56 Royal Astronomical Society. 



Such, briefly, is the well-known history of the discovery of those 

 planets of our system which required the aid of optical resources 

 and of persevering search among the stars for the detection of their 

 existence ; and it is interesting to find that we are indebted for our 

 knowledge of Astrjea to a similar sagacity and perseverance. The 

 discoverer, M. Hencke, of Driessen in Prussia, is a gentleman who, 

 at one period of his life, was employed in the post-office of that 

 town, but who, being gifted with a taste for astronomical pursuits, 

 has, for the last fifteen years, been rendering himself familiar with 

 the general features of the heavens, for the express purpose of dis- 

 covering such a body as has now rewarded his exertions. The cir- 

 cumstances which attended the discovery of the other four asteroids 

 rendered it, antecedently, extremely probable that others yet re- 

 mained to be detected ; and the difficulty lay in conducting a search 

 of such a nature. The body to be discovered would be probably of 

 a brightness equivalent to a star of from the 8th to the 10th magni- 

 tude, and the only sensible circumstance in which it would differ 

 from a star would be its motion. But the motion of a body can be 

 detected only by comparisons between its situations on different 

 days ; and there would be nothing to direct the choice of the objects 

 to be tried amongst the hundreds that one sweep of the telescope 

 would present to the observer. Nothing, then, it is evident, but a 

 complete familiarity with the part of the heavens under review, and 

 a knowledge of the relative positions of all the stars in it, to the 

 limits of the lowest magnitude above-specified, would suffice to as- 

 sure the observer of the presence of an object in a particular posi- 

 tion on one evening which did not occupy that position on a pre- 

 ceding. M. Hencke, while examining a portion of the heavens in 

 the fourth hour of R. A., on December 8, was immediately aware of 

 the presence, directly between two stars of the 9" 10th magnitude, 

 marked on the Berlin maps, and denoted by the positions 



R.A 4 h 18 m 45 a \ 



N.P.D 77° 18' 1"/ 



And R.A 4 h 20 m 20°\ 



N.P.D 77° 28' 9"/ 



of a star of the 9th magnitude, not marked on the maps ; and, from 

 his familiarity with this part of the heavens, he felt assured that the 

 star did not previously exist there. He wrote immediately to Pro- 

 fessor Encke, and soon afterwards . to Professor Schumacher (the 

 letter to Schumacher was received by him December 1.3), announ- 

 cing his suspicions of the discovery of a new planet, and giving the 

 position of the star in question for the time of his observation, viz. — 



Dec. 8, 8 h Berlin mean time. 



R.A =65 25 



N.P.D =77 19 



Professor Encke and Professor Schumacher immediately made public 

 M. Hencke's communication ; and we are indebted to them for the 

 principal sources of our information relating to the discovery, by 



