We at length come to the fifth and last head of constructive modification, as affecting and affected by, the shock, viz., the effects of wall apertures, windows, doors, &c. A few words of illustration will be sufficient for this. Assuming the simplest case, as in Fig. 86, of a normal

wave; whenever the wall is pierced by one or several rectangular or other apertures, it may be viewed as divided into different segments such as that ${\displaystyle ef}$, ${\displaystyle gh}$, ${\displaystyle nk}$, each having a separate moment of stress of its own, and giving rise to a separate dynamic couple, the extremity of one arm being in the centre of gravity, ${\displaystyle c}$, the other at the base and junction with the similar adjoining segments, the ten-