Page:Popular Science Monthly Volume 29.djvu/330

316 longer pendulum is suspended. The four pendulums are each connected just above their bobs to the central pendulum with loose silk threads. Fixed to the center of each of these threads, and held vertically by a light spring, is a needle, so adjusted that each thread is depressed to form an obtuse angle of about 155°. These needles form the terminals of an electric circuit, the other termination of which is a small cup of mercury placed just below the lower end of the needle. By a horizontal swing of one of the pendulums this arrangement causes the needle to move vertically, but with a slightly multiplied amplitude. By this motion the needle comes in contact with the mercury, and an electro-magnet with a lever and pencil is caused to make a mark on a band moved by clock-work. The five pendulums being of different lengths, the apparatus is adapted to respond to seismic waves of different velocities.

Professor Rossi's microphone consists of a metallic swing arranged like the beam of a balance. By means of a movable weight at one end of the beam, this is so adjusted that it falls down until it comes in contact with a metallic stop. The beam and the stop form two poles of an electric circuit, in which is a telephone. The slightest motion in a vertical direction causes a fluctuation in the current passing between the stop and the beam, and announces itself in the telephone.

By observations made with instruments like these, it has been shown that the soil of Italy is in incessant movement, with periods of excessive activity, called seismic storms, that usually last about ten days. The storms are separated by periods of relative calm. They are more regular in winter, and exhibit sharp maximums in spring and autumn. In the midst of such a period or at its end there is usually an earthquake. They have been observed to be generally related to barometric depressions.

Earth-pulsations are slow but large undulations that appear to travel over or disturb the surface of the globe. They are made manifest through variations in the movement of pendulums, changes in the position of the bubbles of levels, eccentricities in the behavior of clocks, the swinging of chandeliers in churches, unusual disturbances in bodies of water, and even of water in tubs, irregularities in the flow of springs, and other phenomena, the occurrence of which, or the peculiar manner of it, while it is consistent with the hypothesis of such movement, can not be accounted for on any other probable supposition.