520 BIBLIOGRAPHY OF HERSCHEL's WRITINGS. 



Synopsis of the Weitixgs of — Continued. 



This measure repeated V 50". 47. 



Tliis measure repeated 1' 53". 437. 



This measure repeated 1' 50".G25. 



This measure repeated 1' 45".937. 



" Maupertuis accounts for the periodical appearances of changeable 

 stars by supposing that they may he 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 v/ith a 



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

 tion on account of the aberrations. 



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



ered with spots and dark bodies, some i^art thereof remaining lucid ; 

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

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



1780 70 507 Astroriommd Ohservations relating to the mountains of the Moon. By Mr. 



Herschel, of Bath. Communicated by Dr. Watsox, juu., of Bath, 



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

 508 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, Lalantje, Fergusox, and 



Keill. 



512 Explanation of the method used by Herschel. 



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



length, (usual), magnifying power 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 excepted a few, the generality do not exceed half a mile 

 in their perpendicular elevation." 



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



the excellent 3^ 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 jirojection somewhat larger than the 

 true, if not guarded against byjiroper limitations jflaced before the 

 object-glass." 



519 Continuation of the same observations 1780, March 11— March 16. 

 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 tive figures (diagrams) to illustrate the methods of 

 observation. 



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



made with a view to determine tvhethcr the earth's diurnal motion is jxr- 

 fectly equable. In a letter from Mr. William IIeeschel, of Bath, to 

 William Watson, 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 iirobably the diffi- 



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

 the standard by which we measru'e all the other motions. 



