544 POPULAR SCIENCE MONTHLY 



the Snake Biver Valley, and while we are not in a position to compute 

 with accuracy their dimensions, we can say with confidence that the 

 volume of the largest of them does not exceed two cubic miles. The 

 great eruption of the Skaptar Jokul, in Iceland, in the year 1783, was 

 estimated by Dr. Thoroddson to have outpoured twelve or thirteen 

 cubic kilometers, or three cubic miles of lava. The greatest eruption 

 of which we have any estimate, and that is a very crude one, was at 

 Tomboro, on the island of Sumbawa, which was estimated to have dis- 

 charged about six cubic miles of lava. This estimate is regarded as 

 very excessive. 



On the same scale as before, these two eruptions are represented, 

 and you perceive how insignificant they are in mass in comparison with 

 the whole of the surrounding earth. 



3. The third general fact is the repetitive nature of volcanic erup- 

 tions. A single outbreak with none following is an exceedingly rare 

 phenomenon. Many eruptions, going often into the thousands, occur 

 before the climax is reached and the decline of activity follows. The 

 reason why a volcano, when its vent is once open, does not discharge 

 all the material in its reservoir in one stupendous belch and then close 

 up forever will be shortly brought up. 



4. The next general fact, which we can not claim to be proven, but 

 for which there is a growing mass of strong and highly concordant evi- 

 dence, is that the seat of the reservoir is very shallow and never more 

 than three miles deep. Very rarely is there any indication of its being 

 more than two and one half miles deep, and it is certain that in many 

 cases the depth is less than one mile. The indications are that most 

 of the volcanic eruptions originate at depths between one mile and two 

 and one half miles. The evidence of this is furnished by the earth- 

 quakes which almost always accompany them and which are associated 

 with them in such a way as to leave no doubt or question that they are 

 produced by the volcanic action. The radiation of the tremors of an 

 earthquake from their source in the earth is governed by substantially 

 the same law as sound. The intensity of these tremors where they 

 reach the earth's surface varies in a manner which is dependent upon 

 their depth of origin. In the discussion of the Charleston earthquake, 

 I pointed out one method by which that depth can be approximately 

 computed from the distribution of critical points of the surface in- 

 tensity. The method has been sharply criticized by able seismologists 

 as being liable to error through refraction of the rays of propagation 

 through rocks and media of variable density. But I observe that all 

 of them use that method with surprising consistency and satisfactory 

 results. 



The efficiency of this method depends mainly upon the accuracy 

 with which the intensity can be estimated along a line radiating from 



