Page:Great Neapolitan Earthquake of 1857.djvu/20

viii for, observed, and discussed by a competent investigator—by one conversant with the dynamic laws of the hidden forces we are called upon to ascertain by means of the more or less permanent traces they have left, as Phenomena, upon the shaken territory.

"Observed without such guiding light, or often passed by unnoticed and undiscovered for want of it—the facts hitherto recorded are in great part valueless—but with this guide such investigation is capable of results of high importance. Thus it was that Dolomieu's elaborate record of the effects of the great Calabrian Earthquake is of so much less value than it might have been.

"Earthquake observations are of two classes—those which must be made before and at the moment of shock (time and space measurements chiefly) and those which may be made at a recent period after it. To the latter belong those numerous and instructive facts treated of under the heads of Secondary and Accidental Phenomena in my First Report on the Facts of Earthquakes ('Reports Brit. Ass. 1850'), and also in the 'Admiralty Manual'—as well as many questions treated of under heads 15' to 24' of the former. In those papers I have stated some of the methods of observation—of shattered buildings—altered water courses and springs—changed relations of level and position—localities of maximum and minimum disturbance—their relations to origin—to formation, &c., &c., and the inferences deducible. I need not, therefore, dilate upon them here.

"I have long looked for the occurrence of an opportunity so favourable for inquiry as that which has been