In this seismograph, the pendulums are provided with pointers, the tips of which touch vertical sheets of paper attached to the sides of an upright rectangular box. When an earthquake occurs, this box is made to descend slowly with a uniform velocity, while the moving pointers trace curves upon the smoked paper. The north-and-south component of the horizontal motion is inscribed on the sheet of paper facing west, and the east-and-west component on the paper facing south. [Illustration: FIG. 36.--Seismographic record of the Riviera earthquake at Moncalieri. (_Denza._)] During the principal Riviera earthquake, the former pendulum furnished an indistinct record, while the other traced the diagram reproduced in Fig. 36. The movement, as here represented, began at about 6h. 21m. 50s. A.M. (mean time of Rome) with a series of small tremors, which lasted for about twelve seconds. Then followed some large oscillations, always in a nearly east-and-west direction, which at 6h. 22m. 21s. gave place to a second series of tremors similar to those at the beginning of the shock, but of greater amplitude. These continued for at least twelve seconds, at the end of which time the motion of the smoked paper ceased. The total duration of the movement at Moncalieri cannot therefore have been less than forty-three seconds. Interesting as this record is, it is doubtful how far it represents accurately the movement of the ground. The Moncalieri instrument was erected before the modern type of seismograph was designed, in which some part remains steady, or very nearly steady, during the complicated movements of the ground that take place in an earthquake. It will be noticed that the curve in Fig. 36 shows no sign of the division of the shock into two distinct parts, and this may perhaps be due to the swinging of the pendulum itself; in which case, the curve described by the pointer would be the resultant of the oscillations of the ground and the proper motion of the pendulum. SOUND-PHENOMENA. The sounds that preceded and accompanied the Riviera earthquake have attracted but little study, although they seem to have been widely observed. No attempt was made to define the limits of the area over which they were audible; but Professor Mercalli states that in the two outer zones (Fig. 33) the sound generally passed unobserved. It was, however, heard near Piacenza in Lombardy and Reggio in Emilia, places which are about 115 and 140 miles from the principal epicentre. In the area in which the shock was most violent, the sound resembled that of trains and vehicles in motion; while, outside this area it generally appeared to be like the hissing of a violent wind. In only a few places was it compared to detonations, the crashes of artillery or distant thunder. Some observers describe the sound as appearing at first as if a strong wind were rising, and then as the roaring of a heavy railway-train passing.