SEISMOMETRY. 33 



the position of the cross-arm A. The arm A rests at one 

 end on two sharp points, jp, 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 ^, and rests through a iine 

 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 w^as 

 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 descrip- 

 tion have yielded good records. 



Record Receivers. — A large number of earthquake 

 machines having been referred to, it now remains to con- 

 sider 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, 



