54Q POPULAR SCIENCE MONTHLY 



them is approximately known already; so that, if the presumed con- 

 nection between them and earthquakes is confirmed, we may be able 

 to predict periods of great earthquake frequency. Such periods would 

 be in some respects analogous to the times of spring-tides. It is a 

 familiar fact that at new and full moon the tides are much greater than 

 when the moon is at the quarters. The reason is that we have two 

 tide-raising bodies, the moon and the sun, which sometimes act in con- 

 cert, and then we get large tides; sometimes in opposition, and then 

 we get small tides. If the influence of these two bodies were more 

 nearly equal, instead of the moon being so predominant a partner, we 

 can imagine times when the tides would be barely perceptible. Sim- 

 ilarly there are apparently two contributors to the variation in our 

 earth's rotation, which sometimes act in unison and sometimes in op- 

 position. They are more nearly equal in influence than are our moon 

 and sun; and consequently there are times when these two contributors 

 nearly balance one another and the axis of rotation remains almost 

 steady. But in due time the contributors reinforce one another and 

 the axis acquires a considerable ' wobble.' Each end of the axis then 

 describes a curve composed of wide sweeps and sharp bends; and the 

 evidence seems to be that at the sharp bends we are particularly liable 

 to earthquakes. The exact statement of the case as given by Professor 

 Milne in his Bakerian lecture, ' Eecent Advances in Seismology,' de- 

 livered before the Royal Society, on March 22, last, is as follows : 



In a period of nearly thirteen years (1892 to 1904) I find records for at 

 least 750 world-shaking earthquakes, which may be referred to three periods 

 continuous with each other, and each two-tenths of a year or 73 days' duration. 

 The first period occurred when the pole movement followed an approximately 

 straight line or curve of large radius, the second equal period when it was 

 undergoing deflection or following a path of short radius, and the third when 

 the movement was similar to that of the first period. The numbers of earth- 

 quakes in each of these periods taken in the order named were 211, 307 and 232 

 — that is to say, during the period when the change in direction of motion has 

 been comparatively rapid the relief of seismic strain has not only been marked, 

 but it has been localized along the junctions of land blocks and land plains 

 where we should expect to find that the stress due to change in direction of 

 motion was at a maximum. Until the magnitude of these induced stresses has 

 been estimated it would be premature to assume that the frequency under con- 

 sideration is directly due to change in direction of pole movement, it being quite 

 as likely that both phenomena may result from a general cause. 



It is eminently to be desired that a mathematical investigation of 

 the point should be undertaken; but the difficulties are very great, and 

 as yet no one has had the time and courage to attack them. It will 

 be seen, then, that the seismologist is as yet not able to give forecasts 

 of any commercial value, though he is by no means without hope of 

 doing so. 



