150 On tlieSolar Spots. 



was througl) more than one fourth of the sun's diameter. Thia 

 was a remarkable phenomenon, and I have every reason to be- 

 lieve, no deception arose from inaccuracy in taking the positions. 

 Fig. 4. showed the places and dimensions of 15 spots, visible on 

 Oct. Sth, but all of which disappeared before 1 Ith. Now it is 

 obvious that in these delineations, it was perfectly indifferent in 

 what direction the axis of the sun lay. ■ In Fig. 2, however, it 

 would have been better to have constructed the true projection. 

 The place of the sun's pole and nodes might have been deduced 

 from 3 positions of the spot, with the chords of the intermediate 

 arcs, and the zenith distances, by Cagnoli's problem ; or by a 

 simple process, having the nodes given. This was deemed 

 unnecessary, as it was evident from inspection, that the path 

 of a spot, passing near the centre, and over the limb at an 

 angle of about 45° above the horizontal line, could not have 

 moved parallel with the equator, even supposing the earth 

 in that part of its orbit when the equatorial diameter is most 

 oblique to view. As this deviation therefore was unusual, it 

 was observed in the sentence immediately following, that 

 " in general the course of the maculae is, with little variation, 

 parallel to the sun's ecjuator ; but the direction in which these 

 preceded, was very singular and curious. The passage from 

 Adams's Lectures was then quoted to sanction this remark. 

 Adams's work is not at hand, but I conceived that the sentence 

 quoted, referred exclusively to the real motion of the spots, and 

 not to the apparent motions occasioned by the revolution of the 

 earth in its orbit ; nor to diurnal parallax. My observations were 

 all made within a few minutes of the meridian, and therefore di- 

 urnal parallax was out of the question. It vvas too much to infer 

 that the effects of the earth's annual motion, and diurnal paral- 

 lax, were unknown to me; and therefore the concluding sarcasm 

 was unnecessary. I maybe allowed, perhaps, to return the com- 

 pliment ; for it is amusing to observe your " Correspondent " 

 calling the luminous spots sometimes seen on the disc fasculte ! 

 Such a mistake, I may retort in his phrase applied to me, " nmst 

 be the result of a very slender acquaintance with the subject." 

 FaculiB and Faecuiae differ as much as the flame at one end of a 

 torch does, from the pitch at the opposite end ; or, in other 

 words, as light and darkness. 



It is always right to take leave in good humour ; and I doubt 

 not that a Latin dictionary will enable this " Correspondent " to 

 laugh at his own error as much as I have done. 



Your obedient servant, 

 Feb. 10, 1821. W. M. Mosjn.EY. 



To Mr. Tillock. 



Mi£R- 



