V. KELVIN ON "LIGHT" AND
"THE TIDES"
By Professor W. M. Davis
SCIENTIFIC essays, like those by Lord Kelvin on Light[1] and The Tides,[2] should be read several times by the studious reader, and each time from a different point of view. In the first reading, the reader seeks for information offered by the author; in the second, the reader examines the scientific method by which the author has gained his information; in the third, the reader's attention should be directed to the style of presentation adopted by the author in telling his story. After an attentive study of Kelvin's essays from these different sides, many a reader will find that he has made a distinct intellectual advance.
THE ESSAYS AS STATEMENTS OF SCIENTIFIC RESULTS
The first reading of either essay will disclose some of the most marvelous results that have been reached by scientific investigation. For example, it has been discovered that light is of an undulatory nature; that the vibrations of light quiver at the rate of several hundred millon of million times a second; that light is transmitted over interplanetary distances with a velocity of nearly 200,000 miles a second; and that for the transmission at such a speed through what seems to us to be empty space, as between the sun and the earth, there must be a continuous, extremely tenuous, and highly elastic medium, all pervading and universally extended, to which the name, luminiferous ether, is commonly given. It is of course not to be expected that all these and many other results, physical, geometrical, and numerical, can be easily acquired; some paragraphs must be gone over more slowly than others, and many of them should be re-
