On an Improved Form of Seismograph. 353 



( Elasticity/ Encyc. Brit., new edition) the breaking-stress is 



Cent. -dynes. 



Best pianoforte steel-wire 2318 x 10 7 



The question as to what is the most probable cause of this 

 increase in strength as the diameter diminishes, presents some 

 difficulty. 



Quincke (Comptes Rend, de V 'Acad, de Berlin, 1868, 

 p. 132) has suggested that the great increase observed in the 

 case of metals is due to a surface tension, analogous to that 

 observed in liquids. If this were the true explanation, the 

 breaking-weight could be expressed by the sum of two terms 

 which vary as the diameter and the square of the diameter 

 respectively. This suggestion does not receive much support 

 from our observations, as the results cannot be satisfactorily 

 expressed by means of such a formula. It is, perhaps, more 

 probable that the heating and rapid cooling undergone by the 

 glass when it is drawn out into a fine fibre produces an 

 increase in tenacity ; and it is at all events certain that no 

 comparisons can be made between the strengths of different 

 materials unless they have undergone similar treatment, and 

 unless the sizes of the rods or wires submitted to experiment 

 are the same. 



XL. On an Improved Form of Seismograph. 

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



[Plate IV.] 



THE apparatus described in this paper is an improved 

 form of a seismograph which was made for Prof. Milne 

 in the beginning of 1883, to be used by him in his investi- 

 gations for the Committee appointed by the British Associa- 

 tion to " Investigate the Earthquake Phenomena of Japan/' 

 That apparatus was exhibited to the Geological Society of 

 London, and a description of it by the present writer was 

 published in the Quarterly Journal of that Society in May of 

 the same year. It consisted of a combination of instruments 

 which had been devised by Prof. Milne and the writer, and 

 descriptions of which had appeared from time to time in the 

 1 Transactions of the Seismological Society of Japan,' and in 

 the ' Philosophical Magazine/ The object of the apparatus 

 was to determine the time of occurrence, the amount, the 

 period, and the direction of the different motions in an earth- 

 quake shock. Arrangements were made for recording three 

 components of the motion, one vertical and two horizontal, at 



* Communicated by the Author. 

 Phil. Mag. S. 5. Vol. 23. No. 143. April 1887. 2 B 



