3 66 THE POPULAR SCIENCE MONTHLY. 



through, the Aleutian Islands to Kamchatka, and thence southward 

 through the Japanese Islands, the Philippines, and the Moluccas, to 

 Sumatra and Java. Another branch seems to run from Sumatra, 

 through New Guinea, to New Zealand, and the closed curve may per- 

 haps be completed through the Antarctic regions, which are known to 

 be volcanic. Returning to the first branch which we traced as far as 

 Java, to the westward the seismic areas become more patchy and less 

 linear. It may, however, perhaps be maintained that the ribbon runs 

 on through India, Persia, Syria, the Eastern Mediterranean, Greece, 

 and Italy. 



This grouping of seismic areas into a ribbon does not comprise all 

 the regions of earthquakes, but it must rather be taken as meaning 

 that there is one great principal line of cracking of the earth's surface. 

 In speaking here of earthquakes, those sensible shocks are meant which 

 are sufficiently severe to damage buildings, for, as will be explained 

 below, there is reason to believe that the whole earth is in a continual 

 state of tremor. 



Seismic areas are not absolutely constant in their limits, and cases 

 are known where regions previously quiescent have become disturbed. 

 It seems likely that the recent disastrous earthquake at Charleston 

 belongs to the West Indian system of seismic activity, but there is no 

 reason to suspect a permanent extension of the West Indian area so 

 as to embrace the Southern States. On the contrary, it is far more 

 probable that this disastrous shock will remain a unique occurrence. 

 The previous experience of great earthquakes, such as that of Lisbon 

 in the middle of the last century, shows, however, that the inhabitants 

 of Charleston must for the next year or two expect the recurrence of 

 slight shocks, and that the subterranean forces will then lull them- 

 selves to sleep again. 



With regard to the distribution of earthquakes in time there is no 

 evidence of either decrease or increase within historical periods, and 

 although physical considerations would lead us to suppose that they 

 were more frequent in early geological times, geology at least can 

 furnish no proof that this has been the case.* 



A great deal has been written on the causes of earthquakes, and 

 many of the suggested theories seem fanciful in the highest degree. 

 It is clear, however, that the primary cause resides in the upper layers 

 of the earth, and that the motive power is either directly or indirectly 

 the internal heat of the earth. The high temperature of the rocks, in 

 those little scratches in the earth's surface which we call mines, proves 

 the existence of abundant energy for the production of any amount of 

 disturbance of the upper layers. It only remains to consider how that 

 energy can be brought to bear. One way is by the slow shrinking of 

 the earth, consequent on its slow cooling. Now, the heterogeneity of 

 the upper layers makes it impossible that the shrinkage shall occur 

 * Geikic, "Contemporary Review," October, 1886. 



