BIBLIOGSAPHY OF nERSCHEL'S WRITINGS. 523 



Herscliel, W.: Synopsis of the Writings of— Continued. 



A.D. Vol. F. ^ . ^ , . , 



1781 71 134 Cousklcration of tlireo sources of possible error: I, a mistake in the 

 whole number of revolutions; II, a mistake in estimating the time 

 ■when a spot conies to a certain place ; III, the [determination of 

 the] time may be in error, 



135 A mistake in the whole revolutions would have maile an error of 1'' in 



the concluded rotation time, and the agreement of tlie three results 

 shows that no such mistake has been made. 

 The second cause of error is of some force. An error of 10™ is not 

 likely, as is shown by the results of some experiments. These wero 

 made by placing dots within circles drawn on paper, the (eccentric) 

 positions of the dots corresponding to positions of a spot on Mars, 

 10, 15, and 20 minutes distant from his centre. These were shown 

 to'various persons, who all agreed in locating the dots on the proper 

 side of the centre. 



136 The time was satisfactorily determined in 1779, but in 1777 not so 



much so, as I was then not provided with an altitude instrument. 



136 Allowing for this, the uncertainty of the deduced rotation period is 



estimated at not above 2^34. 



137 An ephemeris of the times of appearances of a dark spot on Mars has 



been calculated for 1781 [and is given, p. 138]. 



138 Observation of the beginning and ending of the solar eclipse of June 



24, 1778. 



1781 71 492 Account of a Comet. [The planet rra» us.] By Mr. Heksciikl, F. R. 

 S., communicated by Dr. Watsox, juu., of Bath, F. R. S. Read 

 April 26, 1781. 



492 "On Tuesday, March 13 [1781], between 10 and 11 in the evening, 

 while I was examining the small stars in the neighborhood of II 

 Gmiinorum, I perceived one that appeared visibly larger than the 

 rest Beino- struck with its uncommon magnitude, I compared it to 

 R Geminornm and the small star in the quartilo between Auriga 

 and Gemini, and finding it so much larger than either of them, sus- 

 pected it to be a comet." 



492 The magnifying power used -when I first saw the comet was .2.. 

 By applying higher powers (460 and 932) the diameter of the comet 

 increased in proportion to the power, while the diameters of the 

 stars to which I compared it were not increased in the same ratio. 



494 Measures of the comet's diameter (March 17, April 18) ; the measures 

 varyfrom3"53'"to5"20'". _ 



494 -By experience I have found that the aberration or indistinctness 

 o;casioned by magnifying much, provided the object be still left 

 sufficiently distinct, is rather to be put up Avith than the power^to 

 be reduced when the angles to be measured are extremely small. 



496 Distance of the comet from certain telescopic stars. [See diagrams ot 



these stars and the comet in Plate XXIV, 1-6-] 



497 Position-angles. 



498 Miscellaneous observations and remarks. i •.„ «„i.;f 

 March 19: It moves according to the order of the signs, and its orbit 



declines but very little from the echptic. 

 April 6: The comet appeared perfectly sharp at the edges and ex- 

 tremely well defined. 



498 Remarks on the path of the comet. mi 



499 The field of view was bright, the micrometer-thread biacK. 



