A curious effect of earthquakes strong enough to damage buildings is that pillars, monuments, etc., may be fractured and the upper part rotated over the lower without being overthrown. Even in Hereford and the surrounding villages, several pinnacles and chimney-stacks were twisted by the earthquake of 1896. The interest of the phenomenon, which has been known, since 1755,[74] is mainly historical, for the endeavour to discover its cause was the origin of Mallet's views on the dynamics of earthquakes. Partly, also, it lies in the difficulty of finding a satisfactory explanation, or rather in deciding which of three or four possible explanations is the true one in any particular case. [Illustration: FIG. 74.--Twisting of monument at Chhatak. (_Oldham._)] The Indian earthquake offered exceptional opportunities for studying the phenomenon in the large number of examples observed and the variety of objects rotated. None could be more striking than the twisted monument to George Inglis, represented in outline in Fig. 74. Chhatak, where this is situated, lies close to the southern boundary of the epicentral area. The monument is an obelisk, built of broad flat bricks or tiles on a base of 12 feet square, and originally more than 60 feet high. It was split by the earthquake into four portions. The two upper, about six and nine feet long, were thrown down; while the third, 22 feet high, remains standing, but is twisted through an angle of 30° with respect to the lowest part, which is unmoved. The upper of these two parts had evidently rocked on the lower, as the corners and edges were splintered, and below the fracture a slice of masonry about 15 inches thick, which was not bonded into the main mass, was split off by the pressure on its upper end. The plan of the parts still standing is shown in the lower part of Fig. 74. The possible explanations of the phenomenon are at least three in number. According to the first, which was given by Mallet in 1846, the adhesion of the twisted portion to its base is not uniform, and the resultant resistance to motion is not in the same vertical plane as the wave-movement.[75] Some years later, Mallet offered another explanation. The body, he imagined, might be tilted on one edge by the earthquake, and, while still rocking, a second shock oblique to the first might twist it about that edge.[76] In 1880, Professor T. Gray suggested that the column might be tilted on one corner and then twisted round it by later vibrations of the same shock.[77]