THE PLANET EROS. 643 



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, how- 

 ever 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 tele- 

 scopes of the Lick and Yerkes observatories. He has recently pub- 

 lished 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 meas- 

 ured 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 prob- 

 ably not more than twenty-five miles, though no precise determina- 

 tion has yet been made. On such a world the force of superficial grav- 

 ity 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 



