520 BIBLIOGRAPHV OF HERSCHEL's WRITINGS. 



Svxorsis OF THE Writixgs of — Continued. 



This measure repeated 1' 50". 47. 



This measure repeated 1' 53". 437. 



This measure repeated 1' 50".G25. 



This measure repeated 1' 45". 937. 



"Maupertuis accounts for the periodical. apijoarances of changeable 

 stars by supposing that they may be of a flat form, like Saturn's 

 ring, which becomes invisible when the edge is presented to us." 



343 This star "appeared always full and round when I viewed it with a 



telescope " ; this is not necessarily opposed to Maupertuis' explana- 

 tion on account of the aberrations. 



344 " Keill says ' it is jirohablo that the greatest part of this star is cov- 



ered with spots and dark bodies, some part thereof remaining lucid ; 

 and while it turns about its axis, does sometimes shew its bright 

 part, sometimes it turns its dark side to us, etc.'" 



1780 70 507 Astronomical OhservaUons relatitHj io the mountains of the Moon. By Mr. 



IIerschel, of Bath. Communicated by Dr. Watsox, jun., of Bath, 



F. R. S. Read May 11, 1780. 

 503 The method used by Hevelius and others to find the height of a 



mountain in the Moon explained. Figure 1. 

 509-512 Quotations from Galileo, Hevelius, Lalaxde, Fekgusox, and 



Keill. 



512 Explanation of the method used by Heusciiel. 



513 The instrument used was a Newtonian reflector 6 feet 8 inches focal 



length, (usual), magnifying jiower 222 diameters, the aperture used 

 was four inches. " I believe that for distinctness of vision this in- 

 strument is perhaps equal to any that was ever made." 



514 Observations in detail from November 30, 1779, to February 19, 1780. 



517 " From these observations I believe it is evident that the height of 



the lunar mountains in general is greatly overrated; and that when 

 we have escejrted a few, the geuerality do not exceed half a mile 

 in their jjerpendicular elevation." 



518 "One caution, I would beg leave to mention to those who may uso 



the excellent 3J feet refractors of Mr. Dollond. The admirable 

 quantity of light, which on most occasions is so desirable, will prob- 

 ably give the measure of the projection somewhat larger than the 

 true, if not guarded against by proper limitations placed before the 

 object-glass." 



519 Continuation of the same observations 1780, March 11— March 10. 

 522 Additional memoranda of the manner in which Mr. Herschel made 



his observations taken from a letter of his to the Astronomer Royal. 

 Plate XI contains five figures (diagrams) to illustrate the methods of 

 observation. 



1781 71 115 Astronomical Oiscrvations on the rotation of the planets ronnd their axes, 



made with a view to determine whether the eartWs diurnal motionis per- 

 fectly equahJe. In a letter from Mr.WiLLiAM Heesciiel, of Bath, to 

 William Watsox, M. D., F. R. S. Read January 11, 1781. 



115 While every one of the motions of the earth that arise from the actions 



of the sun, moon, and planets, etc., have been investigated by as- 

 tronomers, there is one motion which has hitherto escaped the 

 scrutiny of observers — the diurnal rotation round its axis. 



116 The reason why this has not been looked into is j)robably the difli- 



culty of finding a proper standard to measure it by ; smce it is itself 

 the standard by which we measure all the other motious. 



