in place. See 
that the string operating the driving gear is in place, and that the india-rubber tube 
under the apparatus is connected to the composition tube going to the telescope. 
Place the two halves of the octagon house in position, and fill up the open gap where 
it overhangs the table at the back with a duster. See that the little electric lamp 
inside the house is properly placed to illuminate the vernier. Remove all superfluous 
apparatus from the table. Place the felt screen in position, and, when all is proved 
to be in working order, leave, if possible, for three days to acquire a uniform 
temperature. 
“The angle through which the lead balls must be turned in order to produce the 
maximum deflection I had found in a preliminary calculation, completed before the 
apparatus was made, in which 2 R was made 6 inches, 27 1 inch, and the difference 
of levels 6 inches, to be 61° 15’, and the effect of an error of 15’ at this position to be 
1 part in 280,000. With the actual apparatus, in which 27 is less than 1 inch, I 
found by experiment the angle of greatest effect to be 65°, which is that adopted. 
Knowing the vernier reading corresponding to the observed scale reading when there 
is no deflection observable with the optical compass, 7.e., when the lead and gold balls 
are in the same plane, or in the neutral position, and taking this provisionally as a 
position of no deflection, move the lid in one direction through an angle of 65° by 
turning the handle of the wheel d by the telescope round 115 times. ‘The exact 
setting of the lid is finally accomplished by lighting the electric lamp in the octagon 
house, and observing the vernier with the small telescope t. Three or more elonga- 
tions of the apparently moving scale are now read in the large telescope, and then the 
wheel d is turned 125 times back, and, when the mirror is approaching its position of 
rest, the remaining 105 turns are given to the handle, which leaves the balls 65° on 
the other side of the neutral point, and the mirror oscillating through 50 divisions of 
the scale, or even less. The vernier reading must be correctly set by the use of the 
electric lamp, little telescope, and handle d, as before. Three or more elongations are 
again taken. The elongations are corrected, and from the corrected elongations the 
positions of rest, when the lead balls are in their + and in their — positions, are 
calculated. If the supposed neutral position had been accurately found, its scale 
reading would be exactly half way between the + and — position of rest. If, as is 
probable, it is not quite accurate, then, since the variation of the position of rest 
of the mirror is hardly observable when the lid is moved even so much as 1° from its 
position of maximum effect, while such an uncertainty of position is not possible in 
the provisional setting, all that has to be done is to bring the lid into such a position 
that the mirror is at rest exactly half way between its extreme + and — positions 
already observed. It is not sufficient to move the lead balls through an angle 
corresponding to the error, because, though at the neutral position the smallest 
variation produces the greatest effect upon the gold balls, they do not follow it abso- 
