When the large cannons in the forts on our coast are discharged during target practice, there are usually a lot of windows broken in the nearby houses. In Jersey City, N. J., several freight cars and boats loaded with dynamite and ammunition full of high explosives furnished the power for an explosion which, in July, 1916, broke considerably over a hundred thousand dollars worth of windows in the lower part of New York City.

The force of an explosion, whatever its source, throws back the air in huge waves, very much like the waves of the ocean, and whatever they come in contact with must have a sort of a tug-of-war with them, the weaker side being crumpled up and pushed back by the other. Broad expanses of glass, unprotected and without any support, except at the extreme edges, present an easy mark for air waves, therefore, and the amount of damage done to windows by explosions is usually only limited by the power of the explosives which produce the force of air waves.

The earth beneath, and the roof and walls of a building above, all receive the effects of these air waves in exactly the same way as do windows, and the resulting disaster is in direct proportion to their resisting capacity as against the pressure caused by the explosion. Many striking examples of the power of explosives have been accidentally furnished of late, in the course of making munitions for the European war.