86 EEroRT — 1895. 



screws having a millimetre pitch), the correspouding deflection of the pen- 

 dulum may be noted. The turning is done by a lever projecting from the 

 head of the screw. 



The boom of the pendulum is an aluminium tube 4 feet (120 cm.) in 

 length, carrying a sliding weight, W, and a movable point to which the 

 suppoi'ting tie can be attached. This tie, which is of thin brass wire, at 

 its upper end terminates with about an inch of untwisted silk. On the 

 inner end of the boom there is a quartz cup which bears on a steel needle 

 projecting slightly upwards from the base of the cast-iron stand. The 

 suggestion that the needle should project from the stand rather than from 

 the boom is due to Dr. von Rebeur-Paschvvitz. It gets over the difficulty 

 of having anything which may be markedly magnetic in motion ; and 

 secondly, in case of violent disturbance, the relative verticality of the 

 points of support is less liable to alteration. 



The instrument is adjusted so that the needle bears normally on the 

 centre of the quartz cup, or so that the centre of gravity of the system 

 falls about G. 



At the outer end of the boom a stiff wire rises vertically upwards. 

 Clamped to this at the required height is a horizontal wire 15 cm. long, 

 carrying a thin zinc plate ji^, measuring 6 cm. l)y 10 cm. In the centre of 

 this, and parallel to the length of the boom, there is a slit about O'O mm. 

 broad and 2 cm. long. As the boom moves to the right and left, this slit 

 floats over a second slit about 5 cm. long in the lid of the box covering 

 the drum which carries the recording paper. These two slits are at right 

 angles to each other, so that the liglit from a lamp reflected downwards 

 by a plane inirror only reaches the drum as a spot. 



A well-defined spot, which means a clear, shares line on the plioto- 

 graphic film, can be obtained without fine adjustment. That is to say, 

 the distance between the film and the slit, or between the stationary and 

 moving slits, may be anything between 1 and 5 mm. Projecting an inch 

 or so beyond the moving plate and attached to it is a pointer moving 

 over a scale fixed on the cover of the box containing the clock of the 

 recording drum. This can be inspected and the position of the boom at 

 any time noted by looking through tiie glass plate at in. 



The recording drum, on which the photograph -paper is fixed with a 

 spring clamp, as in a recording barometer, is of thin sheet brass 5 cm. 

 wide and lOo cm. in circumference (some are much less). It is turned at 

 the rate of lo cm. per 21 hours, and a film therefore lasts one week. 



The clocks, which are an American type intended to run 8 days, 

 have fitted to the slowest moving arbour four wheels, the last of which 

 turns a disc with slots round its edges once a week. The recording 

 drum, which can be dropped into its bearings, carries a large crank. 

 When in position the clock is slid in a groove until one of the slots catches 

 the outer end of the crank arm ; after this the cover is put over the clock 

 and drum, and the whole is pushed on grooves into the end of the case 

 covering the pendulum. 



Hollow wooden drums, which are easily driven by the clock-work, 

 have a tendency to warp, and this may result in a want of uniformity in 

 the motion. 



Brass drums in the damp atmosphere of a cave in a month or so tend 

 to rust, and this rust may act upon the photographic film to such an 

 extent as to render it illegible. 



U]p to the present time ordinary kerosene lamps have been used, but 



