[89] Familiar Lectures on Scientific Subjects: London, 1867; pp. 9, 10.
[90] It would be more strictly correct to say that the rate of increase varies considerably in different places, though the main fact is everywhere palpably apparent that the deeper we descend into the Earth the higher the temperature becomes. Sir Charles Lyell records a number of careful experiments made in England, France, Germany, and Italy, which seem to show that an increase of one degree Fahrenheit for every sixty-five feet of descent would represent pretty correctly the general average. See his Principles of Geology, vol. ii., pp. 205, 206.
[91] See Sir John Herschel, Familiar Lectures on Scientific Subjects, pp. 26, 27.
[92] See the elaborate work of Sir William Hamilton, entitled Campi Phlegraei, in which he gives a full account of the formation of Monte Nuovo, accompanied with colored plates. He has preserved two interesting narratives of the eruption written at the time by eye-witnesses. See also Lyell, Principles of Geology, vol. i., pp. 606-616.
[93] Sir John Herschel, Familiar Lectures on Scientific Subjects, p. 34; see also Lyell, Principles of Geology, chap. xxvii.; Mantell, Wonders of Geology, pp. 872-4.
[94] See Herschel, Familiar Lectures on Scientific Subjects, pp. 34-6. Lyell, Principles of Geology, vol. ii., pp. 104-6.
[95] Familiar Lectures on Scientific Subjects, pp. 31, 32.
[96] Principles of Geology, vol. ii., pp. 59, 60.
[97] Principles of Geology, vol. ii. pp. 69, 70.
[98] For the account of these various Earthquakes we are mainly indebted to the indefatigable industry of Sir Charles Lyell, who has collected the facts with great care partly from the descriptions of eye-witnesses, and partly from authentic documents written upon the spot. See his Principles of Geology, vol. ii., chap, xxviii., xxix., xxx. See also Mr. Mallet’s Earthquake Catalogue; and the first of Sir John Herschel’s Lectures on Familiar Subjects.