Rojal Society of London. 5% 



paring its apparent difc with that of the Georgian planet. It 

 was Thin ihoucrht very extraordinary that the earculatious, 

 ibunJed upon iuch a comparifon, and upon the molt proba- 

 ble diliance of Ceres from the fun, made the rea! diameter 

 of that newlv-difcovered celeftial body eertainly lefs than 

 5-8fhs of the diameter of our moon. By Dr. Herfchel's 

 Vubfequent obfcrvations, however, detailed in the firfl of the 

 two papers above mentioned, and which were made in much 

 more favourable weather, the diameter of Ceres was found 

 much fmaller than what he inferred it to be from his former 

 obfervations of the 1 1 th of February. His later obfcrvations 

 confiited cliiefly in his taking repeated meafures by means of 

 his lamp-micrometer; an apparatus long ago contrived by 

 Dr. Herfchcl, and publiilied in the Philofophical Tranfac- 

 lions, for eftimating the extent of angles much fmaller than 

 a fecond of a degree. By fimilar oblervations he found the 

 apparent diameter of Pallas to be lefs than that of Ceres ; and 

 from thofe apparent diameters, and the beft data for the di- 

 Itances from "the fun, Dr. Herfchcl has computed their real 

 diameters to be about 160 miles for Ceres, and about So for 

 i'allas; from which it appears, that out of ihe quantity of 

 matter which the planet Mercury contains 73.839 fuch bodies 

 as Pallas could be formed. The exiltence of Iuch relatively 

 minute bodies in the heavens belonging to the folar fyftcm, 

 and moving according to the law of gravitation, is a cireum- 

 ftance which appears to us extremely interelling; the know- 

 ledce of which muft Itiil further diliinguifli the prefent itra 

 of aftronomy, which has already been rendered fo illuUrious 

 by Dr. HerfchLl's own labours m the field of difcovery. 



' In his paper of the 6th of May he gives an account of 

 feveral obfervations of a fmall coma or hazinefs which fur- 

 rounded both Ceres and Pallas, the appearance and extent of 

 which feemcd to vary according to the ftate of the air. Au 

 account is alfo given of the method he followed lo fatisfy him- 

 felf that thefe two (tars have no latellites belonging lo tliem. 

 In the fame paper Dr. Herfchcl points out the advantage 

 of diliin'juidmic; fuch new celeltial bodies from the planets 

 and conTets i)v"fome appropriate name wliieh will admit of 

 a full and precife definitit)n. liy reafon of their diflering fo 

 much from planets and comets as fcarcely to be perceivable 

 from minule fixed liars, even by very good telcfcopcs, Doctor 

 Herlclitl has adi)p!ed the term ajlerouis to denote them; but 

 rcfcrving ilie liberty of fubftituling afterwards any other ap- 

 pellation which he may think more ajipolile, and to which 

 the prccife and ct)|)i<)us dclinition laid down in his paper may 

 c<iMallv apply. He next remarks, that from the nature of 



ajUroids 



