CHAPTER XXX.
FIRST APPROXIMATE CALCULATION OF THE DEPTH OF FOCUS OF THE EARTHQUAKE.
I had now the first decisive proof, that I had passed, beyond the point of the surface, vertically above the origin of the shock; and on comparing the emergence and wave-path ascertained for Polla, with those here obtained, I saw that that point, must be somewhere between the two towns (Polla and Vietri di Potenza), and not very far to the westward of the line joining them. With much curiosity, I made the calculation, for the first approximation to the depth of the focus, ever attempted for any earthquake. On the assumption that this point was in the vertical plane, passing through both Polla and Vietri, we have the emergences:
At Polla
At Vietri .
The angle made at the focus by their respective wave-paths therefore
.
The distance from Vietri to Polla, viewed as a right line, is 5·35 geographical miles. Then calling = the distance from the focus to Vietri,
