thanks to the device for starting and stopping the pendulums from outside, and to 

 the arrangement of right-angled prisms shown in Fig. 2. These have adjusting screws 

 to enable the images from the mirrors of the three observing pendulums to be seen 

 side by side through the same window. 



The agate planes are levelled by means of auxiliary dummies with knife-edges, 

 carrying levels in their heads. 



THE CLOCK. 

 Strasse ami Bolide, 101. 



The break-circuit clock lent by the Geodetic Institute was a second pendulum 

 clock by Strasse and Rohde in a heavy iron case, arranged to be conveniently hung 

 on a pillar or wall. The break-circuit arrangement consisted of a short lever arm, 

 which was lifted by another arm carried on the pendulum, and had an attachment 

 whereby the duration of the contact could be adjusted. This clock was used through- 

 out the series of observations except for those at Potsdam itself, where a break-circuit 

 clock belonging to the Geodetic Institute was used both for the initial and final 

 comparisons. 



THE COINCIDENCE APPARATUS. 



This apparatus was of the usual electro-magnetic type, consisting essentially of a slit 

 in a plate carried on a lever, pivoted behind another fixed slit. Under the influence of 

 the current through the break-circuit clock this lever is drawn down by an electro- 

 magnet against the action of a spring and again released, in isochronism with the beat 

 of the pendulum clock. At every " make " and at every " break " of the current, 

 a flash of light is allowed to pass through the two slits and is observed, after reflection 

 from the pendulum mirrors, in a telescope with cross wires, fixed above the coincidence 

 box. When the observing pendulum is hanging freely without oscillation each flash 

 appears in the same place with reference to the cross wires, but, when the pendulum 

 is swinging, the flash appears to move progressively up and down as a result of the 

 varying position of the observing pendulum when the flash passes through the two 

 slits. Since the period of the pendulums is slightly greater than i second, the pendulum 

 falls further behind the clock at each swing. The time between two successive transits 

 of the flash over the horizontal cross wire in the telescope is called the coincidence 

 interval C. The pendulum thus makes 2 C 1 vibrations in C seconds (measured by 

 the break-circuit clock) and the period of vibration is given by 



C 

 2C-1 ' 



The amplitude of the vibration is determined by noting the reflection in the 

 pendulum mirror of a graduated scale fixed on the front of the box. It is obtained 

 from consideration of the distance-mirror to scale, and of the number of divisions of the 

 scale which are seen to pass over the cross wire of the telescope. 



5 A3 



