Form of Seismograph. 357 



record sheet. It will thus be seen that the instrument is 

 intended to be absolutely self-acting, so long as its supply of 

 paper lasts and the driving mechanism continues to go. The 

 supply-drum can take as much paper as is required in a week 

 on the slow speed. 



The record is made in ink by means of fine glass siphons, 

 in very much the same manner as that which was introduced 

 by Sir William Thomson in his siphon-recorder for submarine 

 telegraph -cable work. This is extremely well adapted for the 

 continuous ribbon method of working, and, besides, gives an 

 excellent clear record which requires no further preparation 

 before it is filed for reference ; and, what is of great im- 

 portance, the record is obtained with exceedingly little dis- 

 turbance from friction at the marking-point. 



The siphons which write the horizontal components of the 

 motion are controlled by two pendulums, the suspending wires 

 of which are held out of the vertical by horizontal struts ter- 

 minating in knife-edges which rest against the bottoms of 

 flat V-grooves fixed to a cast-iron pillar rigidly attached to 

 the sole plate of the instrument. These pendulums, when set 

 in vibration, describe cones, and hence they have been called 

 " conical pendulums." The degree of deflection from the 

 vertical can be varied from about one and a half inches to a 

 foot, by sliding the pendulum-bob along the strut. The strut 

 is made in two pieces, so that a part of it can be removed 

 when high sensibility is required, and in consequence the 

 mass is used near the knife-edge. The bob of the pendulum 

 is suspended by a fine platinum or steel wire from an arrange- 

 ment which permits the suspending wire to be lengthened and 

 shortened, and also allows the pomts of suspension to be put 

 in such positions above the knife-edges as causes the struts to 

 place themselves in positions at right angles to each other, 

 and at the same time provides the means of adjusting their 

 periods of free vibration to any desired length *. 



It is of great importance in apparatus of this kind that the 

 mass which, through its inertia, enables the record of the 

 motion of the earth to be written, should be as far as possible 

 from the knife-edge or poino fixed to the earth ; a long 

 period of free vibration can thus be obtained combined with 

 considerable stability of position, while the greatest motion to 

 which the knife-edge is likely to be subjected does not turn 

 the strut through a large angle. If this latter condition be 



* This pendulum is a modification of one designed by the Author in 

 the beginning of 1880, in which the weight was supported by a thin wire 

 in line with a rigid vertical axis fixed to the end of the strut and resting 

 against bearings so as to keep the strut horizontal. 



