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Page:Popular Science Monthly Volume 20.djvu/559

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rhythm in the movement of the falling water, were observed. This was found to occur only at certain times. If the river was either too high or too low, the water poured over the dam with regular and uniform motion and sound; when the vibration of the houses occurred, the pulsatory movement of the falling water was also noticed.

The vibrations of the water were estimated at eight or ten per second, or about half of the number required to give the lowest continuous sound. These facts being established, it still remained to account for the enormous mechanical effect necessary to produce the vibration of distant buildings.

The long stretch of steadily flowing water above the dam, whose sloping edge only partly arrested its course, suggested the divided current of air thrown into the pipe of an organ. And, as the small column of air in the pipe, when thrown into vibration, is known to produce in some cases a perceptible tremor in the massive walls of a church, it is probable that a column of water half a mile in length, when in rhythmic vibration, would have a mechanical effect in proportion to its mass. But, in order that such vibration of a considerable body of water should be produced, a peculiar combination of physical conditions must exist, and this I have never observed in any other place.

Of course, the fall of such masses of water as pour over Niagara and other great cataracts will cause a tremor of the earth to a certain distance. But this is due to the simple mechanical force of impact, and is in relation to that force only. The phenomenon which I have described is intermittent, and that fact points to the conclusion that it is due to a definite relation between the vibrations of the river and what may be called the keynote of the bedrock over which it flows.

The distance to which such motion may be transmitted will, of course, depend upon the mass of water in vibration. The direction will depend upon the continuity of the underlying rock. Where this is broken by seams, the vibrations will cease, and therefore the line of these remarkable tremors affords an indication of the structure of the granite bed which is the basis of the picturesque hills around Ellicott City.



HENRI-√ČTIENNE SAINTE-CLAIRE DEVILLE, one of the most distinguished of French chemists, was born at St. Thomas, in the Antilles, of French parents, March 11, 1818, and died at Boulogne-sur-Seine on the first day of July last. He went to France while still a boy, with his brother, Charles Deville, the meteorologist, and had his attention drawn early in his career in school to chemical studies, which were then enjoying high credit under the brilliant results