14 PROCEEDINGS OF THE NEW HAVEN MEETING 



Dutton made a special study of the Charleston earthquake in 1886 and 

 devised a new method of ascertaining the depth of the earthquake focus, 

 and measured with greater accuracy than ever before attained the rate at 

 which an earthquake wave is propagated. His isoseismal method of com- 

 puting the depth of focus involves the determination of two critical 

 points : First, the epicentrum, and, second, a point on a radius from the 

 epicentrum at which the intensity of shock diminishes most rapidly. A 

 line drawn around the epicentrum through the points of most rapid 

 change of intensity Dutton called index circle, and pointed out that the 

 focal depth is the product of the radius on the index circle multiplied by 

 the square root of three. 



The Charleston earthquake had two foci. The depth of the Wood- 

 stock focus he computed to be twelve miles and of the Eantowles focus 

 nearly eight miles. The determination of the index circle, as Dutton 

 himself recognized, is a matter of difficulty, and the conclusions must be 

 regarded as only approximate. 



Concerning the rate of propagation, he remarks (22, page 211) : 



"After a careful study of all discussions of this particular problem, based 

 upon the observations made in other earthquakes, I have no hesitation in de- 

 claring my opinion that the result from the Charleston earthquake far out- 

 weighs them all, and that all preceding determinations of this quantity are 

 wholly invalid or wide of the mark." 



The average speed of propagation of the Charleston earthquake Dutton 

 determined from three groups of observations to be 5,184 meters per 

 second. He devoted much consideration to the nature and mechanism 

 of the earthquake wave motion. 



After his retirement, with abundant time at his disposal, his active 

 mind was much employed in the further study of volcanoes and earth- 

 quakes. His latest publication on the latter subject is a book entitled 

 "Earthquakes in the Light of the New Seismology," a most comprehen- 

 sive, instructive, and useful contribution to popular knowledge. To 

 quote his own words : 



"Chapter I sets forth the nature of an earthquake according to the modern 

 concepts. It defines the technical terms used in discussion, and describes the 

 action taking place on the surface of the ground during a quake of great 

 energy. Chapter II is a general discussion of the causes of earthquakes. Two 

 causes are recognized, apparently quite distinct, though possibly they may 

 have interrelations not yet recognized. The first cause is volcanic ; the second 

 is that force which is presumed to be always active in disturbing the rocks 

 which form the outer shell of the earth, resulting in the building of moun- 

 tains, the folding and shearing of the strata, and the elevation and depression 



