Robert Mallet advocated the use of fallen objects and cracks in buildings as aids in the study of earthquakes. He made a detailed investigation of the Neapolitan earthquake of 1857, in which he paid particular attention to the way buildings were cracked, walls overthrown, and soft ground fissured. Mallet believed that an earthquake consisted primarily of a compression followed by a dilatation. For such a shaking, he suggested, the resulting cracks in structures would be transverse to the direction of wave propagation. Overturned objects would fall along the horizontal projection of the direction of wave propagation. By observing the directions of arrival from a number of different points, he plotted an origin from which the wave seemed to spread. Mallet also published a set of formulae for calculating the velocities necessary to overturn structures of various simple shapes. From these, and observations of overturned objects, he estimated the velocity of particle motion at different sites.
Robert Mallet's assumption that earthquakes consisted mainly of longitudinal motion was proven invalid as soon as seismometers were built which recorded the large transverse component of ground motion.