8() 



> 84. I'e/orifij of Transit ol' Aiiiticia/ Earflupi a }:!'■■<. 



The method in questit>n ha.< been applied bv Messrs. Milne and Gray to 

 measure the speed of transit of artificial disturbances.* The bearing of the 

 origin being known, two stations were of course sufficient, and at them horizontal 

 pendulum seismographs were set. The soil was hardened mud. The velocities 

 found were 438 feet per second for normal waves, and 357 feet per se<'ond for 

 transverse waves. 



The earlier experiments of Malletf gave the following velocities: — 



In .sand 825 feet per second 



In jointed granite 1306 „ „ 



In solid granite 1665 „ „ 



But from the known elasticity and density of solid rock, and from other 

 observations, it is probable that the last named velocity is much too low ; and 

 that in a continuous mass of rock as high a velocity a^ 8000 or even 12000 feet 

 per second may be attained. 



ri 85. Experimental Tests of Sewnographs.% 



To find experimentally the accuracy with which a seismograph registers 

 motions of any assigned form and periixl, let the seismograph to be tested, with 

 it.« receiving plate and driving clock, complete, be ])laced on a shaky table. Side 

 by side with the multiplying lever of the instrument let a second lever be 

 arranged, pivotted by an altogether independent fixed support, and connected to 

 the table at one point, so as to give the same ratio of multiplication as the actual 

 in.^trument. This lever is to be provided with a point which writes on the plate 

 close by the marking ])oint of the seismogra]>h. If the table is now shaken, the 

 component of earth-motion transverse to the levers is recorded by both pointers; 

 and the agreement of the two records will show the accuracy of the seismograph. 

 To make the test as conclusive as possible the table should be forced to move in 

 such a manner as to give records resembling those which experience show> actual 

 earthi|nakes give. For a vertical-motion seismometer the same jilan may he 

 followed, if we use, instead of a table, a spring board capable of vertical shaking. 



In applying the test to a pair of horizontal pendulums with pivotted bobs, a 

 convenient plan is to place the two pendulums parallel or facing each other and 

 so that their marking points come on the same radius of the plate. Then let the 

 bol) of one be lieltl fixed i)y means of a bracket from a neighbouring wall : this 

 will ensure that its "steady line" is rigorously .steady. The steadiness of the 

 other is then tested by shaking the table, and seeing how closely accordant tiie 

 two records are. 



* Phil. Mag., Nov. 1881. Also Proe. Royal Society, Vol. XXXIII, p. 139. (Dec. 1881). 



t British Assoc. Reports for 1851 and 1852. 



t See also Proc. Royal Society. Vol. XXXI. p. 4J4. 



