186 The East Anglian Earthquake.
" If a building were constructed of a material like steel,
we can imagine that it might be set rocking to and fro,
which, if it were shaken by an earthquake, it might do in a
manner something similar to that of an inverted pendulum,
without any danger of its being broken. It would simply
rock back and forth with a definite period of its own. In
ordinary houses, however, instead of having a single vibration
of the whole building to consider, we have to investigate the
vibration of a number of parts, the periods of which are all
more or less different. These parts, although they are tied
together in various manners, owing to differences in elasticity,
height, thickness, and load they carry, do not tend to syn-
chronise in their swings. Whilst one portion of the building
is endeavouring to move towards the right, another is pulling
towards the left, and, in consequence, either the bonds
which join them, or else they themselves are strained or
broken."93
In accordance with this principle, a crack in a building has
no necessary relation to the line of emergence of the seismic
wave, and the actual inspection of the damaged buildings led
us to conclude that any number of angles of emergence, from
90° to the horizontal, might be found from the cracks in the
solid masonry of the buildings round the main axis of dis-
turbance. The angles of some of the more conspicuous cracks
have been given in the descriptive report, but, for the reasons
stated, we have not thought it advisable to give any calcu-
lations of the depth of the origin of the disturbance, being
convinced that under the present circumstances such deter-
minations would only give a fictitious semblance of certainty
to the results.94
93 Trans. Seism. Soc. Japan, vol. i., part 2, p. 71. The principle of
relative vibrational periods was first enunciated by my present colleagues,
Profs. Perry and Ayrton, in a pamphlet on ' Structures in an Earthquake
Country,' published at the Imperial College of Engineering, Japan, 1878.
94 The difficulties attending the determination of the depth of origin of
an earthquake are well illustrated by the wide limits within which the
determinations vary, even when instrumental records have been obtained
by a skilled observer. Thus Milne found that the angles of emergence of
the Japanese earthquake of February 22nd, 1880, as given by observations
at Tokio and Yokohama, were 1.5°, 6.25°, 10.5°, 11°, 18°, 31°, or 45°,
these corresponding to depths of 0.3, 1.6, 2.8, 3.2, 6.3, 9.7, or 16 miles.