628 



NATURE 



[Oc/. 26, 1882 



£X M u? that L^SL*^ * *" ««-»!» boTA^he lower ^ndTt^x 



having taught us that the older instruments told us ™ 

 little about the actual movements which were going on at 

 the time of an earthquake, a large number of instruments 

 to replace them have been gradually invented. Of these 

 I may mention the three following tvpes. 



#~ 



}■ Pendulums, so far controlled by friction a, to be 

 ' dead-beat," provided with an index which magnifies 

 the earth's motion. The general construction of one of 

 these instruments will be understood from Fig. 2 B B is 

 a heavy lead ring, used as a pendulum; p a slidiii" 

 pointer loaded with lead (so that it may give sufficient 



friction), resting on a glass plate on the pendulum. This 

 pointer is earned by an arm attached to the side of the 

 box containing the pendulum. Its object is to control 

 the swinging of the pendulum, p the index, axled near 

 the centre of gravity, c, of the pendulum, and again a 

 snort distance below at a in an arm attached to the side 



. ,j° , w«»-i tuu ui nils iiiuex 



there is a sliding needle, n, to write on a moving or 

 stationary glass plate. The magnification cf the actual 

 motion of the earth in this instrument is as c a : A n 



2. Bracket Machines (see Fig. 3). b is a heavy weight 

 pivoted at the end of a small bracket, c A K, which 

 bracket is free to turn on a knife edge K above and a 

 pivot A below, in the stand S. At the time of an earth- 

 quake b remains steady, and the index p forming a con- 

 tinuation of the bracket, magnifies the motion of the 

 stand, which is to that of the earth in the ratio of A c • c N 

 These instruments are used in pairs, the brackets in each 

 being placed at right angles to each other. 



1 



3. Rolling Spheres (Fig. 4). s is a segment of a large 

 sphere with a centre near c. Slightly below this centre a 

 heavy weight, B, which may be a lead ring, is pivoted 

 At the time of an earthquake c is steady, and the earth's 

 motions are magnified by the pointer c a n in the propor- 

 tion of c A : a N. The working of this pointer or index is 

 similar to that of the pointer in the pendulum. 



The indices of all these instruments, of which there are 

 many modifications, are allowed to write on smoked glass 

 plates, which at the time of an earthquake are either bein<r 

 moved by clockwork or else are stationary. For vertical 

 motion, sunken buoys, the water in a can with a flexible 

 bottom, and a weight suspended at the end of the Ion.. 



Fig. 4. 



arm of a lever and stretching a stiff spring, have all been 

 used with more or less success as steady points in the 

 recording of vertical motion. 



For the invention of the greater number of these in- 

 struments, which I may remark have already done very 

 much in writing down actual earthquake motion, we are 

 indebted to Mr. T. Gray. Messrs. Ewing, Chaplin, 

 Wagner, and other members of the Seismological Society 

 of Japan, have also made valuable contributions to this 

 part of the subject. 



Some of the more important results which have been 

 arrived at by the use of these and other instruments 

 are : — 



