Page:Popular Science Monthly Volume 26.djvu/57

Rh not more changed the conditions of business and industry than the speculations of Darwin and Helmholtz, and their compeers, have affected those of philosophy and science.

But, although this return to our birthplace suggests retrospections and comparisons which might profitably occupy our attention for even a much longer time than this evening's session, I prefer, on the whole, to take a different course; looking forward rather than backward, and confining myself mainly to topics which lie along my own line of work.

The voyager upon the inland Sea of Japan sees continually rising before him new islands and mountains of that fairy-land. Some come out suddenly from behind nearer rocks or islets, which long concealed the greater things beyond; and some are veiled in clouds which give no hint of what they hide, until a breeze rolls back the curtain; some, and the greatest of them all, are first seen as minute specks upon the horizon, and grow slowly to their final grandeur. Even before they reach the horizon-line, while yet invisible, they sometimes intimate their presence by signs in sky and air; so slight, indeed, that only the practiced eye of the skillful sailor can detect them, though quite obvious to him.

Somewhat so, as we look forward into the future of a science, we see new problems and great subjects presenting themselves. Some are imminent and in the way—they must be dealt with at once, before further progress can be made; others are more remotely interesting in various degrees; and some as yet are mere suggestions, almost too misty and indefinite for steady contemplation.

With your permission, I propose this evening to consider some of the pending problems of astronomy—those which seem to be most pressing, and most urgently require solution as a condition of advance; and those which appear in themselves most interesting or likely to be fruitful from a philosophic point of view.

Taking first those that lie nearest, we have the questions which relate to the dimensions and figure of the earth, the uniformity of its diurnal rotation, and the constancy of its poles and axis. I think the impression prevails that we already know the earth's dimensions with an accuracy even greater than that required by any astronomical demands. I certainly had that impression myself not long ago, and was a little startled on being told by the superintendent of our "Nautical Almanac" that the remaining uncertainty was still sufficient to produce serious embarrassment in the reduction and comparison of certain lunar observations. The length of the line joining, say, the Naval Observatory at Washington with the Royal Observatory at the Cape of Good Hope is doubtful, not to the extent of only a few hundred feet, as commonly supposed, but the uncertainty amounts to some thousands of feet, and may possibly be a mile or more—probably not less than a ten-thousandth of the whole distance; and the direction of the