42 



NA TURE 



[May io, 1906 



twenty-six plates obtained between November 9, 1901, and 

 March 23, 1905, with the No. iv. spectrograph and the 

 325 cm. refractor of the Potsdam Observatory. 



The values obtained for the velocity, referred to the sun, 

 vary between +5-3 km. (on November g, 1901) and 

 — i6-g km. (on December 11, 1902). 



E.iRLY Observations of Eros. — No. 10, vol. liii., of the 

 Harvard College Observatory Annals contains the details 

 of a number of observations of Eros made at Harvard from 

 twenty-one photographs obtained during the period 1893 

 (October) to 1896 (June). 



The measurements of these plates were published in 

 Circular No. 51 of the observatory, but in the present 

 publication the whole of the data relating to the plates, 

 the original measurements of the photographs, the positions 

 of the standard stars employed, reproductions of the photo- 

 graphs, and many other important matters are dealt with 

 in great detail. 



As this number forms the concluding part of vol. liii. 

 of the Annals, several reproductions previously given in the 

 text are now reproduced on plates in a much more satis- 

 factory manner, and published as an appendix. 



Oeserv.^tions of Satellites in 1904 and 1905. — In 

 No. 94 of the Lick Observatory Bulletins Prof. R. G. 

 .^itken publishes the results of the observations of satellites 

 made at Lick during 1904 and 1905. 



Forty-seven observations of the satellites of Uranus were 

 made, the position angle and distance of each object being 

 referred to those of another satellite. 



The second part of the publication refers to the observ- 

 ations of Saturn's satellites during 1905, which were, in 

 some measure, a continuation of Prof. Hussey's work in 

 previous years. Only those combinations most likely to 

 improve our knowledge of the orbits of the inner satellites, 

 I.e. Rhea with Dione, Tethys with Enceladus, and, as a 

 check, Tethys with Rhea, were, however, measured. Four 

 eclipses of Saturn's satellites were also observed. 



Observations of Jupiter's fifth satellite, made during 1904 

 and 1905, referring this object to the three inner satellites, 

 form the subject of the concluding section of the Bulletin. 



New Variable Stars in Orion. — From a study of the 

 Heidelberg 6-inch plates, Prof. Max Wolf has discovered 

 seven new variables in Orion. 



Photomicrograph'ic reproductions, through a microscope, 

 of the regions containing the stars on the 6-inch plates are 

 given, together with the positions and observed variations 

 of the seven objects, in No. 4085 of the Astronomische 

 Nachrichten. 



RECENT ADVANCES IN SEISMOLOGY.-' 

 'T'iiE most remarkable development in modern seismology 

 is not the seismic survey of a city, or even of a 

 country, but of the whole world. This branch of inquiry 

 is now in active progress. Since the time of the great 

 earthquake of Lisbon in 1755 it has been known that dis- 

 turbances of the magnitude of that event, although not 

 directly recognisable as earthquakes in regions distant from 

 the origin, have nevertheless given evidence of commotion 

 by causing the water in lakes and ponds to oscillate. By 

 observing and timing the movements of the bubbles of 

 sensitive levels, astronomers have recorded unfelt pulsatory 

 movements of the ground which they showed to be the 

 result of seismic disturbances in far distant countries. In 

 Japan these unfelt movements have been automatically 

 recorded since 1884 (Seis. Soc. Trans., vol. x., p. 6). Thev 

 were recognised to have originated at a great distance, but 

 the centres from which they sprang were not determined. 

 Some years later, while seeking for a gravitational influence 

 of the moon, the late Dr. E. von Rebeur-Paschwitz found 

 on his records abnormal movements, several of which he 

 traced to definite but very distant seismic centres. Before 

 this, indeed, it had been predicted that a large earthquake 

 occurring in any one part of the world would produce move- 

 ments which, with proper instruments, would be recorded 

 in any other part," but it was not until after von Rebeur's 

 announcement that serious attention was directed to what 



1 Abridged from ihe Bakerian Lecture delivered by Prof. John Milne, 

 F.R.S., at the Royal Society on March 22. 



