VIBRATORY MOVEMENTS AND THEIR EFFECTS. 43 
CRACKS IN WALLS AND CHIMNEYS. 
In Mallet’s great report on the Neapolitan earthquake of 1857 he assumed that the 
waves were propagated thru buildings just as thru the earth below, and concluded 
that the cracks made in the walls were at right angles to the direction of propagation of 
the waves. From this he deduced the direction of propagation and the position of the 
focus. But the length of buildings is only a very small fraction of the length of a seismic 
wave; and in them the proper conditions for the rectilinear propagation of waves do not 
exist; so that Mallet’s assumption and his conclusions can no longer be accepted. Lines 
drawn at right angles to the cracks on the floor are probably at right angles to the direc- 
tion of propagation of longitudinal waves, for these cracks are practically formed in the 
ground, like those described above; but cracks in walls can not be lookt upon as at 
right angles to the direction of the movement, even when no windows are present to cause 
special weakness in some directions. We shall form a better conception of the mechanical 
conditions if we look upon the wave as divided into two components, one producing a 
horizontal vibration of the house and the other a vertical vibration. If, as is usually the 
case, the house is longer than it is high, the inertia opposing the motion will produce a 
horizontal shear and it may be shown, by the method used on page 34, that cracks have 
a tendency to form at an angle of 45° with the horizontal in walls running in the direction 
of the vibration; as the motion is first in one direction and then in the other, two sets of 
cracks would be formed at right angles to each other, and each 45° with the horizontal. 
The vertical component produces vertical compression and expansions which may slightly 
modify the direction of the cracks. This is exactly what was observed; many walls 
exhibited the double system of diagonal cracks. An excellent illustration of these cracks 
in the St. James Hotel at San Jose is given by Professor Omori. 
Chimneys, and walls running at right angles to the direction of the vibrations, were 
affected in a different way; they are high in comparison with their breadth and conse- 
quently were set into vibrations, like long rods. As they swayed back and forth they bent 
and were comprest on the concave and stretcht on the convex sides. If this stretching 
exceeded the elastic limit of the material, a horizontal crack was made. It was in this 
way that chimneys were overthrown and the tops of walls and gables were thrown out. 
In practically all cases of brick walls and chimneys the break occurred at a joint and the 
bricks which were thrown down were usually unbroken, but entirely detached from each 
other, showing that the mortar was very weak. Chimneys of uniform thickness would 
naturally break at their lowest free point, which, in the case of the chimneys of houses, 
is where they pass thru the roof; and walls would break where they are not well braced, 
which was usually near their tops or in the gables or at the corners. The high chimneys of 
factories are thicker near the ground and gradually diminish in diameter and thickness 
from the ground up. They did not break at the ground, but at some point about a third 
of the way up where the bending moment was greatest in comparison with their strength. 
It by no means follows that a broken chimney will fall; in regions where the shock was 
not so very strong, many short chimneys were broken, and the detached part rocked on 
the lower part without overturning; the very small power of stretching possest by brick 
and mortar caused chimneys to break before they were sufficiently inclined to lose their 
balance and fall. 
ROTATORY MOVEMENTS AND THE ROTATION OF OBJECTS ON THEIR SUPPORTS. 
It has been a matter of frequent observation that during the shocks of large earthquakes 
a twisting motion is felt, and after the shock, chimneys which were not thrown down, 
monuments in cemeteries, ornaments, etc., are found to have been rotated on their 
1 Bull. Imperial Earthquake Investigation Commission, vol. 1, No. 1, plate tv. 
