CHAPTER I.
OF THE SUPERFICIAL POSITION OF THE SEISMIC VERTICAL, OR OF THE POINT ON THE SURFACE VERTICALLY ABOVE THE FOCUS.
The two preceding parts, have been necessarily tedious and lengthy, as describing not only the development of a method, but the numerous facts elicited with a view to its application; the colligation of these facts, and the generalizations and conclusions to which they lead, may be more briefly given. I purpose treating them under distinct heads, and first, of the point on the surface vertically above the focus.
Referring to the seismic Map A, transcribed from Zannoni's great map of the Two Sicilies; this map, embracing the whole of the observed country, is laid down upon Mercator's projection, to a uniform scale both in parallel and meridian, of 0·64 inch English, or 0·162 metre French, to the geographical mile, and is divided into squares of ten geographical miles on the side each, and numbered consecutively from Naples Observatory as Zero. The original map by Zannoni was produced between 1809 and 1812, from trigonometrical survey of the chief points, and is generally of great accuracy, and from its large scale (more than half an inch to the English mile) admits of very exact angular protraction and distance measurement.
