Form of Seismograph. 359 



William Thomson has recently suggested to the writer that a 

 flat spring, which in its normal state is bent to such a curva- 

 ture that it is brought straight by supporting a weight on its 

 end, might be found a good arrangement for a vertical motion 

 seismometer. This would certainly have considerable advan- 

 tage in the way of simplicity, and with proper compensation 

 applied, say to the index-lever, so as to lengthen the period, 

 may be found very suitable. The only doubtful point seems 

 to be whether the want of rigidity in the spring may not lead 

 to false indications in the record due to the horizontal motions. 



The application of a rigid horizontal lever, pivoted on knife- 

 edges and supported by springs as a vertical-motion seismo- 

 meter, was first described in the earlier of the two papers to 

 the Seismological Society of Japan, quoted above. The ad- 

 vantage of this arrangement, as rendering it possible to obtain 

 a long period of free vibration by placing the intermediate 

 point of support below the line joining the other two, was also 

 pointed out. The advantage obtained by the lever itself, 

 without compensation, over an ordinary stretched string was 

 more specifically pointed out in the other papers referred to ; 

 and a method of obtaining very perfect compensation, either 

 for a lever or an ordinary spring arrangement, by means of a 

 liquid, was then given. The idea of increasing the period of 

 a vibrating system by the addition, as it were, of negative 

 stability, which was first brought forward in these papers, has 

 been worked out in various ways ; but the method described 

 in this paper is the most perfect yet adopted. Its application 

 to the ordinary pendulum was also brought forward and dis- 

 cussed at a subsequent meeting of the Seismological Society 

 of Japan*. 



The apparatus above referred to for recording the horizontal 

 components of the motion during an earthquake may, when 

 properly adjusted, be used for registering minute tremors and 

 slow changes of level of the earth's surface. It is, however, 

 absolutely necessary for such a purpose that friction of the 

 different parts should be reduced to a minimum ; and hence 

 the siphons, or the marking-points when a smoked surface is 

 used, are only brought for a few seconds at a time into contact 

 with the paper, thus recording a series of dots close enough 

 together to form practically a continuous line. Another method, 

 which gives excellent results and is simple, has been much 

 used by Prof. Milne in Japan. It consists in passing from the 

 point of the index, through the paper, to the drum a series of 

 sparks from an electric induction-coil. The sparks can be 



* "On a Method of Compensating a Pendulum so as to make it 

 Astatic," by Thomas Gray, Trans. Seis. Soc. Japan, vol. iii. p. 145. 



