None of these theories, Mr. Oldham argues, can give by itself a complete explanation of the phenomena observed in the central district of the Indian earthquake; and he therefore favours an extension of the second theory, which, though first proposed in 1882,[78] was thought out independently and in greater detail by himself. When the focus is of considerable dimensions, the shock at neighbouring places is constantly varying in direction, owing to the arrival of vibrations from different parts of the focus. Thus, instead of the two separate shocks required by Mallet's second explanation, we have a number of closely successive impulses frequently changing in direction and giving rise to what is known in the South of Europe as a vorticose shock. And, instead of a single twist of the pillars about one centre only, we have a series of small twists round a number of different centres, accompanied in consequence by a much smaller displacement of the centre of gravity than would have occurred had the same rotation been accomplished in one operation. The theory, it will be seen, accounts for the twisting of the pillar without overthrow, and for the splintering of the edges during the rocking of the column. It explains why in any district a number of similarly placed objects are generally twisted in the same direction. Moreover, a low column rocks to and fro more rapidly than a tall one similar in form and position, so that, at the instant when a later impulse comes from a different direction, two such columns might happen to be tilted on opposite edges, and would then be twisted in opposite directions. In certain cases, then, as occurred at several places during the Indian earthquake, an object may rotate in one direction, while others, similar in every respect but size, may be twisted in the opposite direction. AFTER-SHOCKS. _Frequency of After-Shocks._--For some days after the great earthquake, the after-shocks by their very frequency and by their wide distribution baffled close inquiry. During the first 24 hours, hundreds were felt at all points of the epicentral area; indeed, it is not too much to say that for several days the ground was never actually at rest. At the Bordwar tea-estate, which is traversed by one of the great fractures to be described in the next section, the surface of a glass of water on a table was for a whole week in a constant state of tremor; and at Tura a hanging lamp was kept continually swinging for the first three or four days.