These few examples of artificially produced tremors, to which many more might be added, have been given because they teach us something respecting their nature. Hitherto earth tremors have only been regarded as intruders, which it was necessary to escape from or destroy. From what has been said they appear to be a superficial disturbance which is propagated to an enormous distance. This distance appears to depend upon the propagating medium, upon the intensity of the initial disturbance, and upon its duration. In the observation of these artificial disturbances, which are accessible to every one, and which hitherto have been so neglected, we have undoubtedly a fruitful source of study. _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.