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the preceding observations of course have taken no account as yet of the reactions produced on the walls by roofs and floors: they refer to the walls considered as standing alone. The actual extent of elastic flexibility of stone and brick masonry, especially of the former; is not commonly considerable; and unfortunately, as yet, no precise measures of these exist for any class of masonry. Were it not for this property, however, no building would stand, even a very moderate shock; and were the velocity of the wave confined within the limits of the velocity of the centre of oscillation of the structure, considered as an elastic compound pendulum, whose time of vibration is due to the length of a simple pendulum equal to the height of that centre above the base, and were the amplitude of the shock within the limit of elastic displacement of the masonry, &c., at that centre, no building would be thrown down.

A well-constructed brick and mortar wall, of 30 or 40 years' induration, and 40 feet in height, unsupported, of two bricks, or 1·60 feet in thickness, has been observed by myself, to vibrate nearly 2 feet transversely at the top,