20 



NATURE 



[November 3, 1904 



October 29, 1900, in Europe and Japan, he arrives at the 

 conclusion that if the earth consists of a central core and 

 an outer shell, each of uniform composition, the outer shell 

 must have a thickness of not more than 500 km. Ihis 

 result would fall in with Milne's hypothesis, but as this is 

 considered to be inconsistent with the facts of astronomy, he 

 adopts the conclusion that there is a continuous increase in 

 the rate of propagation from the surface to the centre of 

 the earth, this increase being much more rapid near the 

 surface than at greater depths; this condition would result 

 in the wave motion being propagated along curvilinear 

 paths, and give rise to a small apparent rate of propagation 

 near the origin as compared with that found at greater 

 distances. The value of Prof. Liska's conclusion is 

 diminished bv the fact that it is based on the consideration 

 of only a single earthquake, the time of origin of which 

 is not known by direct observation. 



In the Boll, dell Accademia Gioenia di Scienze Naturali in 

 Catania of February, 1904, Prof. Ricco returns to the con- 

 sideration of the gravitational anomalies he has detected 

 under Mount Etna, and shows that they are accompanied by 

 corresponding irregularities in the course of the lines of equal 

 magnetic force. Prof. Ricco merely records the fact of these 

 magnetic irregularities, but the observation is important in 

 its bearing on the explanation of the gravitational anomaly, 

 which is equivalent to the removal of more than 1000 metres 

 in thickness of rock, at sea level, from under the summit of 

 the mountain. It is inconceivable that this can be due to 

 the existence of huge cavities in the earth ; more probably 

 the effect is due to the existence of a " root " of the moun- 

 tain, depressed into a denser magma, by the buoyancy of 

 which the visible mountain is supported. There is indepen- 

 dent geological evidence that Mount Etna lies over a region 

 of special subsidence, the basis of sedimentary rock on which 

 it was heaped up having been depressed during its form- 

 ation, and if we suppose this depression to have caused the 

 displacement of denser by less dense rocks to a considerable 

 depth, we get an explanation of both gravitational and 

 magnetic anomalies. A rough calculation shows that the 

 buoyancy of the downward protuberance would, on the most 

 favourable supposition, be inadequate to support the whole 

 weight of the mountain, and it must be concluded that 

 Mount Etna is not in a condition of complete isostacy, but 

 partially supported by an upward force. 



In No. I of the tenth volume of the Bolletino of the 

 Italian Seismological Society Prof. Grablovitz discusses the 

 vexed question of the nature of the wave motion in the 

 third phase of the record of a distant earthquake. The 

 occasion is the series of earthquakes which originated in the 

 Balkan peninsula on April 4, 1904 ; as registered at Ischia, 

 the great waves had a period of about 8 seconds, and, if the 

 records of the horizontal pendula are interpreted as due to 

 tilting, they indicate angular movements of as much as 

 100 seconds of arc, and this means a vertical movement of 

 more than 2 metres; in the same earthquakes the instru- 

 ment for recording the vertical component of the movement 

 gave only negative results. From this Prof. Grablovitz 

 concludes that the records obtained from the horizontal 

 pendula and the vasca sismica are not due to tilting ; he 

 admits that there may have been a small amount of vertical 

 movement which the instrument failed to record, but this 

 must have been much smaller than that obtained by calcu- 

 lation in the ordinary way. 



The same number contains a description, by Dr. 

 Agamennone, of a new form of very delicate seismoscope, 

 adapted for the detection of both near and distant earth- 

 quakes ; and an account, by D. Vassalo, illustrated by a 

 sketch plan, of the condition of Stromboli in June, 1904. 



Dr. R. von Kovesligethy, of Budapest, has made an 

 ingenious calculation of the work done by great earth- 

 quakes. Regarding the observed irregularities in the dis- 

 placement of the poles as compounded of a regular epicycloid 

 movement, and an irregular movement, which has been 

 shown by Prof. Milne to vary with the frequency of great 

 earthquakes, he calculates that each of the 200 great earth- 

 quakes registered during the eight years 1895-1902 caused 

 an average displacement of the pole through -o"-oo275; 

 the negative sign is interesting, as showing that the tendency 

 of great earthquakes is to diminish the departure of the 

 instantaneous from the mean axis of revolution. The work 

 done by this displacement is calculated as equivalent to that 

 NO. 1827, VOL. 71] 



which would be required to raise a mass equal to that of 

 the earth through 1.2 mm. at its surface {Die Erdbeben- 

 ■warte, iii., 1904. pp. 196-202). 



