May 20, 1922] 



NA TURE 



653 



which a discussion is feasible, and the most promising 

 of these seems to depend on the fact that the downward 

 pressure is greatest at the time when the attracting 

 body is on the horizon, and least when it is on the 

 meridian. If, then, we divide an earthquake record 

 into two groups, one containing all shocks which occur 

 within six hours before a meridian passage, and the 

 other all that happened within six hours after, one 

 of the two groups will cover a period during which 

 the downward pressure is, on the average, increasing, 

 while the other will cover the period during which it 

 is decreasing. As the amount of the change so intro- 

 duced is known, with sufficient accuracy for the present 

 purpose, and as it should, on the hypothesis being used, 

 influence the frequency of earthquakes, it follows that 

 we have here a method, which should enable us to 

 make an estimate of the rate of growth of the strain, 

 to which fracture is due. 



Although simple in principle, the method is difficult 

 in application. To begin with, a record is required of 

 sufficient extent and continuity to give a trustworthy 

 average, not merely of the general frequency, but also 

 of the frequency in each of the two sections into which 

 it is divided, and this in practice means that the record 

 must contain at least two thousand shocks and ought 

 to contain double that number or more. Then it must 

 be reasonably accurate as to times and complete as to 

 occurrences, or at least must be fairly uniform in its 

 incompleteness over the whole period investigated. 

 There are not many records which fulfil these primary 

 requirements, but there is another even more important. 

 In all records there is a noticeable variation in frequency 

 at different times of the day, moreover, the nature of 

 this diurnal variation has been found to vary in different 

 regions, but appears to be constant and characteristic, 

 in each region, over the period of record. The cause 

 of this periodicity may reasonably be attributed to 

 some efifect, meteorological or other, connected with 

 the daily course of the sun, but its nature, no less 

 than its variability, shows that it can only be attributed 

 in part, if at all, to gravitational attraction. It is 



only, therefore, by a conversion of the record from 

 solar to lunar times that the influences of these other 

 effects can be eliminated, and the gravitational attrac- 

 tion of the moon be detected and estimated, and, 

 for the satisfactory application of this method, it is 

 necessary that the record should cover a complete 

 lunar cycle, or a period of nineteen years. There are 

 only two records extant and available which fulfil 

 this requirement, and of these the Italian is not only 

 the most complete and accurate, but is the only one 

 to which the conversion into lunar times has been 

 applied. 



From the summary of the figures obtained, published, 

 in our Quarterly Journal, it appears that in the six 

 hours preceding and following a meridian passage the 

 mean departure, from the general average for six 

 lunar hours, is almost exactly i per cent, of the mean. 



Passing over details of calculation, the average rate 

 of growth of strain is found to be such that the breaking 

 point would be reached in about two months from 

 the start, with a wide variation on either side. Some 

 other relations between the frequency of earthquakes 

 and the diurnal variation of the tidal stresses might 

 be, and have been, investigated ; all give fairly con- 

 firmatory results, the longest period indicated as 

 required for reaching the breaking strain being just 

 about a year. 



It must not be supposed that value can be attached 

 to the precise figures. As is invariably the case, in all 

 calculations regarding physics of the earth, many 

 considerations are involved of a very uncertain nature, 

 but the reasoning does show that the increase of strain 

 must have taken place at such rate that the breaking 

 point was reached in a period measurable at most by 

 months, and shows that the period could not have 

 been of such length as to be measurable by years or 

 decades, for, had this been the case, the disparity 

 dealt with would have been much greater than that 

 actually found. 



{To he continued.) 



Obituary. 



Prof. G. S. Boulger. 



BY the death of Prof. G. S. Boulger on May 4, 

 botanical science has lost an accurate and 

 advanced observer who did much to popularise the 

 study of his favourite science, and left his impress on 

 several generations of devoted students. He was an 

 active botanist to the end. On April 26 he attended, as 

 chairman, the meeting of the Botanical Section of the 

 South-eastern Union of Scientific Societies, and he 

 himself was struck by the difficulty in breathing which 

 he experienced in mounting a number of stairs to 

 the meeting-room. Almost his last words on his 

 death-bed had reference to the preparation of the 

 report to be presented to the Congress of the Union 

 at Southampton in June. He died literally in harness. 

 Prof. Boulger was born in 1853, and was educated 

 at Wellington and Epsom Colleges ; at an early age 

 he became Professor of Natural History at Ciren- 

 cester College, holding the chair for thirty years. 

 Among other appointments which he held were those 



NO. 2742, VOL. 109] 



of professor of geology and botany at the City of 

 London College, and in recent years he was a guide at 

 the Imperial Institute. But his students were a wider 

 public than institutions afforded. He was in con- 

 siderable request as a public lecturer, and frequently 

 visited, as such, various local natural history societies. 

 He was closely connected with the Selborne Society, 

 of which he was a vice-president, and the magazine 

 of which. Nature Notes, he edited for some years. In 

 recent years he was much interested in what has come 

 to be known as Regional Survey, and in this connection 

 did good work for the Gilbert White Fellowship, an 

 up-to-date survey of Selborne being now in progress. 



As an author Prof. Boulger had an attractive style. 

 For some time he edited the Proceedings of the 

 Geologists' Association. His knowledge of geology 

 enabled him to throw considerable light on the origin 

 of the British flora. His " Biographical Index of 

 British and Irish Botanists," with Mr. J. Britten, is a 

 valuable book of reference. His " Familiar Trees," 



