1894.] on the Newtonian Constant of Gravitation. 369 



the octagon house separated. Here a further system of screens con- 

 sisting of concentric brass tubes may be seen, but the lower one, 

 which surrounds the window, has been removed and placed upon the 

 table. The driving gear is also seen in this photograph, and a pipe 

 coming from the screw under the instrument which holds the central 

 tube, which pipe is also seen in Fig. 1. This enables me to con- 

 trol the motion of the mirror from the telescope without approaching 

 the corner in which the apparatus is set up. This is done as 

 follows : the back window at the level of the mirror is made of metal, 

 with a hole in it in which is screwed a metal tube lightly filled with 

 cotton-wool. This is not central, but opposite one end of the mirror. 

 The pipe on the table does -not fit the screw, but is merely bent up 

 and enters it loosely. By gently drawing air from the end of the 

 pipe at the telescope a very feeble draught is produced in the appara- 

 tus, for nearly all the required air is supplied by leakage round the 

 pipe near the screw, very little entering through the window tube, in 

 consequence of the resistance offered by the cotton-wool. In this way, 

 if the mirror is moving it may be gently brought to rest without 

 impact, or it may be given a swing of any desired amplitude. So 

 perfectly does this work, that the mirror may be steadied very quickly 

 so as to move through less than a scale division, an amount which 

 corresponds to six or seven seconds of arc, or to a force of less than 

 one thousand-millionth of the weight of a grain. 



The operations for any complete experiment are fourteen in number. 

 I do not intend to go through these seriatim, as time will not allow 

 me to do so. It is sufficient now to say that the first eight are 

 necessary to get the instrument and scale relatively fixed and ad- 

 justed, the vertical measures made, and generally all ready for 

 operation 9, in which the optical compass is employed. This is a 

 most important one, for not only are the horizontal measures made, 

 on which so much depends, but in addition the planes of the wires 

 and fibres are made identical, the corresponding scale reading is 

 found, and any eccentricities are measured and may be corrected. 



Fig. 8 is a view of the apparatus with the optical compass in 

 position, and with the microscopes focussed upon the wires. They 

 are then ready to be withdrawn by the focussing slide, so as to 

 transfer the distances directly to the small glass scale, as already 

 described. 



When this is completed the proper windows are put in position, 

 the screen tubes, the octagon house and the felt screens are all 

 placed ready for operation 10, in which the deflections are measured, 

 and the period with the balls is determined. As this is the operation 

 in which variations of temperature produce so serious an effect, I 

 prefer to leave everything undisturbed for three days, to quiet down. 

 A few hours are quite useless for the purpose. 



In operation 11 the period with the counter- weight in the place 

 of the gold balls is measured ; also the deflection, if any, due to the 

 lid and lead balls upon the mirror alone. This is only 1/10 division, 



Vol. XIV. (No. 88.) 2 o 