Prof. Omori contributes a note on the variations of sea 

 level on the east coast of Japan to part xiii. of vol. ii. of 

 the reports of the Tokio Physico-Mathematical Society. 

 The curves of barometric pressure and sea level are very 

 similar, and approximately reversed; the maximum sea level 

 is in September and the minimum in February, while the 

 minimum barometric pressure is in July and the maximum 

 in November. The range of barometric pressure is 93 m., 

 corresponding to 126 mm. of sea level, while the range of 

 sea level amounts to 276 mm. at Misaki and 219 mm. at 

 Ayukaua ; these figures show that while the local variations 

 of barometric pressure doubtless influence the level of the 

 .sea, this is also dependent on the variations of barometric 

 pressure over the Pacific Ocean. The net result is that the 

 variations of pressure on the bed of the sea are the opposite 

 of those on the adjoining land, and Prof. Omori correlates 

 this fact with the observed variations in frequency of earth- 

 quakes originating off the east coast of Japan. 



The Deutschen Rutidscliau, vol. xxvii., part i., contains 

 an interesting note, originally printed in the Honolulu 

 Evening Bulletin of June 21, 1904, by Dr. Otto Kuntze on 

 the present condition of Kilauea, which he describes as 

 being now dormant or extinct. There are no longer any 

 " lakes of fire "; the old lake of lava has cooled, and is 

 covered by a sheet of rock, and though steam issues from 

 some of the cracks in this, no molten, or even red-hot, rock 

 is now visible. A remarkable statement in the note is that 

 the lava lake, formerly visible, did not mark an active vent, 

 but was merely a reservoir of slowly cooling lava, which 

 had (lowed from the crater of Halemaumau and accumulated 

 in the lowest part of the caldera of Kilauea. There is no 

 authentic record of this crater, which rises from the floor 

 of the caldera, having been in eruption since June 24, 1S97, 

 and the paper contains some strongly worded comments on 

 the mis-statements regarding the present condition of the 

 crater, printed in the guide books issued by the tourist 

 agencies, mis-statements which are unnecessary, as Kilauea, 

 even in its existing condition, is nevertheless one of the most 

 interesting sights in the world, of which Dr. Kuntz claims 

 that few have seen more than himself. 



In No. 17 of the Publications of the Earthquake Investi- 

 gation Committee in Foreign Languages, .Mr. s. Kusakabe 

 continues his investigations of the modulus of elasticity of 

 rocks, and publishes some interesting results. He finds that 

 all rocks show a marked hysteresis, that is to say, when 

 e.\posed to a stress they go on yielding, apparently to an 

 indefinite extent, though after a while the effect is masked 

 by that due to changes of temperature, and when released 

 from the stress the recovery takes place at a continuously 

 decreasing rate, but apparently is never complete. Rocks 

 in a state of strain have a higher modulus of elasticity than 

 in the unstrained condition, and if exposed to a series of 

 alternating stresses, increasing and decreasing in opposite 

 directions, the mean modulus for the whole cycle is dis- 

 tinctly greater than that obtained by the usual method of 

 determination. The mean modulus of elasticity decreases 

 with the increase in amplitude of the cycle, from which it 

 is concluded that the rate of transmission of earthquake 

 waves is a function of their amplitude, and is less for a 

 larger than for a smaller amplitude. The tnodulus of 

 elasticity was found to have a maximum value at about 

 9° C, and to decrease by about half per cent, of its value 

 for each rise of one degree of temperature ; from this it 

 is inferred that there is a tendency towards a decrease in 

 the rate of transmission as the depth of the wave path 

 increases. On the other hand, the average rate of trans- 

 mission is higher in Archaean and Palaeozoic th.in in the 

 newer rocks, and from these two considerations the deduc- 

 tion is drawn that there is a level of maximum velocity of 

 transmission. We may point out that in arriving at this 

 conclusion no account is taken of the increase in pressure 

 with depth, and the consequent increase in compression of 

 the rocks. 



Prof. IiTiamura, in the Tokio Sugaktt-Butsurii;alikuiai 

 (Tokio Physico-Mathematical Society), vol. ii.. No. 13, 

 adopts the same notion that there is a level of maximum 

 rate of propagation, and places this level at .1 depth of a 

 few hundred kilometres. The estimate is based on the 



