150 



KNOWLEDGE. 



[July 2. 1900. 



second. With this estimate, the other observations 

 are in fairly close agreement. 



The IJnfelt Earthquake. 

 Far beyond the limits of the disturbed area, the earth- 

 (juake was recorded by many of the delicate instmments 

 constructed for the registration of distant shocks. § 

 All over Italy, from Ischia. and Catania in the 

 south to Pavia in the north, these instruments 

 began, one after the other, to write their records of the 

 movement, as the unfelt earth-waves sped outwai-ds from 

 the centre. Italy passed, the tale was taken up by 

 magnetographs at Potsdam and Wilhelmshaven, 

 Pawlovsk (near St. Petersbm-g), Copenhagen, Utrecht, 

 and Pare St. Maui- (near- Paris); by horizontal pen- 

 dulums at Strassburg and Shide (in the Isle of Wight), 

 and by a bifilar j^e^dulum at Edinburgh. Shide is 

 4891 miles from the centre of distui'bance, but the move- 

 ment could be traced for a distance greater even than 

 this. 



In the more complete records, and especially in those 

 given by the Italian microseismogi'aphs, Mr. Oldham 

 distinguishes three phases of motion. The first consists 

 of nearly horizontal rajjid displacements of the in- 

 strumente without any undulating movement of the 

 ground. In Italy, it begins at about 11.17 a.m. 

 (G.M.T.), that is, about I2h minutes after the com- 

 mencement of the shock at the epicentre. Without 

 any break in the movement, and after the lapse of about 

 8^ minutes, the second phase begins ; the vibrations are 

 similar to the preceding, but they are larger and more 

 open, and are accompanied by an unmistakable tilting 

 of the sui-face of the ground. Lastly, after the lajDse of 

 nearly 20 minutes more, the second phase gives place, 

 without interruption, to the third, consisting of well- 

 marked slow undulations, which have been aptly com- 

 pared to the movements caused by an ocean-swell. As 

 they travelled over Europe, the surface of the ground 

 was thrown into a series of flat waves, 34 miles in 

 length, and 20 inches in maximum height, the complete 

 period of each wave being 22 seconds. This phase is 

 by far the longest of the three; in the more sensitive 

 instruments, two or three hours elapsed before theii' 

 traces ceased to show signs of movement. 



As we know the distances of the different observa- 

 tories from the epicentre, and the times taken by each 

 phase to reach them, we can form some idea of the rates 

 at which they travelled. If the early tremors moved 

 in straight lines, their mean velocity for the first phase 

 was 9.0 kilometres per second or about 345 miles a 

 minute, and for the second 5.3 kilometres per second or 

 about 200 miles per minute. But if, as is probable, 

 they moved along curved paths through the body of the 

 earth, their mean velocities must have exceeded these 

 amounts. For the first undulations of the third phase, 

 the velocity would be 2.9 kilometres per second or 109 

 miles per minute if they travelled along straight lines, 

 or 3.0 kilometres per second or 115 miles per minute if 

 they moved along the surface of the earth. 



The existence of the second phase was noticed for 

 the first time by Mr. Oldham in the records of the 

 Indian earthquake, but he has since detected it in 

 those of other shocks. || He believes, in common with 

 most other seismologists, that the first phase corresponds 



§ For desoviptions of the more important see Srif. Assoc. Rep 

 1893, pp. 291-308 ; 1895, pp. 85-86 ; 1896, )>p. 40-19 ; ] 897. pp. 10-11. 

 Nature, Vol. 1... 1894, pp. 246-249. Natural Science, Vol. VIII., 

 1896, pp. 233-238 



i; I'hU. Trans., V.K*.K A., pp. 135-174. 



to waves of elastic compression travelling through the 

 body of the earth ; and he attributes the second to 

 waves of elastic distortion travelling in the same way, 

 in which the pai-ticles move at right angles to the 

 direction in which the wave travels, thus causing a 

 slight tilting of the surface. It is probable that the 

 waves of both phases move along curved, rather than 

 straight, lines through the earth, that the curves are 

 concave towards the surface, and that the velocities of 

 the waves increase with the depth of their path below 

 the surface. On the other hand, the surface-velocity 

 of the first undulations of the third phase is practically 

 constant for all distances from the epicentre, and, in 

 the case of the Indian earthquake, it agi-ees almost 

 exactly with that obtained for the velocity within the 

 disturbed area and as far as Bombay. It is therefore 

 difl&cult to resist the conclusion that the third phase 

 consists of undulations which travel along the surface 

 of the earth. 



If this be the case, we can imagine these undulations 

 speeding outwards from the epicentre in ever-widening 

 circles, until they have passed over a quarter-circum- 

 ference of the earth, when they should begin to converge 

 towards the antipodes. Here they should cross each 

 other, and again spread out as circular waves, once more 

 in their course passing the same obsei-vatories where 

 they were first recorded, but in the opposite order. It 

 has been resei-ved for the most violent earthquake 

 of modern times to verify this interesting conclusion. 

 Faint, but decided, are the traces of the second crossing. 

 At Edinburgh they occur at 2.6 p.m., and at about the 

 same time at Shide, at Leghorn 2.10, Catania 2.12|, 

 while at Ischia there are several movements between 

 2 and 3 p.m. At Rocca di Papa, near Rome, the time 

 is slightly earlier, but the undulations, like those at 

 the first crossing, have a complete period of about 

 20 seconds. The distances traversed by the waves are 

 more than 20,000, instead of less than 5000, miles ; 

 but the mean velocity of travel is almost exactly the 

 same as at first — namely, 2.95 kilometres per second, or 

 111 miles per minute. 



{To he concluded.) 



AMERICAN INDIANS. 



By R. Lydekker. 



Although now used in a totally different sense, the 

 title of Americans undoubtedly belongs by right of birth 

 to the aboriginal tribes who were in possession of the 

 New W^orld previous to the incursion of the white man 

 and their modern pure-bred descendants. But to change 

 this usage is now clearly impossible, and some other 

 general title must consequently be sought. By a curious 

 misapplication of terms the American aborigines ai-e 

 almost invariably spoken of as " Indians," while the 

 natives of Hindustan, to whom that name by right be- 

 longs, are scarcely ever so called, excejJt indeed by those 

 well-meaning enthusiasts who seek to claim " India for 

 the Indians." Still it is, on the whole, the wisest coiu'se 

 to bow to custom and accept the current name of 

 American Indians ; the alternative designation of 

 American Aborigines, or the Aborigines of America, 

 being too cumbersome for ordinary use. 



There is, it is true, the popular title of " Redskins " ; 

 but this, however expressive it may be, is somewhat too 

 " slangy " for present pui"poses. Moreover, it has 

 been objected to as inappropriate — but of this more 

 anon. 



