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 Rantowles 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 retir emen t, with abundant time at his disposal, his active 
mind was much employed in the further study of volcanoes and earth- 
quakes. His„ lates t publication on the latter subject is a book entitled 
“Earthquakes in the Light of the Hew 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. Ch apter 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 A-olcanic ; 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 
