CHAP, in.] The Third Comet seen. 85 



nation also of the parallel can hardly err so much as 15 

 seconds of a degree. 



This, and several evenings afterwards, I viewed the comet 

 again with such powers as its diluted light would permit, 

 but could not perceive any sort of nucleus which, had it 

 been a single second in diameter, I think, could not well 

 have escaped me. This circumstance seems to be of some 

 consequence to those who turn their thoughts on the inves- 

 tigation of the nature of comets, especially as I have also 

 formerly made the same remark on one of the comets dis- 

 covered by Mr. Mechain in 1787, a former one of my sister's 

 in 1786, and one of Mr. Pigott's in 1783> in neither of 

 which any defined, solid nucleus, could be perceived. 

 I have the honour to remain, 

 Sir, &c., 



WM. HERSCHEL. 

 SLOUGH, NEAR WINDSOR, 



March 3, 1789. 



The third comet was discovered on the 7th January, 

 1790 ; the fourth on the 17th April of the same year, 

 during her brother's absence from home. It was 

 announced to Sir Joseph Banks in the following 

 letter : 



April 19th, 1790. 



SIR, 



I am very unwilling to trouble you with incomplete 

 observations, and for that reason did not acquaint you 

 yesterday with the discovery of a comet. I wrote an 

 account of it to Dr. Maskelyne and Mr. Aubert, in hopes 

 that either of those gentlemen, or my brother, whom I 

 expect every day to return, would have furnished me with 

 the means of pointing it out in a proper manner. 



But as perhaps several days might pass before I could 



