Descriptive Report. 85
square brick shaft, with its corners directed to the cardinal
points, was twisted slightly round from the N. towards the
Bath Street, as the line of vision falls on to the roof about 3 feet above
the highest part of the window on the Bath Street side of the workshops.
Now at the moment of the shock Mr. Wilkins asserts that he was enabled
to see through both windows, and observed the fall of the chimney and
brickwork from a house in Bath Street opposite to the workshops.
Taking as data his own height and distance from the buildings, and the
height of the opposite windows above the ground, Mr. Wilkins's diagram
shows him as lifted vertically to the extent of 3 feet, and the building
depressed to the same extent, and tilted over towards him at an angle
of 5°. This construction carries the line of sight just across the two
windows. I have not thought it necessary to reproduce the drawing
here, but I may add that its accuracy is unquestionable, and the fact
observed can hardly be denied, coming as it does from an observer whose
daily professional avocation leads him to deal with exact measurements.
Without venturing to offer any explanation of my own, I will only
add, by way of comment, that both the wave length and the angle of tilt
of the building are arbitrarily assumed, and could not be found by the
method given. It is moreover assumed that the building swayed as a
whole, whereas the opposite faces may have been displaced with respect to
each other in opposite directions. A certain amount of elasticity must
be allowed to exist in a building, into the construction of which a large
amount of wood enters, as in the present case; this elasticity, together
with the unknown wave-length and angle of tilt, entirely vitiates any
calculation based on the supposition that the structure simply rocked to
and fro as a whole, without undergoing any deformation. I at first
thought it possible that Mr. Wilkins had been mistaken in identifying
his position, but he satisfactorily removed every doubt on this point, and
whilst expressing my thanks to this gentleman for the courtesy with
which he supplied me with all the information required, I feel bound, in
placing his observation upon record, to express dissent from his in-
terpretation.
(Since writing the above note I have received a letter from Prof. J. A.
Ewing to whom I submitted the diagram referred to. This well-known
authority in seismology confirms the objections raised by me, and points
out " the general truth that a building may assume a slope, by shearing,
during an earthquake, although the ground at its base is not sensibly
sloped. A series of small horizontal movements of the base will often
produce a large accumulated vibration on tho part of the superstructure,
especially if the roof is heavy, and the construction such as to admit of
distortion. In a frame house, without cross-bracing, and with a heavy
roof, there is nothing to prevent the swing of the roof being a foot or two
in amplitude or even more, though the movements of the base may be
fractional parts of an inch. To explain the case in question, however,