‘100 REPORT—1858. 
certain zones or areas of surface, more or less irregular, present themselves, 
within which the destructive effects upon buildings and other objects capable 
‘of overthrow are manifested much more intensely, than upon similar objects 
situated upon other portions of the superficies of the country. These zones 
of maximum disturbance (as yet ill observed) have been remarked to run in 
curvilinear directions of surface, to approach more or less, according to the 
means of (i. e. the objects afforded for) observation, to closed curves, and to 
be wholly distinct from those variations of destructive agency, irregularly 
“parsemé over large shaken areas, which depend upon differences of geologic 
surface-formation, configuration of country, &c., construction of buildings, 
and many other conditions, which modify the direction and effects of the 
shock at points often very little removed from each other, and the analysis 
of which, and extrication of the true primary movement from the entangle- 
ment of such minor phenomena, constitute the greatest difficulty of earth- 
quake observation. The physical conditions which give rise to such zones 
of maximum disturbance are easily explained. 
Fig. 7. 
: 
x 
a 
ia 
= 
; 
: 
; Referring to fig. 7, let h! h be the horizon (which we may assume a 
right line) cut by a vertical plane passing through a great circle of the earth, — 
and through A, the centre of impulse of the earthquake. The blow from 
this origin is propagated outwards in all directions, through the elastic mass _ 
of the earth (here assumed homogeneous), in spherical concentric shells, — 
which the circles 1, 2, 3, 4, &c. denote, at similar phases of the wave. The 
elastic wave starts from the impulse with one normal and two transversal 
vibrations. Its vs viva must remain constant, and (in the same medium its | 
dimensions being very great) the velocity of translation also. The mass in — 
Wwave-movement, at any moment of its transit, is therefore the same, and the 
thickness of each successive spherical shell decreases from the centre of im- _ 
pulse as the square of its mean distance. This is the measure of the normal — 
excursion of any particle, from any given phase of the wave, in its passage out- 
wards, to the recurrence of the same phase, and is also the measure of the nor- 
mal intensity of the shock, or that in directions AB, AC, AZ, &e. Neglect- — 
ing for the present the effects of the transversal wave, the normal intensity or _ 
direct overthrowing power of an earthquake shock varies inversely as the 
square of the distance from origin. But the surface capability of the shock 
