May 27, 1922] 



NATURE 



685 



its authority and prestige would have been very 

 different. It is a high honour to be selected one of a 

 small elite. It is a doubtful compliment to be included 

 in a miscellaneous crowd, and one may not feel very 

 proud to add the letters F.R.S.E. after one's name. 



It is not too much to say that the history of the 

 Royal Academy of Brussels for the last hundred years 

 is the history of Belgian science and Belgian learning. 

 Nothing is more remarkable in its activities than the 

 universality of their range. The Academy may claim 

 among its members men of world-wide reputation 

 in every branch of activity. It has produced mathe- 

 maticians and physicists like Quetelet and Stas ; 

 naturalists and chemists like van Beneden and Spring ; 

 jurists like Willems and Thonissen ; historians like 

 Gachard, Kurth, Cumont, and Pirenne. Looking at 

 the record of those eminent men, Belgium is entitled 

 to a high place amongst the smaller powers of Europe. 

 In one important respect the Royal Academy of 

 Belgium has set an example even to larger and more 

 ancient institutions. It has encouraged research and 

 learning not only by the number and value of its 

 publications. The publications of the Academy con- 

 stitute indeed a considerable library and are perhaps 

 only rivalled by the editions of that illustrious national 

 seat of learning, the Academy of Cracow. It has 

 published over two hundred volumes of Memoirs, one 

 hundred volumes of bulletins, a collection of about one 



hundred volumes of the national writers of Belgium, 

 including the Chronicles of Froissart and Commines 

 and Jehan le Bel. In addition it has published a 

 Dictionary of National Biography of which twenty-two 

 volumes have already appeared, and last but not least, 

 it has issued one hundred and thirty volumes of 

 Belgian Chronicles. 



The literary activity of the Academy has been all the 

 more useful because Belgian writers, owing to the exiguity 

 of the national territory, can appeal only to a small audi- 

 ence and are generally depending on the support of the 

 French reading public. That activity has been all the 

 more creditable because hitherto the Belgian Academy 

 has had no legal status and for that reason it cannot 

 administer directly any funds or property which may 

 be left to it. 



There are good reasons to hope that in the near 

 future the Belgian Academy will at last be endowed 

 with its civil and legal personality. Under that new 

 regime, under the patronage of an enlightened 

 sovereign, and with a national conscience which has 

 been quickened by the tragedy of the war, the Royal 

 Belgian Academy may look forward to an enlarged 

 sphere of scientific and literary usefulness. The 

 present historical record of the institution may 

 therefore be accepted not only as a fitting tribute to 

 the past, but as the promise of an even more brilliant 

 future. 



The Cause and Character of Earthquakes.^ 

 By R. D. Oldham, F.R.S. 



THE stress-difference required to produce fracture 

 in average hard rocks, as they are met with at the 

 surface, is round about 1,000,000 grammes per centi- 

 metre square, and, allowing for the greater strength 

 at depth, which is indicated by the experiments of 

 Prof. Adams and the computations of Prof. Barrell, 

 we may put the breaking strength of the earth's crust 

 at about double this quantity, so that to reach this point 

 in one year from starting, the strain would have to 

 increase at the rate of about 1400 grammes per centi- 

 metre square in each quarter of a day. According to 

 the late Sir George Darwin the stress-differences set 

 up by the moon in the latitude of Italy would amount 

 to about 20 grammes per centimetre square in an 

 incompressible Earth, and in a compressible Earth 

 with an incompressible crust, a condition much more 

 akin to what we have reason to suppose is the reality, 

 the stress-differences would be many times this figure ; 

 but even the lower amount is nearly \\ per cent, of 

 the growth required to reach breaking point in one 

 year ; it would be close on 15 per cent, if the period 

 is increased to ten years, and, with anything approach- 

 ing this proportion, a periodicity would result which 

 could not have escaped detection before now. 



The figures, therefore, give us a lower limit of the 

 rate of growth of strain ; it must have been something 

 faster than that needed to reach the breaking point 

 in one year from starting, if the differences on which 

 the argument is based are real. But are they real ? 

 The actual amount of difference, barely one per cent, 

 of the mean, is so small that it may well be fortuitous, 

 and the true interpretation may be that the gravita- 



' Continued from p. 653. 



NO. 2743, VOL. 109] 



tional stresses, and the stress-differences produced by 

 them, have no effect whatever in determining the time 

 of occurrence of an earthquake. If this be so, then 

 the rate of growth of strain becomes infinite, and each 

 earthquake becomes the result of a development of 

 strain, akin to an explosion in its suddenness. 



The truth may lie anywhere and must lie some- 

 where between these extremes, so we reach the con- 

 clusion that there is no support for the commonly 

 accepted notion of a continuous, slow growth of strain, 

 extending over years, decades, or even centuries, before 

 the breaking point is reached ; on the contrary, it 

 appears that the cause of earthquakes must be a rapid 

 growth of strain. This strain cannot be developed 

 without some deformation, but the magnitude of this 

 has no relation to the frequency or magnitude of the 

 earthquakes ; if change of form is slow and prolonged, 

 reUef may be provided by gradual yielding, if rapid, a 

 very small amount of distortion may lead to fracture, 

 and on the extent, form, and position of this fracture 

 will depend the character of the resulting earthquake. 



This study of the rate of growth leads to the 

 question, which is the really important one in its 

 bearing on geology, of how the strain is produced. It 

 can scarcely be the result of those tectonic processes 

 which result in folding, for these must necessarily be 

 slow in their action ; the change of form involved in 

 the bending of solid rock from its original shape into 

 complicated folds, without breach of continuity, can 

 only have been a slow one, and, as we have seen, the 

 deformation which produces earthquakes must be a 

 rapid one. With faults the case is different ; many 



