APPENDIX. 



ASTATIC SrSPEXSrOX BY LINKWORK. 



At a meeting of tin- Seisnioloicical Society of Japan, held while thi? niomoir 

 ha.« been passing through the ])ress, Prof. C. D. West exhibited a model of a 

 new method of astatic suspension, ap]>lical)le to seismoraetry. A massive bar 

 a (fig. 40, Plate XXIII) is hung by a system of links bbcc, like the sooalled 

 j)arallel motion of AVatt, whose effect is to give the bar freedtmi to move in an 

 approximately straight horizontal line, in the direction of its own length. A 

 small degree of stability is secured by jilacing weights dd at the bottom of the 

 links bb, or by making these slightly shorter than the lower links cc. To guard 

 against motion at right angles to the plane of the links a guide-bar ee is added, 

 which pas.ses through holes in tlie u])rigiit supports. This, however, is liabk' to 

 intnxluce an objectionable amount of friction during an earthquake : and, ai>art 

 from it, the numerous joints can scarcely be prevented from causing more fric- 

 tional resistance than is permissible in a good seismometer. The up]>er links, 

 which are ties, may be made of flexible cord or wire instead being of jointed, but 

 even then so many joints will remain as to constitute a .serious defect. 



More recently several methods of astatic suspension have occurred to the 

 present writer, one of which, in particular, seems well adai)ted to the measure- 

 ment of earthquake motions, especially when the.se are great. 



It is ol)vious that the ma.-s who.se inertia is to give steadiness in a sei.-mo- 

 meter may be hung in neutral ef|uilibrium by any system of links which ])roduces 

 an exactly or apjjroximately straight-line motion, provided the links are placed 

 so as to make this line of motion horizontal. We might, for example, suspend 

 a ma.*s i)y a jiair of Peaucellier linkages, with freedom to move horizontally, and 

 thereby obtain al).<olute a.-^taticism throughout the whole range of its motion ; or, 

 by reducing the distance between the two pivot points in each linkage, wc might 

 compel the mass to move in an arc of very large radiu.«, and therein' give it the 

 slight >tai)ility needful in a seismometer. But this jilan would be o]icn to the 

 .same objection as the susjK'nsion by Watt's linkage propo.-jcd by Mr. West, — the 

 multiplicity of joints would give rise to an intolerable amount of friction. 



To avoid frif-tifin we should s»-lect a form of linkage with as few links as 

 l)os>ibli-, and the»«.' all ties, in order that we may easily sut)stitute ile.xilile cords 

 for rigid pieces with joints. No linkage sati.sfies these conditions better than the 

 approximate .straight-line motion of Tchebich"ff, illustrated in the sketch. AÄ 

 are fixed sii|)fK)rts. f) n are two equal links, whirji, when thr apparatus takes 

 this fcrni, may Im- flexible cords. Tliey cro.ss each other and are connected to 

 the end of a hanging Iwr C. The vertical distance of the middle of C from the 

 Iineyl.i4 is ocjual to the di.-lanc«.' A A, while the length of thi- hangin'.; bar is 



