Page:Popular Science Monthly Volume 58.djvu/651

Rh photographs of the same region for comparison. To pick out the trails, however, is the work of an hour. The finding of the images on the photographs is only a small part of the work involved. First, one must know whether the object seen is new or old. This implies tables giving the positions of all known asteroids, the computation of which involves a great amount of labor, and, in most cases, the results in themselves seem to be of small value. With the greater telescopes and more sensitive plates of the future, it seems probable, unless some kind Providence prevents it, that the number will become so great that astronomers will grow weary of the enormous labor involved in making ephemerides of them all. Twenty-two of them are, as Professor Young expresses it, 'endowed.' These were discovered by Professor Watson, who, at his death, left a fund to bear the expense of taking care of them. These favored ones will evidently be followed carefully, however unobserved their less aristocratic sisters go sweeping on in their neglected orbits.

It is probable that all the larger asteroids have already been found. Professor Barnard has made many micrometric measurements of the diameters of the largest of these baby worlds, using the great telescopes of the Lick and Yerkes observatories. He has recently published in the 'Monthly Notices' of the Royal Astronomical Society the following results:

The albedo, or light-reflecting power, is referred to that of Mars as unity. The values in the third column are derived from the measured diameters and the known brightness of the asteroids. Vesta, though not the largest by the above measures, is the brightest of them all, and is sometimes visible to the naked eye. Probably none, except the four given above, has a diameter as great as 100 miles, and the vast majority perhaps not more than ten or twenty miles. Eros itself, at its nearest approach, will perhaps present a disc of sufficient size to permit measurements in the most powerful instruments. Its diameter is probably not more than twenty-five miles, though no precise determination has yet been made. On such a world the force of superficial gravity would be about one three-hundredth of that at the surface of the earth, and a person might almost throw a stone with sufficient velocity to make it fly off into space and become an independent planet. To