24S 



NA TURE 



[July 12, 1S94 



ima^e oi i wire in front of the disc coincides with the 

 cross-wire of the telescope. The change of readings 

 gives the tilt of the earth's surface about a line parallel to 

 the plane of the wires supporting the mirror. Or, keep- 

 ing the source of light fixed, a continuous record can be 

 taken on a moving piece of photographic paper, and the 

 \-alue of the instrument will of course be greatly increased. 



If the instrument be tilted about a horizontal Ime at 

 right angles to the plane of the suspending wire, the 

 only effect is to alter the horizontal distance between the 

 points of support, P P', and therefore to change the 

 sensitiveness. If the tilt be about any other horizontal 

 line, it mav be resolved into two components, one about 

 a line parallel, and the other about a line perpendicular, 

 to the plane of the wire. The tirst of these deflects the 

 mirror, the second alters the sensitiveness ; but this 

 change of sensitiveness is nearly always very small, and 

 may as a rule be neglected, because the displacement of 

 p is always very small compared with the horizontal 

 distance between P and P . It will be obvious, however, 

 from these remarks that the instrument is only capable 

 of measuring lilts, or components of tilts, in one definite 

 direction. For a complete knowledge of any given 

 movement, it would be necessary to have a combination 

 of two such instruments placed with the planes of their 

 suspending wires at right angles to one another. 



The body of the instrument consists of a copper tube 

 fixed into a heavv gun-metal casting, containing a small 

 chamber very little larger than the mirror, and covered 

 in front by a glass window. Each end of the fine silver 

 wire is attached to a small pulley, and the axles of the 

 pulleys are gripped tightly so that the weight of the 

 mirror is not sufficient to turn them. A strong copper 

 frame carries the pulleys, and can be easily removed 

 from the instrument, thus rendering possible the delicate 

 operation of manipulating the silver wire. The bottom 

 of the frame rests in a hollow cone inside the instrument 

 just above the window ; and the length of the silver 

 wire is adjusted by turning the copper pulleys so that 

 the mirror hangs in the small chamber facmg the window. 

 The top of the frame is pressed by a spring against the 

 p-iint of a micrometer screw, and can move only in a 

 straight line in the direction of the screw, which is at 

 right angles to the plane of the suspending wire. 



A turn of the screw thus tilts the frame of the 

 instrument about a line parallel to the plane of the wire. 

 Knowing the dimensions of the frame and the pitch of 

 the screw, the tilt caused by one turn, or given fraction 

 of a turn, of the screw can be calculated, and ihe corre- 

 sponding change of scale-reading determined. The 

 screw is turned through a small angle by means of a 

 lever (Fig. 4,, and adjust- 

 able screw-stops arS ar- 

 ranged at the bottom of 

 the lever so as to allow 

 it to move through only 

 that amount which will 

 make the frame tilt 

 through an angle of two 

 seconds. The lever is 

 moved by a rocking-arm 

 turned by means of two 

 small bellows, into which 

 air is forced by squeez- 



iwo india-rubber balls connected with them by long 



Ihe instrument stands on three screw-feet, which are 



turned by two tangent-screws fixed to long rods. The 



'if!s of these rods are shown in Fig. 4. The tangcnl- 



■ r-ws are arranged so that turning one will tilt the 



■ --• about an axis parallel to the plane of the 



: wire, and will therefore move the spot of 



1.^-. .. .-.vays, so that, when it has nearly left the 



photographic paper, it is easily brought back to the 



NO. I 289, VOL. 50] 



required position without approaching the instrument. 

 Turning the other screw tilts the instrument about an 

 axis perpendicular to the plane of the suspending wire, 

 and this alters the sensitiveness. 



When the ends of the silver wire have been fixed to the 

 pulleys, and the frame, to which the latter are attached, 

 has been inserted in the instrument, the copper tube is 

 completely filled with paraffin oil. If the frame is lilted, 

 the mirror then comes to rest very slowly, and rapid 



vibrations, like those caused by passing carts and trains. 

 do not affect it. Thus the pendulum is designed, not so 

 much for the study of earthquakes, as for the investiga- 

 tion of slow tilts and pulsations of ihe earth's crust, by 

 whatever cause thev may be produced. 



Ikliouy and Uses of Ihe Bijilar Pendulum.— \n the 

 penduluni with which the preliminary experiments were 

 made at IJirmingham, the vortical distance between the 

 points of support of the silver wire is one foot, while the 

 borizonial distance between them is about j!,, of an 



