Again, the towns, for greater security from attacks in early times, are generally perched upon the summits and steep flanks of hills, especially of the lower spurs that skirt the great mountain ranges; and the rocking of the hill-sites, in Mallet's opinion, greatly aggravated the natural effects of the shock. The streets, moreover, are steep and narrow, sometimes only five feet, and not often more than fifteen feet, in width; and the houses, when shaken down, fell against one another and upon those beneath them. As Dolomieu said of the great earthquake in 1783, "the ground was shaken down like ashes or sand laid upon a table." Of the total amount of damage, not even the roughest estimate can be made. The official returns are clearly, and no doubt purposely, deficient, and obstacles were placed in Mallet's way when he endeavoured to ascertain the numbers of persons killed and wounded. Taking only the towns into account, he calculated that, out of a total population of 207,000, the number of persons killed was 9,589, and of wounded 1,343.[8] A few towns were marked by an excessively high death-rate. Thus, at Montemurro, 5000 out of 7002 persons were killed and 500 wounded; at Saponara, 2000 out of 4010 were killed; and, at Polla, more than 2000 out of a population of less than 7000. GENERAL OBJECTS OF INVESTIGATION. The principal objects of Mallet's investigation were to determine the position of the epicentre and the depth of the seismic focus. If, in Fig. 3, F represents the seismic focus (here, for convenience, supposed to be a point), the vertical line FE will cut the surface of the earth in the epicentre E.[9] The dotted lines represent circles drawn on the surface of the earth with E as centre and passing through the places P and Q. [Illustration: FIG. 3.--Diagram to illustrate wave-path and angle of emergence.] When the impulse causing the earthquake takes place at the focus, two elastic waves spread outwards from it in all directions through the earth's crust. The first wave which reaches a point P consists of longitudinal vibrations, that is, the particle of rock at P moves in a closed curve with its longer axis in the direction FP. Mallet supposes this curve to be so elongated that it is practically a straight line coincident in direction with FP. In the second or transversal wave, the vibration of the particle at P takes place in a plane at right angles to FP. These vibrations Mallet, for his main purpose, neglects.