viewed more than once; some are difficult of comprehension because they are without the vivid experiments by which they were illustrated in the original lecture; and others because they are compressed into terse statements without explanation. But at the end of what is here called the "first reading," many of the conclusions announced regarding the nature of light should be fairly familiar. Similar examples may be drawn from the lecture on the tides; the larger share of mathematical considerations here encountered may make the second essay more difficult than the first; if some readers do not clearly understand, for example, the statement regarding diurnal inequality (p. 291), they may be excused, for the statement is very brief; similarly, the account of the tide machines (pp. 293–297) is too dense to be really comprehended by a non-mathematical reader, previously uninformed on such matters as harmonic analysis.
THE ESSAYS AS EXAMPLES OF SCIENTIFIC METHOD
The second reading of the essays, directed to an examination of the scientific method employed by the author, should have for its most valuable result a better appreciation of the nature of "theorizing" than most persons possess. The immediately observable elements of such phenomena as light and tides are called "facts"; but an intelligent inquirer is soon persuaded that the facts of observation are really only a small part of the total phenomena. For example, some invisible factors must determine that the noonday sky overhead is blue, and the horizon sky near sunset or sunrise is yellow or red. Or, some unseen factors must determine the strength of the tides and their hour of occurrence varying from day to day. How can light travel at its incredibly rapid velocity? How can the moon cause changes of sea level on the earth? The true answers to such questions would acquaint us with phenomena that, in spite of their invisibility, take place just as truly as the phenomena that we observe. Such unseen phenomena might be called "facts of inference," to distinguish them from "facts of observation." To discover the facts of inference and to demonstrate their connection with the facts of observation is the effort of all theorizing. A theory is, in brief, a statement in which the supposed facts of inference are reasonably
