360 Messrs. Milne and Gray's Experiments 



the weight causes the bracket to turn, while it itself remains 

 nearly steady. This frame is firmly attached to the head of a 

 stake driven into the ground. A light prolongation of the 

 bracket forms an index, which writes on the surface of a smoked 

 glass plate a magnified representation of the motion given to 

 the frame by the ground. Two of these instruments, placed 

 at right angles to each other, are used in conjunction for the 

 purpose of writing two rectangular components of the motion. 



The principle involved in this instrument (namely, that of 

 a mass supported in neutral equilibrium at the end of a 

 bracket) was, we believe, first used by Prof. W. S. Chaplin, 

 of the Tokio University — and subsequently in the instrument 

 described above, which was invented by one of the authors of 

 this paper, who was not then aware that the principle had been 

 previously applied to this purpose. An instrument involving 

 the same principle has also been invented by Prof. J. A. Ewing, 

 and is described in the Proceedings of the Royal Society for 

 February 1881. 



(/) Double-Bracket Seismograph. — This instrument con- 

 sists of two brackets hinged to each other, and one of them to 

 a fixed post. The planes of the two brackets are then placed 

 at right angles to each other, so as to give to a mass suspended 

 at the end of the outermost bracket two degrees of horizontal 

 freedom. (See Phil. Mag. for September 1881.) 



(g) Rolling-Sphere Seismograph ; 



(A) Rolling- Cylinder Seismograph. 



For descriptions and drawings of both these instruments 

 see Phil. Mag. for September 1881. 



Ball- and- Plate Seismograph. — In this instrument three 

 balls rest at the corners of a triangle on a " surface plate." 

 On the top of these balls is placed another " surface plate." 

 When the lower surface-plate, which is fixed to the earth, is 

 moved, the balls roll and the upper plate is left behind. Pivoted 

 on the upper plate is the end of a long light index. A short 

 distance below this pivot the index passes through a universal 

 joint fixed to the lower plate. The lower end of this index 

 carries a sliding needle, the point of which rests on a smoked 

 glass plate, and there writes a magnified representation of the 

 earth's motion. 



3. Instruments for recording Vertical Motion. 

 (a) Ordinary Spiral Spring. — The earliest form of instru- 

 ment used to record vertical motion was a spiral spring with 

 a weight at the lower end, the weight being supposed, on 

 account of its inertia, to remain steady at the time of a shock. 

 No satisfactory result has ever been obtained from such an 

 instrument; for it is extremely inconvenient to make the 



