538 POPULAR SCIENCE MONTHLY 



questions of the gravest importance; and it is well worth while to 

 review the little knowledge already accumulated with the utmost care 

 to see whether it will give us even provisional answers to them. 



Professor Milne, in the tenth report of the British Association com- 

 mittee, refers the ' world-shaking ' earthquakes observed in the six 

 years 1899-1905 to thirteen great earthquake regions, designated by the 

 first thirteen letters of the alphabet. Three of these, I, J and L, are 

 responsible for only five, three and two shocks respectively, .and are 

 thus of small importance compared with the others, which average 

 about forty shocks each. Excluding them for the present, the remain- 

 ing ten regions lie approximately in two rings on the earth's surface, 

 a configuration whicb is most strikingly apparent when the regions are 

 marked on a globe. The more important ring includes the following 

 seven regions: A (Alaskan coast), B (Californian coast), C (West 

 Indies), D (Chilian coast), M (South of New Zealand), F (Krakatoa 

 region), E (Japan). Its center is among the conspicuous group of 

 islands which includes Tahiti, and the radius of the ring is about 65 

 degrees. The other ring has its center at the opposite point of the 

 earth, which is in the Sahara desert ; and at a radius of 50 degrees from 

 this center lie regions G (between India and Madagascar), H (the 

 Azores) and K (Tashkend). Now, this is not merely a convenient 

 geographical summary, but a physical fact of vital importance, accord- 

 ing to recent researches by Professor Jeans. In a remarkable paper 

 read before the Eoyal Society in 1903 be gave reasons for believing 

 that the earth is by no means a sphere or a spheroid, as we have been 

 accustomed to think, but is of a pear-shape. Under gravitational stress 

 it is continually approaching the spheroidal form — the pear is being 

 crushed into a sphere by its own attraction; and the result is a series 

 of earthquakes. These naturally occur in the weakest places, and if 

 any one will experiment in crushing a pear towards a spherical shape, 

 or even draw a diagram and consider where the weakest points would 

 be, the reasons for the existence of two rings of greatest weakness will 

 readily suggest themselves. The ends of the pear are the centers of 

 these rings, one in Africa, one in the Pacific; and when once this is 

 pointed out, the pear-shape of the earth is, according to Professor 

 Sollas, ' obvious to mere inspection ; it is a geographical fact and not 

 a speculation.' Professor Sollas is indeed responsible for the particular 

 suggestion above sketched; for Professor Jeans had originally proposed 

 a different axis, which he withdrew in favor of the obvious improve- 

 ment. The confirmation of Professor Sollas's view from the distribu- 

 tion of earthquake centers is remarkable. It does not seem, however, 

 quite certain which is the blunt end of the pear; it has been hitherto 

 placed in Africa, but there seem to be several reasons for regarding 



