CHAPTER VZII.

Angles of emergence and mean depth of foci. (1)

Much that has been said with regard to azimuths of the wave path isapplicable to the angle that the emergence of the latter forms with thehorizon.

The irregular distribution of rock masses differing in form and density,the uneven surface of the ground and want of uniformity of construction inthe various buildings examined , all render the observation of angles ofemergence even more difficult than that of simple azimuths. Probably thegreatest error, however, arises from the fact that the focus in this case isnot a mathematical point, but probably a fissure, whose area is largecompared with that of the district in which observations are made. Besidesthis, in calculating the mean depth of the focus, we are obliged to employa vertical line instead of a plate-like area, a form which the fissure probablyassumes, and the size of which is only approximately known.

The observations taken in 1881 were so scanty that hardly anything morethan a rough guess at the depth of the focus could be expected, and eventhe observations of 1883 are really small in number for the purpose required.

One fact worthy of notice in the azimuth charts is, that the farther werecede from the seismic vertical, the shallower is the focus indicated by eachobservation, or in other words, the angle of emergence diminishes more rapidlythan it should do in proportion to the distance. The explanation of this apparentanomaly, it seems, is not far to seek. The farther we descend below the earth’ssurface, the greater will be the superincumbent pressure, and the greater

(1) See Plate V.

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