PREFACE.

THE most marked improvement in the method of recent text-books on physics is the growing tendency to emphasize the essential continuity of the science. It is true that physics, as a matter of convenience, still has to deal with certain phenomena of the sensations of sound and of light, which logically belong to physiology or psychology; but there is comparatively little difficulty in so isolating these portions of the text from those which may be dealt with in a purely objective way that there need remain no reason for the mental confusion which so often arises from the natural tendency to accept a sensation as a just measure of its awakening cause. Thus, if we set aside Chapters XXXIV and XLIV of the following pages, which treat primarily of the relations of sensations to their physical causes, and which will doubtless long be wanting in the scientific precision that characterizes other portions of physics, we find that all the remainder can be described as a strictly quantitative study of various transferences and transformations of energy. An understanding of energy is, therefore, absolutely essential to a satisfactory intellectual grasp of physics. This can only be attained by sustained study of dynamics, whence elementary mechanics must be regarded as the logical basis of the whole science of physics. No pains should be spared on the part of the student in attaining clear notions on this portion of his This conviction has prompted the writers to make

course.

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