_Parallel motion Instrument._—A machine which writes its record as two components, and which promises great stability, is one suggested by Professor C. D. West. Like the bracket machines it consists of two similar parts placed at right angles to each other, and is as follows: A bar of iron A is suspended from both sides on pivots at C C by a system of light arms hinging with each other at 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 together 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 earthquake 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 seismographs. Here, again, we have a class of instruments for the various forms of which we are indebted to the ingenuity of Mr. Gray. The general arrangement and principle of one of these instruments will be readily understood from the accompanying figure. S is a segment of a large sphere with a centre near C. Slightly below this centre a heavy weight B, which may be a lead ring, is pivoted. At the time of an earthquake C is steady, and the earth’s motions are magnified by the pointer C A N in the proportion of C A : A N. The working of this pointer or index is similar to that of the pointer in the pendulum. [Illustration: FIG. 5.] Closely connected with the rolling sphere seismographs, are Gray’s rolling cylinder seismographs.