2 See " Earthquakes," p. 226, International Scientific Series, 1883. 



NO. 1906, VOL. 74] 



has proved to be a line of research open to workers in all 

 countries. Many instruments have been designed to record 

 these unfelt breathings of our earth, but there is still much 

 uncertainty in the interpretation of all their records. 



Observations also show that large earth-waves are from 

 time to time propagated over the whole surface of the globe. 

 These far-reaching commotions lead to the inference that 

 their originating impulse must have been delivered over a 

 large region. Harboe has shown that within a meizo- 

 seismic area blows of varying intensity have been struck 

 in quick succession at points long distances apart. A 

 district appears to have given way, not simply along the 

 line of one large fault, but along many minor faults. 

 Oldham estimated that the Assam earthquake of 1897 had 

 been accompanied by the bodily displacement of 10,000 

 square miles of country along a thrust plane. If we inter- 

 pret the time observations made in connection with this 

 disturbance in the light of the suggestion made by Harboe, 

 then this relief of seismic strain originated over an area 

 of 500,000 square miles. 



Although a large block of the earth's crust may thus be 

 fractured, our knowledge of the depth to which the effects 

 of fracturing descend is largely one of inference. From 

 the observations hitherto published, which are now in pro- 

 gress at Przibram, it would seem that a seismogram 

 obtained at a depth of 1150 metres differs but little from 

 one obtained on the surface. This is contrary to observ- 

 ations on small earthquakes, which, although they may 

 alarm the inhabitants of a town and shatter chimneys, 

 may pass unnoticed in shallow mines. 



The fact that the large earth-waves have what is prac- 

 tically a constant arcual velocity of approximately 3 km. 

 per second, whether the path be across continents, over 

 ocean floors, or over districts which vary greatly in their 

 geological structure, suggests the idea that the crust of the 

 earth is moved as a whole, and that under the influence 

 of its own elasticity and gravity it behaves in a manner 

 similar to a sheet of ice upon an ocean swell. An 

 alternative view is to assume that the wave motion is due 

 to energy retained within the crust itself, the heterogeneity 

 of which is superficial. W^hichever be the case, we may 

 picture a crust yielding irregularly, and possibly through 

 its total thickness, until it gives up its energy to a medium 

 which transmits undulatory movements with uniform 

 velocity. 



Many hypotheses have been adduced which suggest thick- 

 nesses for the superficial covering of our globe. To these 

 as an outcome of recent seismological research we may add 

 one more. Preceding the large waves of a teleseismic dis- 

 turbance we find preliminary tremors. These are appar- 

 ently propagated through the body of the globe with an 

 average speed along paths which are assumed to be chords 

 at about 10 km. per second. This high and nearly 

 constant rate of transmission, however, only obtains for 

 paths which represent arcs greater than 30°. For chords 

 which lie within a depth of thirty miles the recorded speeds 

 do not exceed those which we should expect for waves of 

 compression in rocky material. This, therefore, is a maxi- 

 mum depth at which we should look for materials having 

 similar physical properties to those we see on the earth's 

 surface. Beneath this limit the materials of the outer part 

 of this planet appear rapidly to merge into a fairly homo- 

 geneous nucleus with a high rigidity. Following closely 

 on the heels of the preliminary tremors, but in advance of 

 the large undulations, a second phase of motion appears, 

 the chordal velocity of which up to distances of 120° is 

 approximately 6 km. per second. These are tentatively 

 regarded as the outcrop of distortional waves. When these 

 are better understood it may be expected that they also will 

 play their part in shedding fresh light upon the physics of 

 the earth. 



I will now turn to a consideration of the regions in 

 which these sudden accelerations of geological change are 

 in operation. Thev may be grouped as follows ; — 



Regions which lie on the western suboceanic frontier of 

 the American and the eastern frontier of the Asiatic con- 

 tinents, and regions which lie on a band passing from the 

 West Indies through the Mediterranean to the Himalayas. 



In addition to these there are two minor regions, one 

 following the eastern suboceanic frontier of the African 

 continent, which I have called the Malagasy region, and 



