NEWTONIAN CONSTANT OF GRAVITATION. 39 
these the distances dg can be determined when the fibres are stretched by the weight 
of the balls. Also find their individual torsion periods by the method of coincidences, 
Fig, B. 
poy 
| 
Beene, 
oS 
watching them with the cathetometer telescope, and listening to the clock. This latter 
is needed for the purpose of calculating the very small correction (p. 35) which should 
be evanescent. The cathetometer measures can be made with an accuracy of two or 
three-hundredths of a millim., and this, as will appear when the figures are 
examined, is more than abundantly accurate. 
Operation 4. 
Put the lead balls in the apparatus, fix the central tube in place, with its front 
window facing the scale, let the gold balls down the central tube, and hang them on 
the side hooks, place the lid in position ; having marked one side of the lead balls in 
some distinguishing manner (a very small spot of shellac varnish is what I used), 
suspend them with these marks pointing in some definite direction, e.g., inwards ; 
let down the mirror by its torsion fibre, and adjust the torsion head so that it hangs 
approximately centrally, and at a convenient height. 
Operation 5. 
Find, as already described, the division at which a perpendicular from the axis of 
the tube falls upon the scale. Twist round the central tube, if necessary, until the 
window reflects a light placed behind this division into the large telescope. 
Operation 6. 
Find the optical distance in terms of the corrected scale division from the silvered 
surface of the mirror to the foot of the perpendicular upon the scale, and thus find 
the angular measure corresponding to any observed deflection. 
The details of this process are sufficiently described under the heading The Steel 
Tape and its Accessories (p. 17). 
Operation 7. 
Make cathetometer readings of a, a and d, d. Subtract from these the corre- 
