On Measuring and Recording Earthquake-Motions. 199 



In a paper by Captain Abney and Colonel Festing, recently 

 read before the Physical Society and printed in the Philoso- 

 phical Magazine for June, on the Transmission of Eadiation 

 through Ebonite, reference is made to our original experi- 

 ments; and the authors say that, judging from the figure 

 accompanying our Note, they should think that the thickness 

 of the ebonite prism traversed by the intermittent beam must 

 have been about one fourth of an inch. We are afraid that that 

 figure is liable to give this misconception ; in drawing it we 

 were merely paying attention to the directions of the incident 

 and refracted beam, and not to the actual thickness of the 

 ebonite, which was in fact very small indeed where the inter- 

 mittent beam passed through it. 



XXV . On Instruments for Measuring and Recording Earth-* 

 quake-Motions. By Thomas Gray, B.Sc, F.R.S.E* 



[Plate III.] 



I. Rolling-Sphere Seismograph. 



THE instrument which I have called a rolling-sphere seis- 

 mograph will be readily understood from the accompa- 

 nying sectional drawing (fig. 1). A sphere of lead, iron, or any 

 other heavy substance rests on a flat plate, B, made truly plane 

 and furnished with three levelling-screws, L. An arm, A, fixed 

 to the base, B, is so formed that a circular ring fixed to its end 

 is held in a horizontal position with its centre vertically above 

 the highest point of the sphere. This ring carries a species of 

 spring universal joint, consisting of four very light bent springs, 

 ^arranged at right angles to one another and meeting in a small 

 round disk, b, at the centre. The lower end of the lever, I, 

 passes through this ring b, and is fixed to it at such a point 

 that its lower end, which is rounded, just fits a small hole in 

 the top of the sphere S. Between S and b a small sphere, s, 

 is fixed to the lever Z, and is so proportioned that the lever I, 

 when pushed at b, tends to rotate around a point a little above 

 its lower end, thus diminishing the push on the sphere S. 

 The springs j serve to allow the lever / to turn in any direc- 

 tion, and are made so light that they can only make the ball 

 roll with a very long period. When thus proportioned they serve 

 the purpose of a universal joint, and at the same time give a 

 little stability to the parts, thus preventing the plate P, 

 if it be put in motion, from causing the ball to roll over. 

 The lever Z is a rod of bamboo which is at the same time 



* Communicated by the Author. 



