220 PROFESSOR FORBES ON A SEISMOMETER FOR MEASURING 



greater length by means of the screw E. It is evident that, by adjusting the stiff- 

 ness of the wire, or the height of the ball C, we may alter to any extent the relation 

 of the forces of Elasticity and of Gravity, and consequently render the equilibrium 

 of the machine in a vertical position stable, indifferent, or unstable. Since, then, 

 a lateral movement, which carries forward the base of the machine, can only 

 act upon the matter in C through the medium of the elasticity of the wire, the 

 stiffness being diminished, or the weight increased, the tendency of the rod to 

 right itself may be diminished in any proportion, and that irrespectively of the 

 dimensions of the instrument. 



The wire D being cylindrical, the direction of the displacement occasioning the 

 shock will at once be indicated by the plane of vibration of the pendulum, which, 

 being once put in motion, will oscillate backwards and forwards many times before 

 coming to rest. The pendulum is adjusted to the vertical position by four anta- 

 gonist screws e e e e, acting on a ball and socket arrangement/! 



The self-registering part of the apparatus, which Mr DAVID MILNE has termed 

 a Seismometer, was arranged by that gentleman and by Mr JAMES MILNE, the 

 ingenious artist who constructed it. It consists of a spherical segment H I K of 

 copper lined with paper, against which a pencil L, inserted in the top of the pen- 

 dulum-rod, is gently pressed by a spiral spring. The marks thus traced on the 

 concave surface indicate at once the direction and maximum extent of the pen- 

 dulum's vibration. The arrangement of the pencil is seen upon a larger scale in 

 Fig. 4, where L is the pencil as before loosely fitting the cylinder b c, and pressed 

 upwards by the spring a. The whole pencil-case moves stiffly on the extremity 

 B of the pendulum-rod, so as to adjust the pressure against the paper. 



HA.RDY'S instrument was intended simply for ascertaining the stability of the 

 support for a clock. The spring was a piece of flat watch-spring the plane of 

 its motion was parallel to that of the pendulum of the clock whose influence was 

 suspected, and the time of oscillation being adjusted accurately to seconds by 

 screwing the bob up or down, the repetition of impulses always isochronal, 

 though individually feeble, at length urged it into considerable arcs of vibration, 

 if the beam or wall on which it stood was not perfectly stable. The instrument 

 under consideration, on the other hand, has a free vibration in every vertical 

 plane, the time of its oscillation is immaterial, except in so far as the sensibility 

 is increased as the time is greater ; HARDY'S instrument collects the effect of a 

 series of isochronous impulses, this one registers the maximum effect of a single 

 and insulated one in direction and in intensity: HARDY'S was an indicator of in- 

 stability, this (as we shall see) furnishes a measure of the cause of a concussion. 



The admirable advantage which the balance of the gravitating and elastic 

 forces affords will appear from the following considerations : 



I. We must first attend to the friction which must be overcome in order to 

 carry the pencil across the surface receiving the trace. The moving force of the 



