Of the crowd of vibrations that agitate the ground during an earthquake, part only combine to form the perceptible shock. Some are insensible owing to their small amplitude, others to the slowness of the motion. An interesting observation belonging to the latter class was made by an engineer near Midnapur, a place which lies just within the area of damage. At the time of the earthquake, he was taking levels on a railway bank, and was about to take a reading when he noticed the bubble of the level oscillating. In five or ten seconds the shaking began and appeared to last three or four minutes; but, for more than five minutes after it had apparently ceased, the level showed that the ground continued to rock. Again, in Burmah, at a place nineteen miles east of Tagaung and close to the border of the disturbed area, the water in a shallow tank, about 300 yards in length, was seen lapping up against the side in a manner that was at first attributed to elephants bathing. No shock was felt, but the shaking of the trees at the same time showed that the disturbance was due to the earthquake. Far beyond the limits of the disturbed area, however, the earthquake was recorded by many of the delicate instruments which have been employed during the last few years for the registration of distant shocks. Among the more important of these instruments are long vertical pendulums, horizontal pendulums of various forms, and magnetographs. In the vertical, and some of the horizontal, pendulums, especially in those used in the Italian observatories, the masses carried are heavy, and the movements of the ground are magnified by lightly-balanced levers ending in points which trace their records on bands of smoked paper driven by clockwork. In the other horizontal pendulums and in the magnetographs, the method of registration is photographic. The paper required for the mechanical records being inexpensive, a high velocity (half-an-inch or more per minute) can be given to it, and the resulting diagrams are open and detailed. The Italian instruments also respond more readily than the others to the earlier and slighter tremors: while the apparatus in which photographic methods are used are sometimes so violently disturbed by the later undulations that the spot of light fails to leave any trace on the photographic paper. It is therefore from the Italian observatories that the more interesting records come. One of these, given by a horizontal pendulum at Rocca di Papa near Rome, is reproduced in Fig. 71; while the curve of the bifilar pendulum at Edinburgh (Fig. 72) is a good example of those obtained by the photographic method of registration.[71]