If we assume that the first tremor observed in the mercury is to determine the true rate of transmission, General Abbot tells us that we must reject all observations made with type A, inasmuch as they do not reveal the velocity of the leading tremor. However, he also tells us that a still higher power above 12 might have detected still earlier tremors.
When gunpowder was the explosive, the observers noted that the disturbance observed in the mercury took a much longer time to reach a maximum than it did when dynamite was employed.
It was also observed that explosions fired beneath deep water gave a higher velocity than similar explosions which took place beneath shallow water. In the latter case much of the energy was probably expended in throwing a jet of water into the air.
Another point which was observed appears to have been that the rate varied with the initial shock. Thus:—
| Feet per second | |
|---|---|
| 400 lbs. of dynamite gave | 8,814 |
| 200 „ „ | 8,730 |
| 70 „ powder (deep) gave | 8,415 |
Also it is probable that the rate of a wave diminished with its advance. For,
| Feet per second | |
|---|---|
| 200 lbs. of dynamite gave for 1 mile | 8,730 |
| „ „ „ „ 5 miles | 5,250 |
| 50,000 „ „ „ 8 „ | 8,300 |
| „ „ „ „ 13½ „ | 5,300 |
General Abbot’s general conclusions are:—
1. A high magnifying power of telescope is essential in seismometric observations.
2. The more violent the initial shock the higher is the velocity of transmission.