Mallet, in his account of the Neapolitan earthquake of 1857, approximated to the amplitude of an earth particle by observing the width, at the level of the centre of gravity, of fissures formed through and remaining in great masses of very inelastic masonry. Taking stations situated on or very nearly on the same line passing through the seismic vertical (_epicentrum_), Mallet observed the amplitude increased as some function of the distance, as will be seen from the following table:— +-------------------------+------+-------+-------+---------+-------+ | Station |Polla |La Sala|Certosa|Tramutola|Sarconi| +-------------------------+------+-------+-------+---------+-------+ | Distance from Seismic } | | | | | | | Vertical in } | 3·45 | 11·60 | 16·50 | 20·60 | 26·7 | | geographical miles } | | | | | | | Amplitude in inches | 2·5 | 3·5 | 4·0 | 4·5 | 4·75 | +-------------------------+------+-------+-------+---------+-------+ The possibility of a law such as this having an existence for places at a distance from the seismic vertical comparable with the vertical depth of the centrum will be shown farther on. With regard to the maximum displacement of an earth particle. Mallet was of opinion that there was evidence to show that it had in some cases been over one foot. M. Abella, in an earthquake which occurred in the Philippines in 1881, made a rough observation of the motion of the earth to a distance of about _two metres_. This, as might be expected, was beyond the elastic limits of the material, and caused fissures to be formed, which were seen to open and shut. _Intensity of Earthquakes._—In speaking of the strength of an earthquake, we usually employ terms like ‘weak,’ ‘strong,’ ‘violent,’ &c. Although these expressions, accompanied by illustration of the effects which an earth quake has produced, convey a general idea of the strength of a shock as felt at some particular locality, our ideas nevertheless wanting in definiteness; and if we endeavour to compare one shock with another, as a whole, our want of exactness is augmented. We have seen that Palmieri’s seismograph indicates intensity by a certain number of degrees, which, to a certain extent, is a measure of the violence of the motion as indicated at a particular locality. The degrees, as before stated, refer to the height to which in consequence of the shaking, a certain quantity of mercury was washed in a tube, which is a function of the depth of mercury in the tube, and also of the duration of the disturbance.