_Instruments to record Vertical Motion._—The instruments which have been devised to record vertical motion are almost as numerous as those which have been devised to record horizontal motion. The earliest form of instrument employed for this purpose was a spiral spring stretched by weight, which, on account of its inertia, was supposed at the time of a shock to remain steady. No satisfactory results have ever been obtained from such instruments, chiefly on account of the inconvenience in making a spring sufficiently long to allow of enough elongation to give a long period of vibration. Similar remarks may be applied to the horizontally placed elastic rods, one end of which is fixed to a wall, whilst the opposite end is loaded with a weight. Such contrivances, furnished with pencil on the weight to write a record upon a vertical surface, were used in 1842 at Comrie, and we see the same principle applied in a portion of Palmieri’s apparatus. Contrivances like these neither give us the true amplitude of the vertical motion, insomuch as they are readily set in a state of oscillation; nor do they indicate the duration of a disturbance, for, being once set in motion, they continue that motion in virtue of their inertia long after the actual earthquake has ceased. They can only be regarded as seismoscopes. [Illustration: FIG. 6.] The most satisfactory instrument which has yet been devised for recording vertical motion is Gray’s horizontal lever spring seismograph. This instrument will be better understood from the accompanying sketch. A vertical spring S is fixed at its upper end by means of a nut _n_, which rests on the top of the frame F, and serves to raise or lower the spring through a short distance as a last adjustment for the position of the cross-arm A. The arm A rests at one end on two sharp points, _p_, one resting in a conical hole and the other in a V-slot; it is supported at B by the spring S, and is weighted at C with a lead ring R. Over a pin at the point C a stirrup of thread is placed which supports a small trough, _t_. The trough _t_ is pivoted at _a_, has attached to it the index _i_ (which is hinged by means of a strip of tough paper at _h_, and rests through a fine pin on the glass plate _g_), and is partly filled with mercury. Another method of obtaining a steady point for vertical motion is that of Dr. Wagener, who employs a buoy partly immersed in a vessel of water. This was considerably improved upon by Mr. Gray, who suggested the use of a buoy, which, with the exception of a long thin style, was completely sunk.