SEISMOMETRY. 29 



the black dots, between the upper and lower parts of the 

 rigid frame B C. The arms are of such a length that for 

 small displacements parallel to the length of the bar, c C 

 practically move in a straight line, and the bar is in 

 neutral equilibrium. A light prolongation of the bar d 

 works the upper end of the light index e, passing as a 

 universal joint through the rigid support F. A second 

 index e' from the bar at right angles also passes through F. 

 The multiplying ends of these indices are coupled to- 

 gether to write a resultant motion on a smoked glass 

 plate s. 



Conical Pendulums, — Another group of instruments 

 which have also yielded valuable records are the conical 

 pendulum seismographs. The idea of using the bob of 

 a conical pendulum to give a steady point in an earth- 

 quake machine was first suggested and carried into 

 practice by Mr. Gray. The seismograph as employed 

 consists of a pair of conical pendulums hung in planes at 

 right angles to each other. The bob of each of these 

 pendulums is fixed a short distance from the end of a 

 light lever, which forms the writing index, the short end 

 resting as a strut against the side of a post fixed in the 

 earth. The weight is carried by a thin wire or thread, 

 the upper end of which is attached to a point vertically 

 above the fixed end of the lever. 



Rolling Spheres and Cylinders. — After the conical 

 pendulum seismographs, which claim several important 

 advantages over the bracket machines, we come to a 

 group of instruments known as rolling sphere seismo- 

 graphs. Here, again, we have a class of instruments for 

 the various forms of which we are indebted to the 

 ingenuity of Mr. Grray. 



The general arrangement and principle of one of 

 these instruments will be readily understood from the 



