able:

1. Fractures or dislocations (chiefly in the masonry of
buildings), which afford two principal sources and sorts of
information, namely:

_a._ From the observed _directions of fractures or fissures_,
by which the _wave path_, and frequently the _angle of
emergence_, may be immediately inferred.

_b._ Information from the preceding, united with known
conditions as to the strength of materials to resist
_fracture_, by which the _velocity_ of the fracturing impulse
may be calculated.

2. The overthrow or the projection, or both, of bodies large or
small, simple or complex. From these we are enabled to infer:

_c._ By direct observation, the _direction in azimuth_ of the
wave path.

_d._ By measurements of the horizontal and vertical distances
of overthrow or of projection, to infer either the _velocity_
of projection, or _angle of emergence_.

Fractures by shock present their planes always nearly in directions
transverse to the wave path. Projections or overthrow take place (unless
secondarily disturbed) in the line of the wave path, or in the vertical
plane passing through it: but the direction of fall or overthrow may be
either in the same direction as the wave transit (_i.e._, as the motion
of the wave particle in the first semiphase), or contrary to it.

It is thus obvious that the principal phenomena presented by the effects
of earthquake shock upon the objects usually occurring upon the surface
of the inhabited parts of the earth, resolve themselves into problems of
three orders, and are all amenable to mechanical treatment, viz.:

1. Problems relating to the direction and amount of velocity
producing fracture or fissures.

2. Problems relating to the single or multiplied oscillations
of bodies, considered as compound pendulums.

3. Problems referable to the theory of projectiles.

These three may combine in several cases, and on the part of the
observer must combine with measurements, angular and linear, and with
geodetic operations to be conducted in the shaken country.

The methods of application in detail are described fully, as well as
their actual application and results, in my work published in 1862 (2
vols.), entitled "The First Principles of Observational Seismology, as
developed in the Report to the Royal Society of London of the Expedition
made by Command of the Society into