The only other investigations which have been made in this interesting branch of observational seismology are those by Mr. Robert Mallet,[10] and those by General Henry L. Abbot.[11]
Mallet’s Results.—The velocity with which earth vibrations were transmitted as deduced by Mr. Mallet were as follows:—
| Feet per second | |
|---|---|
| In sand | 824·915 |
| In contorted stratified rock, quartz, and slate at Holyhead | 1,088·669 |
| In discontinuous and much shattered granite | 1,306·425 |
| In more solid granite | 1,664·574 |
A striking result which was obtained by Mallet in his experiments at Holyhead was that the transit velocity increases with an increase in the intensity of the initial shock. Thus with a charge of 12,000 pounds of powder the transit rate was 1,373 feet per second, whilst with 2,100 pounds the transit rate was 1,099 feet per second. In these experiments tremors were observed as preceding and following the main shock.
Abbot’s Results.—The important results obtained by General Abbot are contained in the following table:—
| No. of Observation | Date | Cause of Shock | Distance to Station in miles | Type of Seismometer | Velocity in feet per second |
|---|---|---|---|---|---|
| 1 | Aug. 18, 1876 | 200 lbs. of dynamite | 5 ± | B | 5,280 |
| 2 | Sep. 24, 1876 | Hallet’s Point Explosion | 5·134 | A | 3,873 |
| 3 | „ | „ „ „ | 8·330 | B | 8,300 |
| 4 | „ | „ „ „ | 9·333 | A | 4,521 |
| 5 | „ | „ „ „ | 12·769 | B | 5,309 |
| 6 | Oct 10, 1876 | 70 lbs. dynamite | 1·360 | A | 1,240 |
| 7 | Sept. 6, 1877 | 400 „ „ | 1·169 | A | 3,428 |
| 8 | „ | „ „ „ | 1·169 | B | 8,814 |
| 9 | Sept. 12, 1877 | 200 „ „ | 1·340 | A | 6,730 |
| 10 | „ | „ „ „ | 1·340 | B | 8,730 |
| 11 | „ | 70 „ „ | 1·340 | A | 5,559 |
| 12 | „ | „ „ „ | 1·340 | B | 8,415 |
A seismometer of type A means that the telescope used in observing the tremor produced on the surface of a vessel of mercury by the passage of the shock had a magnification of 6, whilst a telescope of the type B had a magnification of 12.
The mean velocity given by six observations with type A is 4,225 feet per second, while that given by the same number with type B is 7,475 feet per second.
Fig. 13.