306 KEPOKT — 1893. 



is mounted on a small iron stand, whose position can be adjusted both 

 vertically and horizontally, and the stand is cemented to a pillar. 



By the side of the lamp is placed the recording apparatus. The 

 photographic paper is wrapped round a horizontal roller, 20 cm. long 

 and 56 i cm. in circumference, which is turned round with considerable 

 friction on its axis once in fifty hours, the paper being held fast to the 

 roller by two bars. Parallel to the roller, and at the same height as its 

 axis, is a cylindrical lens, 20 cm. long and 5 cm. in focal length. After 

 reflexion at the pendulum-mirror, M, and a double passage through the 

 lens in front of it, the pencil of light sent out from the slit is focussed by 

 the cylindrical lens to a fine point on the surface of the photographic 

 paper. An image is also formed by reflexion from the fixed mirror, m', 

 and, by rotation of that mirror, can be brought into any desired position. 

 This fixed point of light records a straight line on the photographic 

 paper, and is also used for marking the time, the light being shut ofl" for 

 five minutes at the beginning of each hour by a small screen moved by 

 the clockwork. The recording apparatus is mounted on a heavy cast- 

 iron plate, through which pass the cords of the weight and the pendulum 

 of the driving-clock. It is covered by a wooden box, closed at the top 

 by a lid, and having in front a horizontal slit for the passage of the light. 

 The lamp is changed twice a day ; and, two being used, a fresh one can 

 be inserted in the case by a slide without appreciable loss of time. Thus, 

 when once well adjusted, the working of the instrument requires very 

 little attention. 



Suggested Alterations. — The sensitiveness of a given horizontal pen- 

 dulum increases with its period of oscillation, and this is greater the 

 smaller the inclination of the axis to the vertical.' The difficulty of 

 obtaining and preserving very fine steel points therefore imposes a limit 

 to the sensitiveness of the instrument. In order to preservre the form of 

 the points, it is desirable that the pendulum should be as light as possible. 

 It should therefore be made of aluminium, whose specific gravity is about 

 Y'oths that of bi'ass. This woald at the same time reduce still further 

 the little friction that exists. The distance between the steel points might 

 also be increased with advantage to double its present valae, or even to 

 20 cm., without altering the distance of the centre of gravity from the 

 axis of rotation. 



The two lateral foot-screws should be removed and their places taken 

 by screws for adjusting the position of the lower of the two steel points. 

 The third foot-screw, which regulates the sensitiveness of the pendulum, 

 should also be replaced by a screw for adjusting the position of the upper 

 steel point. In the original instrument, if the parts of the foot-screws 

 projecting downwards are of unequal lengths, a temperature-correction 

 must be introduced ; but with the removal of the foot-screws this may 

 be rendered unnecessary. 



The head of the screw which regulates the position of the upper 

 steel point should be provided with divisions and an index, so that the 

 axis of rotation may be tilted through a known angle. The period of 

 oscillation (To) of the pendulum in a horizontal position of the axis may 

 then be determined indirectly as follows. If Tj and Tg be the periods of 



' IE T„ be the period of oscillation when the axis is horizontal, T that when the 

 axis is inclined ot an angle, t, to the vertical, T= = T„^ cosec i. 



