which cannot well be explained without them, and they offer far better means of conveying a knowledge of the relative positions of the various countries, seas, etc., than any maps. The great expense of globes has hitherto prevented their very general use, but some experiments are at present being made with a view to lessening the cost of the construction, which it is hoped may be successful. I cannot pass from this subject without alluding to that class of maps which gives life to the large volumes of statistics which are accumulating with such rapidity. On the Continent these maps are employed to an extent unknown in this country, both for purposes of reference and education, and they convey their information in a simple and effective manner.
| THE TRANSIT OF VENUS. |
By Prof. S. P. LANGLEY,
OF THE ALLEGHANY OBSERVATORY.
ON the 8th day of the present month, at a little before nine in the evening of our time, the planet Venus will be first seen entering upon the face of the sun, from that side of the earth on which it is then day, and to observe the event astronomers will have made their way from all the principal countries of the civilized world. The spectacle in itself offers nothing that is imposing; to the naked eye, indeed, nothing of it will be visible, and all that the best telescope can discern will be a small, black, circular spot moving across the upper part of the solar disk, during some four and a half hours. The interest of the occasion, as all know, lies in the rare opportunity it offers for obtaining the sun's distance from the earth; but, as it is not so well understood why this distance is wanted, why it has not been found before, and what Venus has to do with determining it now, it is proposed here to attempt to answer such questions, as fully as it can be done in general and untechnical terms, and in a single article.
The exact object to be obtained can be better understood after considering what we know about the relations of the sun and planets, and what we have yet to learn. We know already, then, with almost entire exactness, the relative distances from the sun of every planet (the earth included), so that, if we wished to make a map of the solar system, on which the position of each member should be laid down with great precision, we have already all the means at hand to do it. Let us suppose such a map to be drawn, in which circles around a central point represent the planetary orbits. Then the planets being ranged in a line from the sun, and the distance of Venus from it being let us say five inches, that of the earth will be seven, and that of
