518 



SCIENCE, 



[Vol. IV., No. 96. 



In fig. 3, Professor Ewing has carefully combined 

 the motion of the two components so as to give the 

 actual path of a point of the earth's surface for a 

 short interval. From p to q, following the arrow- 

 heads, it shows that motion (magnified six times) 

 during an interval of three seconds. 

 This same characteristic wriggling motion is shown 

 in fig. 4, which is the entire record, 

 magnified three and a half times, 

 upon a stationary plate, of an earth- 

 quake which occurred on April 23, 

 18S3, and lasted four minutes and 

 a half. It is quite likely that the 

 earthquakes which recently shook 

 up our middle and western states would have given 

 a somewhat similar record. 



The principal characteristics of the average earth- 

 quake in the plain of Tokio are : 1°. The motion of 

 the ground begins very gradually. 2°. An earth- 

 quake consists of many successive movements, and 

 there is almost always no single large one which 

 stands out prominently from the rest. 3°. The dis- 

 turbance ends even more gradually than it begins. 

 4°. The range, the period, and the direction of move- 

 ment, are exceedingly and irregularly variable during 

 any one earthquake. 5°. The duration of disturb- 

 ance of the ground is rarely less than one minute, and 

 often several minutes. 6°. Even in somewhat de- 

 structive earthquakes, the greatest displacement of a 

 point on the surface of the soil is only a few milli- 

 metres. 7°. The vertical motion is generally much 

 less than the horizontal. 8°. A mass shaken back 

 and forth in the most severe earthquakes of Tokio, 

 would, if it did not slide, be urged by a horizontal 

 force, which, at its maximum, would equal about 

 one thirty-third of its weight. This, regularly re- 

 peated, is sufficient to crack brick walls, and some- 

 times throw down chimneys. 



To the many readers who have had no experience 

 of earthquakes, but are accustomed to think of them 

 as a sudden violent thrust, accomplishing at a blow, 

 as it were, all their disastrous work, the preceding 

 descriptions will be a somewhat new revelation. 



Professor Ewing plainly shows that as seismom- 

 eters, the instruments in use by Palmieri and others 

 are worthless ; for not one of them can be depended 

 upon to give a reliable measure of the direction, 

 period, or amplitude of the vibrations of the ground, 

 most of them being designed to record a single violent 

 thrust in one direction, and nothing subsequent, and 

 the greater part of them being some form of stable 

 pendulum which is almost certain to be set swinging 

 in an earthquake through amplitudes far greater 

 than the earth itself, and thus to mask entirely the 

 motion of the latter. 



One novel ' time-taker,' the invention of Professor 

 Milne, whose work in Japan is so well known, is 

 worthy of note. A clock has its hour, minute, and 

 seconds hands all on the centre of the face, and of 

 different lengths, with their ends turned up into the 

 same plane, and tipped with cork smeared with print- 

 er's ink. In front is a track upon which, when a 

 seismoscope closes a circuit, a carriage travels up and 



presents a disk to the face, and then backs off again, 

 carrying an impression of the instantaneous position 

 of the three hands, and leaving the clock to go on 

 undisturbed. 



The closing chapter treats of the constructive de- 

 tails and requirements of a seismological observatory; 

 and a series of experiments by Professors Milne and 

 Gray are noticed, in which it was sought to deter- 

 mine, by a series of artificial earthquakes (dropping 

 heavy weights in a foundery, and exploding buried 

 cartridges of dynamite) in connection with time- 

 recording seismometers, the velocity of transmission 

 through the ground. These gave 438 feet per second 

 for normal, and 357 feet per second for transverse, 

 waves. This was through hardened mud. Mallet's 

 earlier experiments gave for sand 825 feet, for jointed 

 granite 1,306 feet, and for solid granite 1,665 feet, per 

 second. The last, Professor Ewing remarks, is prob- 

 ably very much too low. 



This element of earthquake motion, the velocity 

 of transmission through the earth's crust, is very in- 

 exactly known ; and the author notes the desirability 

 of extending the observation of earthquakes over a 

 considerable region of such a country as Japan by 

 means of many stations connected by telegraph, to 

 which simultaneous time-signals can be sent, and 

 at which the same earthquake may be recorded on 

 rotating plates, together with a record of the absolute 

 time. These, if sufficiently widely distributed and 

 numerous enough, would give us valuable data re- 

 garding the latitude, longitude, depth, and time of 

 the origin of the disturbance, and the velocity of its 

 transmission to the surface in all directions, suppos- 

 ing it rectilinear and uniform. Regarding the possi- 

 bility of this, Professor Ewing, in the article referred 

 to above {Nature, June 19, 1884), speaks as follows : 

 "But all this depends upon our being able to recog- 

 nize at the various stations, some one wave out of 

 the complex records deposited at each ; and, especial- 

 ly in view of the curvilinear nature of the motion, 

 it would be hazardous to say, without trial, whether 

 this can be done." 



In conclusion it may be said ,that the whole work is 

 exceedingly interesting and valuable; and Professor 

 Ewing is to be highly commended for thus bringing 

 together the best results of modern methods in exact 

 seismometry, and for showing the sources of error 

 and the fallacies in older methods and theories. The 

 work should receive as wide a distribution as possible 

 by the University of Tokio. H. M. Paul. 



EXCURSION MAP OF THE VICINITY 

 OF BALTIMORE. 



The need has long been felt, among those students 

 of the Johns Hopkins university who are especially 

 interested in the study of natural history, of a relia- 

 ble map of the adjoining country, on a suitable scale, 

 and so mounted as to be adapted for convenient pocket 

 use. It is believed that a few words regarding the 

 method by which the want of it has been recently 



