Miscellaneous Concluding Observations. 207
we have been able to meet with is due to Mr. Thomas Gray,
and as this mode of interpreting the results applies also to
chimneys, we may here give the method in question, which
will be found the more especially useful as Milne's paper, in
which it was first published, is contained in a publication not
readily procurable.109
Fig. 16.—Gray's treatment of twisted columns.
In the above figure the basal section of the column is shown
in plan, A, B, C, D. Shock at right angles to A B or C D
would tend to tilt the object over on the edges A B or C D,
according to the direction of the shock. Similar considera-
tions obviously apply to shocks at right angles to A D, B C,
and the corresponding edges. A shock in the direction of one
of the diagonals, such as D B, would overturn the column on
the corner B. If the shock has some intermediate direction,
such as e G (G being the centre of gravity), the impulse may
be supposed to be given towards e, and its effect would be to
cause the body to bear heavily on B and at the same time to
109 Trans. Seism. Soe. Japan, vol. i., part 2, p. 33.