218 



REPORTS ON THE STATE OF SCIENCE, ETC. 



It will be seen that the adopted tables are in the main correct with the following 

 exceptions : — 



(a) For values of A from 20° to 50° there appears to be a double maximum in 

 both P and S. The principal maximum is not far from the tables, though both P and S 

 tables require a small negative correction near A=35°. The subsidiary maximum 

 may either be real and distinct, involving a considerable departure from the tables, 

 in which case the question arises which are the true P and S. Or it may be that the 

 dupUcity of maximum is spurious, and in that case the correction to tables must lie 

 between the two values, much nearer the numerically smaller. 



The most interesting hypothesis is that the weaker and earlier maximum represents 

 the true P and S, or at any rate provided the angles of emergence measured by 

 Galitzin. If that is so, there is no difficulty in explaining why he could not reconcile 

 his values for the angle of emergence with the adopted tables for P, for the corrections 

 indicated above introduce a point of inflexion into the graph of 8P. 



(b) For values of A from 70° to 115° observations of S are liable to be observations 

 of [Sj or S C P C S, Gutenberg's wave which goes through the central core of the earth 

 as P. Attention was drawn to this phenomenon in the last Report. 



Various suggestions have been made for the improvement of the existing tables, 

 many of them based on the results for a single earthquake. The present discussion 

 of the results for a large number of earthquakes scattered over the earth during five 

 years suggests that there are several difficult questions to be considered before any 

 change in the adopted tables is made. Any such change is bound to cause confusion, 

 especially if it is made while our knowledge is still imperfect. 



Deep Focus. 



From the above-mentioned discussion of the large earthquakes in 1918-1922 the 

 cases of abnormal focus were excluded. They represent another fundamental 

 question on which opinion is divided. The cases of abnormal focus are a small 

 percentage of the whole, but are by this time sufficiently numerous to constitute a 

 considerable body of evidence in favour of the hypothesis put forward. The following 

 is a summary of the cases. The four quarters of the year are denoted by I, II, III, IV. 



Cases of ABNOEMAii Focus. 



In many of these cases a full discussion is given (in the International Summary 

 itself) of the evidence for the hypothesis. Three principal points are generally 

 examined with care. 



(1) The time T is shown to be well determined by the observations at a number 

 of stations near the epicentre. 



(2) The time of transmission to stations at the opposite side of the earth, as 

 measured from this T , is shown to differ sensibly from the average time, being less 

 than the average when the focus is deep, greater when the focus is higher than normal. 



(3) The observations at stations well distributed in azimuth round the epicentre 

 are all shown to require correction of one sign, applied as corrections to A. The 

 equations for correction to the position of the epicentre are usually solved both with 

 and without the corrections to A, and it is shown that one solution will work, the 

 other will not. 



