CHAPTER VI. 



MISCELLANEOUS INSTRUMENTS. 



§ 63. Fluid Pendulums. 



In Mallet's " Fourth Report <iii tlie Facts and Tlicmy of Eartliqiiake 

 Phrnomona "* an account will he fouud of a nuniher of early s^cis^lometer^;, most 

 of which may he classed either as solid jicndulums or as fluid jiendulums. To 

 the first class l)elong the simjile pendulum and the inverted pendulum of Forbes, 

 to wlii'h ullu>ion has already Ijeen made (§ 43). Under the second head we 

 may include all arranj^ements in which a mass of fluid is caused to oscillate 

 relatively to the earth hy the earth's movements. These last are nut well 

 adajited to the alisohitc measurement of earth(juakes, and are mentioned here 

 chiefly for the sake of showing tlicir inahility to act as anything more tlian 

 seismoscopes. Ijikc most other early seisnuimeters, they seem to have heen hased 

 on the idea tliat an earth(juake consists essentially of a single sudden impulse. 



( 1 ). Probably the oldest of these is that of Caceiatore, described thus by 

 Mallet '.^ — " It consists of a wooden circular dish about 1(1 in. diameter, placed 

 horizontallv and filled with mercury to the hrim-lcvcl of eight notches that face 

 the cardinal points and the bisecting rhumbs between, and are cut down through 

 the lip of the dish, equally in width and depth all round. Beneath each such 

 notch a small cup is [)laced, to receive such mercury as may be thrown out of 

 each notch by an oscillatory displacement of the main mass of mercury, due to a 

 ireneral oseillatit)n of the whole svstem. Either the volume or the weiirht of 

 mercury found in each cup is supposed to measure the value of the displacement, 

 and hence of the shock in its direction in azimuth." (2). Another, .suggested by 

 Babbage, is a bowl of mola-sses or other viscous liquid. (3). Another (Mallet) 

 is a cylindric tub with chalked or whitewashed sides, jiartially filled with liquid, 

 which is coloured .so that it may show the height to which it is washed up 

 during the distmhance. (4). An improvement on any of these is a set of U- 

 shajied tubes, placed in different azimuths, and partially filled with mercury 

 (Palmieri and Mallet). 



All these are liable to the objection already urged against a common short 

 ])endulum, — that of having too great stability. This makes the amount of their 

 movements dejiend .so greatly on the agreement or non-agreement of the liquid's 

 period of oscillation with that of the earthquake as to deprive these of all value, 

 even as indications of the relative intensity of different shocks. If we avoid 

 accumulated oscillation by using a viscous liquid, (2), we introduce a friction 

 error of great but, generally, very unequal amount in different cases. The 



* British Association Report for 1868, p. 72 et seq. 



