From this it seems possible that a very slow motion of small amplitude, continuing over a sufficient period of time, might, if it agreed with the period of the mercury, indicate an earthquake of many degrees of intensity, whilst residents in the neighbourhood might not have noticed the disturbance; and, on the other hand, a short but intense shock creating considerable destruction might have been recorded as of only a few degrees of intensity. Although objections like these might be raised to such a method of recording intensity, in practice it would appear that such results are not pronounced, and the indications of the instrument usually give us approximate indications of relative intensity. In writing about the Neapolitan earthquake of 1857, Mallet says that ‘area alone affords no test of seismic energy.’ The area over which a shock is felt will depend not only upon the initial force of the disturbance, but also upon the focal depth of a shock, the form and position of that focus, the duration of the disturbance, and the nature and arrangement of the materials which are shaken. From observations in Japan, it is clearly shown that massive mountain ranges exert a considerable influence upon the extension of seismal disturbances. On one side of a large range of mountains large cities might be laid in ruins, whilst on the other side the disturbance creating this destruction might not be noticed. _Velocity and Acceleration of an Earth Particle._—We now pass on to methods of determining the intensity of an earthquake which are less arbitrary than those which have just been discussed. These methods have already been discussed when speaking of artificial disturbances, where it was shown that the intensity of an earthquake as measured by its destructive effects greatly depended upon the suddenness with which the backward and forward motions of the ground were commenced or ended. Amongst the earlier investigators of seismic phenomena who observed that there existed a connection between the distance to which bodies had been projected during an earthquake and the suddenness or initial velocity with which the ground had been moved beneath them, was Professor Wenthrop of Cambridge, Massachusetts, who noted that bricks from his chimneys had, by the New England earthquake of 1755, been thrown thirty feet. From this and the known height of the chimney, he calculates that the bricks had been projected with an initial velocity of twenty-one feet per second.[13] The calculations made by Mallet respecting the maximum velocity of an earth particle at the time of the Neapolitan earthquake in 1857 depended upon the overthrow, projection, and fracture of bodies. The principles which guided him in making the calculations will be understood from the following illustration. [Illustration: Fig. 14.]