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.
Among the other forms of apparatus used to record vertical motion may be mentioned vessels provided with india-rubber or other flexible bottoms, and partially filled with water or some other liquid. As the vessel is moved up and down, the bottom tends to remain behind and provides a more or less steady point. Pivoted to this is a light index, which is again pivoted to a rigid frame in connection with the earth. Instruments of this description have yielded good records.
_Record Receivers._—A large number of earthquake machines having been referred to, it now remains to consider the apparatus on which they write their motions. The earlier forms of seismographs, as has already been indicated, recorded their movements in a bed of sand; others wrote their records by means of pencils on sheets of paper. Where we have seismographs which magnify the motion of the earth, it will be observed that methods like the above would involve great frictional resistances, tending to cause motion in the assumed steady points of the seismographs. One of the most perfect instruments would be obtained by registering photographically the motions of the recording index by the reflection of a ray of light. Such an instrument would, however, be difficult to construct and difficult to manipulate. One of the best practical forms of registering apparatus is one in which the record is written on a surface of smoked glass. This can afterwards be covered with a coat of photographer’s varnish, and subsequently photographed by the ‘blue process’ so well known to engineers.