ON EARTH TREMORS. 291 



Bifilar Pendulum designed by Mr. Horace Darwin. 



The pendulum used by Messrs. G. H. and H. Darwin in their experi- 

 ments at Cambridge is fully described in the Report for 1881 (pp. 93- 

 112). The principle of this instrument was suggested to them by Lord 

 Kelvin, and may be briefly described as follows. A heavy weight is sus- 

 pended by two converging brass wires, which only allow the cylinder to 

 move in a direction at right angles to the plane they are in. A fine silk 

 fibre is attached at one end to the bottom of the weight and at the other 

 to a fixed support, and passes through two thin wire loops on the edge of 

 a small circular mirror. The ends of the silk fibre are brought close 

 together, so that the two parts are inclined at a considerable angle. 

 Thus, a very small displacement of the pendulum at right angles to the 

 plane of the mirror will cause it, and a ray of light reflected by it, to turn 

 through considerable angles. 



Mr. Horace Darwin has designed the new bifilar pendulum on the 

 same principle (fig. 1). The mirror is circular and ^ inch in diameter, 

 and itself forms the bob of the pendulum. Two hooks about l inch 

 apart are fixed to its upper rim and are hooked on a 

 very fine silver wire which supports the mirror. The ^' 



points to which the ends of this wire are fixed are some 

 distance apart in a vertical direction, one being very 

 nearly vertically above the other, and are attached to 

 supports in the frame of the instrument. 



Any simple tilt of the ground may be resolved into 

 component rotations about a vertical line and two 

 horizontal lines, one perpendicular, and the other parallel, 

 to the face of the mirror. The first rotation produces 

 no appreciable efiect, the second changes the horizontal 

 distance between the two points of support, thus render- 

 ing the instrument more or less sensitive, according as 

 the distance is diminished or increased. By the third 

 rotation, the upper support is moved through a greater 

 distance than the lower, causing a deflection of the 

 mirror about a vertical axis. If, for instance, the mirror 

 be suspended in a north and south plane, facing a gas- 

 jet at some distance to the west of it, and if the north 

 part of the suspending wire be the longer, it is obvious 

 that the pendulum will only indicate those tilts which 

 take place in an east and west direction, or the east and 

 west component of tilts which take place in other direc- 

 tions ; and that a tilt towards the west will cause the 

 reflected beam to deviate towards the south. 



The pendulum is at present placed in this position. The mirror 

 faces west, and it will be convenient in this description to refer to the 

 sides as north or south, &c. The front or west view of the instrument 

 is given in fig. 2, on one-third the real scale. The base. A, is carried by 

 three levelling-screws, 6 inches apart, and is made in two pieces fixed 

 together ; ' but it would be better for the future to have it cast in 

 one, together with the brass box, b, 2 inches broad, 2| inches high, and 



' In order to equalise the temperature in the instrument, thick metal of good 

 conductivity has been used throughout in the construction. Brass and gun-metal 

 have been used, although copper would have been considerably better. 



u 2 



