Vol. 62.] CONSTITUTION OF THE INTEEIOE OF THE EAKTH. 463 



the distance : the discrepancies being due, as has been explained, 

 partly to inaccuracies in the distant records, and partly to errors in 

 determining the time of origin. Another possible source of dis- 

 crepancy is the possibility that the rate of propagation is not uniform 

 in every direction, and that the time taken by wave-motion in 

 travelling, say from Japan to Europe, is different from that taken 

 by the same form of wave-motion in travelling from an equal 

 distance in America. There are some indications that such is the 

 case ; but the difference is small, in comparison with the whole 

 interval, and as the point is not material to the present investigation, 

 it may be ignored, and the irregularities smoothed to a regular 

 time- curve. 



This is best done graphically, as is represented in fig. 1 (p. 462), 

 where the averages of groups, and the single observations not adapted 

 to averaging, have been plotted, and average time-curves drawn for 

 the first and second phases. 



It will be seen that the time-curves of the first two phases are 

 very similar in shape, up to 120° from the origin ; but beyond this 

 they differ radically in form. That of the first phase, after an 

 irregularity between 130° and 140°, becomes very flat and proceeds 

 almost horizontally from 150° to 180° ; that of the second phase 

 comes to an end at 130° from the origin, and is continued some 

 11 minutes farther up. It is the explanation of these irregularities 

 with which this paper is mainly concerned ; and, to simplify the 

 consideration of this problem, I have found it convenient to tabulate 

 the figures represented by the time-curves, as they are more con- 

 cordant and useful than the individual records. This has been done 

 in the appended table (p. 464), which gives the intervals of time, 

 to the nearest even minute, at each 30° of arc from the origin 

 to its antipodes, the value for the last-named being inferred and 

 not the result of direct observation. 



In addition to the intervals of time, the mean apparent rate 

 of propagation along the arc and the chord is given, regarding 

 which a few words of explanation are requisite. The difference 

 between the true and the apparent rate of propagation of an 

 earthquake has long been familiar to seismologists. The first is 

 the rate as measured along the direction in which the wave-motion 

 is propagated, the latter is the resulting apparent rate of propagation, 

 measured in some other direction, usually along the surface of the 

 earth. Neither of these is readily determinable at any point, except 

 by the construction of the time-curves, such as are drawn in fig. 1 

 (p. 462) ; but if the time be known at two points, then the difference 

 in distance from the origin, divided by the difference in time, gives the 

 mean apparent rate of propagation as between those points. In the 

 table (p. 464) one point is supposed to be the origin, and two values 

 of the mean apparent rate of propagation in kilometres per second 

 are given, according as the distance is reckoned along the surface 

 of the earth or in a straight line through it. It is not suggested 

 that the wave-paths lie along either of these lines, but the values 

 calculated are the material from which the true form of the wave- 

 paths can be determined. 



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