1881.] J. Milne and T. Gray. On Seismic Experiments. 139 



Rupture along the lines of greatest tension in shearing may be well 

 illustrated by pasting a piece of paper over two flat boards, with 

 straight parallel edges, about J" apart; if now, preserving this dis- 

 tance, the boards are forced to move past one another in the direction 

 of their edges, folds appear in the paper parallel to the lines of 

 greatest tension, and if the sliding be continued the paper tears at 

 right angles to the direction of the folds (fig. 15). 



VII. On Seismic Experiments.' 1 By Johx MlLXE, F.G.S., and 

 Thomas Gray, B.Sc, F.R.S.E. Communicated by A. C. 

 Ramsay, LL.D., Director-General of the Geological Survey 

 and of the Museum of Economic Geology. Received 

 November 5, 1881. 



(Abstract.) 



This paper is an account of a series of experiments made at the 

 Akabane Engineering Works, Tokio, for the purpose of investigating 

 some points connected with earthquake motion. The mode of experi- 

 ment consisted in creating a disturbance at a point on the earth's 

 surface by allowing a heavy block of iron (1,710 lbs.) to fall from a 

 height (35 feet), and observing the resulting motion produced in the 

 earth at points variously situated relatively to the centre of disturbance. 

 The centre of disturbance was situated near to one corner of a 

 pond about 10 feet deep, and close to the foot of a small steep hill, the 

 remaining ground being very nearly level, and composed of hardened 

 mud, which extended to a depth of from 20 to 30 feet. The configu- 

 ration of the ground here briefly described is clearly shown by means 

 of a map accompanying the complete paper. In the earlier experi- 

 ments a number of similar vessels of mercury were placed at the 

 different points, and the vibrations produced on the surface taken as a 

 rough indication of the intensity of the disturbance at the point. 

 This method of observation showed with considerable definiteness 

 where the motion became insensible. These preliminary experiments 

 showed that the disturbance could be distinctly propagated to a dis- 

 tance of 650 feet (which was the greatest distance available) ; that 

 the pond cut off the disturbance from points beyond its distant side if 

 these points were sufficiently removed from the corner, but that the 

 hill did not cut off the vibrations. 



In subsequent experiments more definite observations were made 

 by using seismographic apparatus, and by this means the following 

 conclusions were reached. 



A disturbance emanating from a centre as above described, pro- 

 duced at least two distinct sets of vibrations. One of these sets has 



