When in use, the auxiliary pendulum is put in the place of No. 21 (position C, 

 Fig. 2), and No. 21 is moved up to position A. It bears a mirror carried on a long 

 a 1'in p, so that its mirror and No. 21 's mirror lie parallel to one another and in the same 

 plane. 



I 

 I 



Fig. 2. Diagram showing, in plan, the relative positions of the pendulums on the stand. 



The stand, which is shown from above in Fig. 2, is a heavy brass casting about 

 2 feet in diameter, most solidly made and supported by three heavy levelling screws 

 with provision for clamping them in position. In order to minimise the effect of 

 draughts, the apparatus is divided by solid vertical brass plates into four compartments, 

 so that the pendulums swing each in its own compartment and on its own agate plane. 

 Arrangements are provided for lowering any pendulum by means of a slow motion 

 screw actuated from outside, until the agate knife-edge of the pendulum rests in its 

 proper position upon the agate plane of the stand. Apparatus, with separate adjustment 

 for regulating the amplitude, is provided for giving the pendulums the initial swing, 

 without the necessity of lifting off the light double-walled aluminium cover. The 

 temperature is measured by a thermometer divided to single fifths of a degree Centigrade, 

 whose bulb projects into the bob of a dummy pendulum with hollow stem, the bob being 

 in metallic connection with the base of instrument. Windows are provided in the 

 cover, one for reading the thermometer and one for observing the mirrors of the three 

 pendulums. The cover of the stand has not to be removed during the observations, 



