148 



NA TURE 



[December 15, 1904 



likeh' that the greater number of earthquakes felt in 

 the world belong to this latter class. All of them 

 represent a relifcf of stress, and the discussion on the 

 sources of earth stresses, commencing with the con- 

 tractional hypothesis and concluding with the results 

 of investigations by Prof. George Darwin, are attrac- 

 tive not only to seismologists but to all who wish to 

 learn something about the inside of the world on which 

 they live. 



Some fifty pages are given up to descriptions of 

 seismoscopes and seismographs, attention being par- 

 ticularlv directed to those which record unfelt tele- 

 seismic movements. We cannot say that the concepts 

 relating to seismic wave motion put forward are 

 generally accepted, but such as they are we may say 

 that thev represent modern views. About the ampli- 

 tudes and periods of earthquake waves seismologists 

 have certain definite information, but about the magni- 

 tudes of these elements, particularly for waves which 

 have travelled over long paths, much has yet to be 

 learned. For this latter class of movement it is pointed 

 out that discordant results are found in tables showing 

 the speeds at which they were propagated. The author 

 inclines to the view that the differences which have 

 been noted are due to variability in the delicacy of 

 instruments employed to pick up a wave or wave 

 group. In great measure, this may be true, but it 

 seems to us that marked errors may also arise in con- 

 sequence of inaccuracy in determining the time at 

 which waves were generated at their origin. 



Then, again, there are those who incline to a belief, 

 which they sustain with arguments deserving close 

 consideration, that within our earth convection currents 

 e.xist ; it would follow from this that along similar 

 paths, or even along the same path, earthquake speeds 

 should vary. 



Notwithstanding these uncertainties, the author 

 holds the opinion that remarkable and unfexpected 

 results which fit well within errors of observation have 

 been reached. 



Two serious difficulties, for the explanation of which 

 we are asked to w-ait patiently, relate to the lengthen- 

 ing of wave periods and the total duration of a dis- 

 turbance as it radiates. We will suggest that the 

 former phenomenon may perhaps be at least partially 

 explained by assuming that in the vicinity of an origin 

 the records refer to forced vibrations, while at a 

 distance the motion represents a periodic natural move- 

 ment of the crust which varies with its heterogeneity. 

 With regard to the second difficulty, now and then we 

 have evidences that a disturbance recorded at a station 

 far removed from an origin may be reinforced and 

 lengthened by a repetition of the first disturbance 

 which has reached the station by travelling in an 

 opposite direction round the world. Generally, how- 

 ever, the record from a horizontal pendulum near to 

 an origin appears to move as long as, if not longer 

 than, a similar instrument at a distant station, which 

 means that in certain instances the author's difficulty 

 is non-existent. Finally, it must be borne in mind 

 that a single impulse at an origin results in the birth 

 of a series of waves which reach a distant station along 

 different paths and with different speeds, with the 

 NO. 1833, VOL. 71] 



result that a blow at an epicentre may at a distance 

 from the same be recorded as a long train of waves. 



When Major Dutton suggests to his readers that 

 the Seismological Investigation Committee of the 

 British Association carries on its work in consequence 

 of financial aid received from the British Government, 

 we recognise that he shares a widespread mis- 

 apprehension. 



Much is said relating to the elasticity of rocks, in 

 connection with which an elaborate table, the result of 

 investigations made by Prof. Nagaoka, of Tokio, is 

 reproduced. A second long table is that drawn up by 

 M. Montessus de Ballore relating to the distribution 

 of seismicity. 



The illustrations, of which there are sixty-three, are 

 for the most part excellent, but there are one or two 

 photomechanical reproductions of instruments which 

 we imagine will give more delight to their authors at 

 the sight of their own shaky caligraphy than to the 

 ordinary reader. 



Taken as a whole, the work is one to be read by all 

 who wish to know what is known respecting the pro- 

 pagation of wave motion in our earth since the in- 

 vention of the seismograph, and it is destined to receive 

 a hearty welcome. 



TECHNICAL MECHANICS. 

 Die technische Mechanik : elementares Lehrbuch fur 

 mittlere maschienentechnische Fachschulen und 

 Hilfsbuch fiir studierende hoherer technischer 

 Lehranstaltcn. By P. Stephan, &c. Erster Teil : 

 Mechanik Starrer Korper. Pp. viii + 344. (Leipzig: 

 Teubner, 1904.) Price 7 marks. 

 T N the very early part of this excellent work there 

 -'- is a certain lack of system, inasmuch as, although 

 the author very properly treats first of the equilibrium 

 of a particle, he assumes the nature of the stress 

 exerted in such rigid bodies as the bars of a frame- 

 work, the crank and connecting rod of an engine, &c. 

 The nature of such forces is never properly appreciated 

 by the student who is truly a beginner in the subject 

 of dynamics — and, indeed, there is no part of statics 

 in which students of even very considerable experience 

 are so apt to go wrong as that relating to the forces 

 exerted by jointed bars. The author treats from the 

 outset the equilibrium of forces acting in space of three 

 dimensions without having previously disposed of the 

 simpler two dimensional case, a course which meets 

 with the approval of many teachers, although it seems 

 to the reviewer to be the less simple method. Herr 

 Stephan enunciates the parallelogram law for the com- 

 position of forces (or vectors generally) at the outset, 

 and assumes it as a result of experiment — which, on 

 the whole, is perhaps the wisest plan for a teacher. 

 Near the end of the book, however, he gives the 

 ordinary Newtonian proof of the proposition. 



He gives very early and very clearly the method 

 of determining the resultant of a system of coplanar 

 forces acting on a body (other than a particle) by means 

 of the force and funicular polygons — a subject in 

 which English students are, as a rule, extremely weak. 

 There is a section on the determination of the centres 

 of gravity of all the bodies usually figured in our 



