[Illustration: FIG. 11.—Records obtained at three stations of the motion of the ground produced by the explosion of 2 lbs. of dynamite.] _The Intensity of Artificial Disturbances._—The data which we have at our disposal for determining the intensity of an earth particle which has been caused to vibrate by the explosion of a charge of dynamite are a series of records similar to that given on p. 60. These disturbances are practically surface movements, and may be compared with the movements of an earthquake which spreads over an area the radius of which is great as compared with its depth. To find the mean acceleration of an earth particle, which quantity has been taken to represent intensity, during any simple backward or forward motion of the earth, it will be first necessary to determine the amplitude of this motion and its maximum velocity, the mean V^2 acceleration being equal to ———. 2A [Illustration: FIG. 12.] The second and third movements in a shock invariably exhibited the greatest intensity, and to a distance of 400 feet from the origin, where about three pounds of dynamite had been exploded in a bore-hole about six feet deep, these intensities decreased directly as the distance from the origin. The less intense movements also decreased directly as the distance from the origin to a certain point, but after that they decreased more slowly. A mean result of the more prominent vibrations in four sets of experiments is shown in the curve, fig. 12, where the horizontal measurements represent distance from the origin in feet, and the vertical measurements mean acceleration in thousands of millimetres per second. This curve approximates to an equi-angular hyperbola. The area between the curve and its asymptotes is proportional to the whole energy of the shock. The area of the diagram is proportional to the energy given up to the ground by the explosion of three pounds of dynamite. If we call the unit shock the effect produced by the explosion of one pound of dynamite, the above artificial earthquake had an intensity equal to three. The only other investigations which have been made in this interesting branch of observational seismology are those by Mr. Robert Mallet,[10] and those by General Henry L. Abbot.[11] _Mallet’s Results._—The velocity with which earth vibrations were transmitted as deduced by Mr. Mallet were as follows:— Feet per second In sand 824·915 In contorted stratified rock, quartz, and slate at Holyhead 1,088·669 In discontinuous and much shattered granite 1,306·425 In more solid granite 1,664·574