A possible cause of disturbances of this order may be small but sudden fluctuations in barometric pressure, which are visible during a storm. During a small typhoon on September 15, 1881, when in the Kurile Islands, I observed that the needle of an aneroid worked back and forth with a period of from one to three seconds. This continued for several hours. With every gust of wind the needle suddenly rose and then immediately fell. At times it trembled. These movements were observed in the open air. The extent of these sudden variations was approximately from ·03 to ·05 inches. Beckoning an increase of barometrical pressure of one hundredth of an inch as equivalent to a load of twenty million pounds on the square mile, during this storm there must have been the equivalent of loads of from 60 to 100 million pounds to the square mile continually placed on and removed from a considerable tract of the earth’s surface. If the period of application of these stresses approximately coincide with the natural vibrational period of the area affected, it would surely seem, especially when we reflect upon the effect of an ordinary carriage, that tremors of considerable magnitude ought to be produced.
An inspection of the following few observations taken from my note-book for the same typhoon will suggest that even the large and slower variations are capable of producing tremulous motions.
| Time h. m. | Barometer reading |
|---|---|
| 12 5 p.m. | 29·02 |
| 12 10 „ | 29·05 |
| 12 12 „ | 29·07 |
| 12 13 „ | 29·05 |
| 12 25 „ | 29·10 |
| 12 50 „ | 29·00 |
| 1 10 „ | 29·00 |
| 1 20 „ | 29·07 |
CHAPTER XX.
EARTH PULSATIONS.
Definition of an earth pulsation—Indications of pendulums—Indications of levels—Other phenomena indicating the existence of earth pulsations—Disturbances in lakes and oceans—Phenomena resultant on earth pulsations—Cause of earth pulsations.
The object of the present chapter is to show that from time to time it is very probable that slow but large wave-like undulations travel over or disturb the surface of the globe.
These movements, which have escaped our attention on account of their slowness in period, for want of another term I call earth pulsations.
The existence of movements such as these may be indicated to us by changes in the level of bodies of water like seas and lakes, by the movements of delicate levels, by the displacement of the bob of a pendulum relatively to some point on the earth above which it hangs, and by other phenomena which will be enumerated.
Indication of pendulums.—Pendulums which have been suspended for the purposes of seismometrical observations have, both by observers in Italy and Japan, been seen to have moved a short distance out from, and then back to, their normal position.