ON THE TRANSIT-VELOCITY OF EARTHQUAKE WAVES. 211 



" Second Report on Earthquakes," &c., Report of Brit. Assoc. 1851, pp. 287, 

 289, &c. 



On the present occasion, as a considerable time elapsed between the suc- 

 cessive experiments, during which the oil on the instrument more or less 

 changed its state, and as some were made in summer and others in winter, 

 it became necessary to rate the chronograph anew for each experiment, or 

 at least to verify the former rating ; for this end it was necessary to provide 

 a suitable loud-beating seconds clock with a divided arc to the pendulum, as 

 none such could be procured at Holyhead. The same weight was con- 

 .stantly used with the chronograph, and the extreme differences found in 

 the rating during the several years that these experiments have been in 

 progress were no more than the following : — 



// 

 Nov. 1856. Value in mean time of one division of the dial (a) = 0*01485 

 May 186]. Value of same = 0-01806 



Taking for illustration the former value of the smallest division of the 

 dial (a), we see that each division of the dial (J)) is equal to one revolution of 

 the index (a), and equal to 



0"-01485 X 30 = 0"-4455, 

 and one revolution of the index (&) equal to 



0"-4455 X 12 = 5"-346,— 

 an absolute rate of movement of the instrument not widely differing from 

 that employed in the experiments of Killiney and Dalkey, with which it is 

 desirable that the present results should be comparable. Half a small divi- 

 sion of the chronograph can be read ; we therefore in these experiments 

 possess the means of recording time to within 0"*0074', or to nearly y^'^'o^ths 

 of a second. 



The additional apparatus of the chronograph consisted merely of such 

 arrangements that the releasing lever {g), when pressed down by the hand 

 applied to the wood insulator at m, should dip at i into a mercury-cup, and 

 so make contact by the wires (h, b') between the poles of the contact-making 

 battery (E). 



It remains to describe the contact-maker (fig. 2, PI. IV.). c is the base of 

 the instrument of mahogany, carrying a vertical and bent arm (d) of cast 

 iron, into the upper forked end of which the central iron bars, of about |^ths 

 of an inch in diameter, of the electro-magnets a, a (seen in plan in fig. 3) are 

 secured by a cotter ; the coils of covered wire round these are continuous, 

 the wire (b) from the e+ pole passing at its further end from the first coil 

 over to the second, and at the extremity of the latter passing off to the e— pole 

 by b', the junctions being effected by mercury-cups in the usual way. n is 

 a sliding piece of wood, secured upon the base c when adjusted in place by 

 the screw at s ; this carries a wrought-iron lever armature (c), whose arms 

 are as 8 : 1, the shorter and rather heavier end being adjusted so as to be 

 beneath the poles of the electro-magnets, and at such a distance beneath 

 them that, upon passing the current through the coils, the magnets shall 

 readily attract the short end of this lever, snatch it up into contact with the 

 poles of the magnets, and in doing so depress the other or remote end of 

 the lever. The latter extremity of the lever is provided, as seen more at 

 large in figs. 4 and 5, with a forked pair of copper poles amalgamated, 

 which, when depressed by the action of the electro-magnets, dip into the mer- 

 cury of the cups/ and/, and in doing so close the holes of the firing-battery, 

 the conducting wires from which {h and Ji) dip respectively into mercury- 

 cups, which by a ttibe bored through the wood are in permanent communi- 

 cation with /and/ (cups) respectively. The lever and forked poles, &c., 



1'2 



