MEAN DENSITY OF THE EARTH. 



637 



DESCRirTION OF THE APPAKATrS. 



T'lg.l 



The peuduhim consists of a drawu brass tube i'" long", 4.15'"' in diam- 

 eter, and 0.]6«"' thick, which is strengthened near its middle by means 

 of a frame, C E F D. At this place are two mortises 

 on op[)Osite sides of the tube, E F, rectangular in 

 shape, 9''"' long and 2*^'" wide, for the insertion of the v ' ' f Jf 



resting place or bed L for the knife-edge S which is 

 tirndy fastened to the pendulum. The upper portion 

 of this bed is a slightly concave piece of agate of 0"" 

 in length and 1*="' in width securely attached to the 

 supporting brass. The knife-edge, also of agate, has 

 the same length as its bed. Its cross-section is an 

 equilateral triangle with sides l.T'^"' long. The edge 

 is not perfectly sharp, and on account of the concav- 

 ity of the bed rests on it only near the end. The slits 

 just referred to serve as a guide to the knife-edge and 

 also admits the air into the interior of the tube so 

 that there may be a quick and equable response to 

 all temperature changes. The ends of the tube are 

 closed by means of brass caps which carry conical 

 pins. Upon these pins balls of brass. A, are attached 

 by means of screws. The pins are prolonged beyond 

 the balls and one of them carries three small mova- 

 ble screws, N, whose determinations will be given 

 later ; at the other end corresponding to this addi- 

 tional weight a brass disk of equal weight is screwed 

 on so that the symmetry of the apparatus is pre- 

 served. Through the upper end of the pendulum rod 

 perpendicular to the length of the knife-edge there is 

 a slender screw, G IT, T*"" long, which serves in ad- 

 justing the position of the center of gravity. Di- 

 rectly before and behind the knife-edge circular mir- D 

 rors of 3.5''-'" diameter are attached, with whose as- 

 sistance the swinging of the pendulum can be observed. In order to 

 avoid magnetic interferences, brass is used throughout the entire in- 

 strument. 



DESCRIPTION OF THE OBSERVING-ROOM. 



The observing-room, 4'" square, is situated in the basement of the 

 west wing of the observatory. As the walls are very thick and the 

 room, being on the southern side directly under the cupola, is protected 

 from the direct rays of the sun, the changes in the temperature are 

 very slight. 



In the middle of the room there was a rigid pillar of brick with its 

 foundation 3 feet deep. On its south side is the cast-iron bracket which 

 supports the penduium, so arranged that when once jilaced in a perpen- 

 dicular position it can be securely held so. The whole is protected 



