Natural tremors.—Next let us turn to those microscopical disturbances of our soil which are due to natural causes. Thus far they seem to have been recorded wherever instruments suitable for their detection have been erected, and it is not improbable that they are common to the surface of the whole globe.
Some of the more definite observations which have been made upon earth tremors were those made in connection with experiments on the deviation of the vertical due to the attractive influence of the moon and sun.
Professor Zöllner, who invented the horizontal pendulum which he used in the attempt to measure the change in level due to lunar and solar attraction, found his instruments so sensitive that the readings were always changing.
The most interesting observations which were made upon small disturbances of the soil were those of M. d’Abbadie, who carried on his experiments at Abbadia, in Subernoa, near Hendaye, 400 mètres distant from the Atlantic, and 62 mètres above sea level. The soil was a loamy rock. Here M. d’Abbadie constructed a concrete cone 8 mètres in height, which was pierced down the centre by a vertical hole or well, which was continued two mètres below the cone into the solid rock. At the bottom of this hole or well a pool of mercury was formed which reflected the image of cross wires placed at the top of the hole. These cross wires and their reflection were observed by means of a microscope. The observations consisted in noting the displacement and azimuth of the reflected image relatively to the real image of the wires. After allowing this structure five years to settle, M. d’Abbadie commenced his observations. To find the surface of the mercury tranquil was a rare occurrence. Sometimes the mercury appeared to be in violent motion, although both the air and neighbouring sea were perfectly calm. At times the reflected image would disappear as if the mercury had been disturbed by a microscopic earthquake.
The relative positions of the images were in part governed by the state of the tide. Altogether the movements were so strange that M. d’Abbadie did not venture any speculations as to their cause, but he remarks that the cause of the changes he observed were sometimes neither astronomical nor thermometrical. These observations, the principal object of which was to determine changes in level rather than earth vibrations, were carried on between the years 1868 and 1872.[141]
Observations at Cambridge.—Another instructive set of observations were those which were made in the years 1880–1882, by George and Horace Darwin, in the Cavendish Laboratory, at Cambridge. The main object in these experiments was to determine the disturbing influence of gravity produced by lunar attraction. The result which was obtained, however, showed that the soil at Cambridge was in such an incessant state of vibration that whatever pull the moon may have exerted upon the instrument which was employed was masked by the magnitude of the effects produced by the earth tremors, and the experiments had, in consequence, to be abandoned.
The principle of this instrument was similar to one devised by Sir William Thomson, and put up by him in his laboratory at Glasgow. As erected by the brothers Darwin, at Cambridge, it was briefly as follows: A pendulum, which was a massive cylinder of pure copper, was hung by a copper wire, about four feet long, inside a hollow cylindrical tube rising from a stone support. A small mirror was then hung by two silk fibres, one of which was fastened to the bob and the other to the stone basement. A ray of light sent from a lamp on to the mirror was reflected to a scale seven feet distant, and by this magnification any motion of the bob relatively to the stone support was magnified 50,000 times. In several ways the apparatus was insulated from all accidental disturbances. The spot of light was observed from another room by means of a telescope. This instrument was so delicate that even at the distance of sixteen feet the shifting of your weight from one foot to the other caused the spot of light to run along the scale. So sensitive was the instrument that, notwithstanding its being cut off from the surrounding soil by a trench filled with water and the whole instrument being immersed in water to damp out the small vibrations, it would seem that the ground was in a constant state of tremor; in fact, so persistent and irregular were these movements that it seemed impossible to separate them from the perturbations due to the attraction of the moon.[142]
As a result of observations like these, the world had gradually forced upon it the fact that the ground on which we live is probably everywhere in what is practically an incessant state of vibration.
This led those who were interested in the study of earth movements to establish special apparatus for the purpose of recording these motions with the hope of eventually discovering the laws by which they were governed.
Experiments in Japan.—The simpler forms of apparatus which have been used in Japan may be described as delicate forms of seismoscopes, which, in addition to recording earth tremors, also record the occurrence of small earthquakes.