ON THE VOLCANIC PHENOMENA OF JAPAN. 427 



Japan has been as great as in any other portions of the world. One period, 

 of unusual activity was between the years 1780 and 1800, a time when 

 there was great activity exhibited in other portions of the world. It was 

 during this period that a portion of Mount Unsen was destroyed and from 

 27,000 to 53,000 persons perished; that many islands were formed in the 

 Satsuma Sea ; that Sakurajima threw out so much pumiceous material 

 that it was possible to walk a distance of 23 miles upon the floating 

 debris in the sea, and that Asama ejected so many blocks of stone, some of 

 which are said to have been from 40 to over 100 feet in diameter, and a 

 lava stream 68 kilometers in length. 



8. The Form of Volcanoes. 



The form I particulai-ly refer to is the regular so-called conical form,, 

 which is very noticeable in many of the Japanese mountains, especially 

 perhaps in those of recent origin. Outlines of these volcanoes, as exhibited 

 either by sketches or photographs, show curvatures which are similar to 

 each other. In the Kurile Islands I have had opportunities of comparing 

 two volcanoes by so altering my position until one of the mountains 

 partially eclipsed another standing at no great distance in the background. 

 One of these mountains was Otosoyama (Mount Fuss). The other 

 mountain, like many of the peaks in the Kurile Islands, is without a 

 name. 



From a collection of photographs, I traced the profiles of a number of 

 important mountains in this country. These profiles are repeated in this 

 paper. From an examination of these figures I found that the curvature 

 of a typical volcano was logarithmic, or in other words the form of such 

 a mountain was such as might be produced by the revolution of a logarith- 

 mic curve round its asymptote. In my original paper on this subject I 

 said that the form agreed with that which would be produced by the 

 piling up of loose material. As pointed out by Mr. George F. Becker, in 

 a paper on the form of volcanic cones, &c. (' American Journal of Science,' 

 October 1885), I ought to have said it was the form due to a self- 

 supporting mass of coherent material. Mr. Becker continues my observa- 

 tions by an analytical investigation of the conditions of such equilibrium. 

 If the height of a column is a, its radius y, the distance of any horizontal 

 plane from the base x, the specific gravity of the material r, and the co- 

 efficient of resistance to crushing at the elastic limit k, then the equation 

 of the curve which, by its revolution about the x axis, will generate the 

 finite unloaded column of ' least variable resistance ' is 



r 



This latter quantity is of course diSerent for different materials. It 

 can be expressed in terms of x and y 



2 ;.• ^ y 



r ~ (tan^i— 1)4 

 d being the angle which the tangent at any point makes with the x 



axis. 



