162 The East Anglian Earthquake.
gave rise to a general increase in level in these water-bearing
beds, from which the wells derive their supply.61 The
gradual return of the water to its old level may indicate that
the widened fissures are being filled up by the infiltration of
solid matter, or that they are closing up by the slow settle-
ment of the disturbed strata.
The Damage in connection with the Superficial Geology.
When an extensive tract of country is shaken by an earth-
quake, it might naturally be expected that the different
formations on which the buildings stand would exert some
influence in determining the distribution of the damage.
Thus, on a damp coherent formation like clay, it seems
probable that the vibrations would produce a greater effect
upon buildings than if these were situated upon loose gravel,
among which the energy of the vibrations would be dissipated
by the numerous internal reflexions and refractions.62 An
instructive illustration of the nature of the ground in deter-
mining the propagation of the vibrations has been furnished
by Mr. Thomas Boyle, F.C.S., who informs me that in his
laboratory at Silvertown, which stands upon clay over peat,
a heavy locomotive passing along the railway at a distance of
forty to fifty feet causes the building to vibrate sufficiently to
dislodge the bottles from the shelves, whereas in neighbouring
61 According to information subsequently furnished by Mr. F. H.
Meggy, a spring which supplies the lake at Boreham House has since the
earthquake only given half its former supply. A similar fact was
mentioned to me by Mr. T. Taylor, M.R.C.S., with respect to a surface
well at Bocking. It is possible that these facts may indicate that the
water formerly derived from supra-cretaceous drainage now partly finds
its way into the Chalk through the increased cretaceous drainage, but
here, as before, this interpretation of the facts is complicated by the long
prevalence of dry weather.
62 According to the results recently obtained by Milne with artificial
earthquakes, it appears that, "in soft, damp ground it is easy to produce
vibrations of large amplitude and considerable duration; in loose, dry
ground an explosion of dynamite yields a disturbance of large amplitude,
but of short duration ; while in soft rock it is difficult to produce a dis-
turbance the amplitude of which is sufficiently great to be recorded on an
ordinary seismograph."—'Nature,' June 4th, 1885, p. 114.