90 
REPORT--1847. 
necessary to cause actual dislocation. In this form the theory would be only 
a modification of that wJiich has been already developed. The primary 
phsenomena would be accounted for in the some manner in either caK, 
leaving the secondary phaenomena only to be affected by the undulating 
movement of the wave. Still the production of a wave of great magnitude 
w'ould present the difficulty already insisted on, and so far as we may admit 
the existence of an undulatory wave at all, I should be disposed to regard it, 
not as the immediate cause of the primary fissures, but as the consequtnet nf 
a great fissure produced by the general tension of the maw, and the sud¬ 
den elevation which might result from it along the line of dislocatiou. It 
is very possible, 1 conceive, that a surface might be thus generated, 
■which, though comparatively small, might produce sensible effects on the 
crust already fissured, or might even in sonte cases be effective iu producing 
new fissures. We may renmrk that fissures attributable to this cause, since 
they must be parallel to the wave producing them, would be generally 
parallel to the great primary fissure, from the formation of which we are 
here supposing the. surface wove to have originated. ^ 
§ On the Observations required for the determination of the CetUre of 
Earthquake Vihrationfi, and on the Requisites of the Instruments to be em¬ 
ployed. 
48. I should exceed the iMujnds proscribed to me in this H©port, if I were 
to enter into any discussion of the mechanical contrivances by which iustru- 
lueuts may be rendercil sufficiently ancurute and sensible to record tliepre- | 
cise character of nu (jarthrjuakc vihriitioii and the instant of it* occurrence; 
but it U3ay be well to make a few remarks suggested by the preceding theo¬ 
retical exposition, respecting the requisites of such in.<<trument». 
Taking the simplest ca.'se of vibration, that of o rectilinear one, three ele¬ 
ments are necessary for its complete determination. These may be, for 
mstance, the w'hoJe amplitude of the vibration ami two angles to determine 
ite direction; or the vertical and horizontal amplitudt^ of vibration with the 
direction of the latter; or the three auijditudcs parullel to throe fixed lines 
perpendicular to each other, of which one may be taken vertical and the 
other two horizoutal. In addition to (me of these sets of elementfi, or of 
some other equivalent set, it would )»e absolutely essential that the instru¬ 
ment should determine the exact instant of the beginnine of the vibratory 
motion by a self-registr-ring apparatus. 
I have already intimated that the most important result in a geological 
jwmt of view, to which we may expect observathujs on earthquake vibra¬ 
tions to le^ us, IS the position of the focus from wliich such vihratlous pro¬ 
ceed; and we may observe that the roughest approximation to this position 
would institute a very imporUut geological element, and might sweep away 
much that is vague and fanciful in geological speculation. The approxi* 
mate determination of the point C (fig. I.S) directly above the focus (sup- 
proceed from .a single centre), would probably prese«‘ 
mtle dithcully, and wouUl only t-equire two instruments placed at points on 
two lines from C, making an angle with each other not too remote from a 
right angle. This would itwjuire instruments cajjaWc of registering merely 
the honzonhd juiyection of the dirccUou iu which the points, where they 
wero respectively placed, should uiovi- at the first instant of the vibratory 
motion. The distinction between lids initial motion and that which any 
petiole may have «ul>sc(^uently, mmt be carcfullv attended to, because, as 
ainmdy explaiued (art. 42), it is only the initial motion which can be de- 
penaea on to indicate the direction of the waves propagation. Other 
