3. This velocity diminishes as the general wave advances.
4. The movements of the earth’s crust are complex, consisting of many short waves first, increasing and then decreasing in amplitude; and with a detonating explosive the interval between the first wave and the maximum wave, at any station, is shorter than with a slow burning explosive.
Results obtained in Japan.—From some experiments made by the author in the grounds of the Meteorological Department in Tokio, the following results were obtained:—
| No. of Explosion | Velocity in feet per second for the first 200 ft. (A to B) | Velocity in feet per second for the second 200 ft. (B to C) | Velocity in feet per second for 400 ft. (A to C) | Number of Cartridges of Dynamite (6 = 1 lb.) | ||
|---|---|---|---|---|---|---|
| Vertical vibrations | { | I. | 464 | 186 | 265 | 8·3 |
III. | — | 211 | — | 10·1 | ||
IV. | 352 | 234 | 281 | 7·1 | ||
V. | 343 | 232 | 277 | 5·0 | ||
| Normal vibrations | { | VI. | — | — | 407 | 10·0 |
VII. | — | — | 516 | 12·5 | ||
| Transverse vibrations | } | VIII. | — | — | 344 | 12·5 |
The general results to be deduced from the above appear to be:—
1. For vertical motion.
(a) For the first 200 feet. The velocity depends upon the initial force—the greater the charge of dynamite the greater the velocity.
(b) For the second 200 feet. The above law only appears in experiments IV. and V., but it must be remembered that the origins of I. and III. were farther removed from A than IV. and V.
The speed of the wave during the second 200 feet is always less than during the first 200 feet.
2. For normal vibrations.