Phenomena which may possibly hold a relationship to earth pulsations are the periodical swellings of the ocean on the coast of Peru. Dr. C. F. Winslow, who made a long period observation upon the coast of Peru, found ‘the highest tides to prevail at Callao and Paita in December and January,’ and ‘also a series of enormous waves or sea-swells to be thrown from time to time upon the coast, varying from twenty-four to twenty-seven hours in continuance, accompanied by unusual height of the tide during the same period.’ During June and July the ocean was unusually tranquil. These phenomena do not appear to be connected with great atmospheric storms, nor do they hold any relation to the prevailing wind. They increase with and accompany the swelling of the tides, and occur generally, but not always, about full moon.
Sometimes they break suddenly upon the coast. ‘They are annual and constant in their periodicity.’
The periodical swellings are most noticeable between Tumbez 3° S.L. and the Chincha Islands 14° S.L.
These oceanic phenomena synchronise with the periodic intensity of earthquake phenomena in that part of the globe, and these with tidal movements.[153]
Other phenomena possibly attributable to earth pulsations.—If we assume that earth pulsations have an existence, these many phenomena which are otherwise difficult to understand meet with an explanation. The curious effects which were produced in the springs at Toplitz at the time of the Lisbon earthquake may have been due to a pulse-like wave. The flow of the principal spring was greatly increased. Before the increase it became turbid and at one time stopped. Subsequently it became clear and flowed as usual, but the water was hotter and more strongly mineralised. Sudden changes in the flow of underground waters which from time to time are observed may be attributed to like causes. Secondary earthquakes such as occurred after the Lisbon earthquake, as for instance in Derbyshire, may have been produced by pulsations disturbing the equilibrium of ground in a critical state.
The falling in of subterranean excavations is also possibly connected with these phenomena.
Possible causes of earth pulsations.—Mr. George Darwin, in a report to the British Association (1882), has shown that movements of considerable magnitude may occur in the earth’s crust in consequence of fluctuations in barometrical pressure. (A rise of the barometer over an area is equivalent to loading that area with a weight, in consequence of which it is depressed. When the barometer falls, the load is removed from the area, which, in virtue of its elasticity, rises to its original position. This fall and rise of the ground completes a single pulsation.)
On the assumption that the earth has a rigidity like steel, Mr. Darwin calculates that if the barometer rises an inch over an area like Australia, the load is sufficient to sink that continent two or three inches.
The tides which twice a day load our shores cause the land to rise and fall in a similar manner. On the shores of the Atlantic, Mr. Darwin has calculated that this rise and fall of the land may be as much as 5 inches. By these risings and fallings of the land the inclination of the surface is so altered that the stile of a plummet suspended from a rigid support ought not always to hang over the same spot. There would be a deflection of the vertical.
In short, calculations respecting the effects of loads of various descriptions, which we know are by natural operations continually being placed upon and removed from the surface of various areas of the earth’s surface, indicate that slow pulsatory movements of the earth’s surface must be taking place, causing variations in inclination of one portion of the earth’s crust relatively to another.