238 THE ASTEROIDS. 



The discovery of these bodies has hitherto been effected simply by 

 patient and assiduous search. The asteroid hunter provides himself with 

 charts of portions of the sky about 2° square near the ecliptic, so choosing 

 his "preserves" as to have some one of them in convenient position for 

 observation at all seasons of the year. On the chart he marks down all the 

 stars visible with his instrument. The principle labor lies in preparing the 

 charts ; these once made, any interlopers are readily detected, and if planets 

 (and not merely variable stars) their motion will reveal their character in 

 a very few hours. The only remaining difficulty is to be sure that the 

 object is really a new planet, and not one of the old ones, for it has hap- 

 pened more than once that a discovery announced with something of a 

 flourish has had to be given up as a mistake for this reason. Hitherto there 

 has been comparatively little difficulty in the matter, because the "Berlin 

 Astronomical Year Book" has published each year ephemerides of all the 

 planets whose opposition occurs during the year. But the labor and ex- 

 pense of the calculations has become so great on account of the increasing 

 number, and the results are of so little importance to general astronomy, 

 that it has been decided to give them up partially, and the ephemerides for 

 1877 contain the places of only 50 out of the whole 125 which come 'to op- 

 position this year. This will often render it necessary, when a supposed 

 new planet is found, to go through a long and laborious computation in 

 order to make sure that it is not one of those already known. It is to be 

 expected, therefore, that unless this difficulty is somehow met, the number 

 of annual discoveries will greatly diminish. 



The race between the planet-hunters is frequently quite exciting. It 

 has happened several times that the same planet has been discovered by two 

 or more independent observers on the same evening, and both Goldschmidt 

 and Peters have been so fortunate as to discover pairs of planets at a single 

 sitting; the latter has done it twice. 



While these planets are personally, so to speak, of trifling account, very 

 valuable results are obtainable from the study of their motions. An excel- 

 lent determination of the solar parallax has been deduced by Galle from 

 observations on the opposition of Flora. The most reliable value of the 

 mass of Jupiter is that derived from the perturbations he. produces upon 

 the orbits of some of them. One or two cases of great prospective interest 

 are presented, where the orbits of two of these bodies so closely coincide as 

 to render it quite possible that some time they may, if they do not actually 

 collide, come to move around each other in an oval orbit like that of a 

 double star. 



Minute as they are, they are not to be despised, and it is more than 

 probable that in some Avay, though as yet beyond prediction, they will' re- 

 pay the labor spent upon them. Very few scientific facts remain forever 

 barren. — Prop. C. A. Young in Boston Journal of Chemistry. 



