TRAXSACTIONS OP THE SECTIONS. 19 



observatories : although, from the ropes and wooden frame M^ith which 

 it was mounted, it was subjected to great hygrometic and pyrometric 

 changes of form and position. These changes, however, by equally 

 affecting the cross of the collimator, and the object itself, were readily 

 detected and corrected. 



On Halley's Comet. By Sir John F. W. Herschel, Bart. 

 F.R.S., ^c. 



" One of the most interesting series of observations of a miscella- 

 neous kind I had to make at the Cape of Good Hope, was that of 

 Halley's comet. — I saw the comet for the first time after its perihelion 

 passage on the night of the 25th of January. Mr. Maclear saw it on 

 the 24th. From this time we both observed it regularly. Its appear- 

 ance was that of a round, well-defined disk, having near its centre a 

 very small bright object exactly like a small comet, and surrounded by 

 a faint nebula. This nebula in two or three more' nights was absorbed 

 into the disc, and disappeared entirelj% Meanwhile, the disc itself di- 

 lated M'ith extraordinary rapidity ; and by examining its diameter at 

 every favourable opportunity, and laying down the measures by a pro- 

 jected curve, I found the curve to be very nearly a straight line, indi- 

 cating a uniform rate of increase ; and by tracing back this line to its 

 intersection with the axis, I was led, at the time, to this very singular 

 conclusion, viz. that on the 21st of January, at 2h. p.m., the disc must 

 have been a point — or ought to have had no magnitude at all ! in other 

 words, at that precise epoch some very remarkable change in the phj'- 

 sical condition of the comet must have commenced. So far all was 

 speculation. But in entire harmony with it is the following fact 

 communicated to me no longer ago than last month by the venerable 

 Olbers, whom I visited in my passage through Bremen, and who was 

 so good as to show me a letter he had just received from M. Bogus- 

 lawski, Professor of Astronomy at Breslau, in which he states that he 

 had actually procured an observation of that comet on the night of the 

 21st of January. In that observation it appeared as a star of the sixth 

 magnitude — a bright concentrated point, which showed no disc, with 

 a magnifying power of 140 ! And that it actually ivas the comet, and 

 no star, he satisfied himself, by turning his telescope the next night on 

 that point where he had seen it. It was gone ! Moreover, he had 

 taken care to secure, by actual observation, the place of the star he 

 observed ; that place agreed to exact precision with his computation ; 

 that star teas the comet, in short. Now, I think this observation every 

 way remarkable. First, it is remarkable for the fact, that M. Bogus- 

 lawski was able to observe it at all on the 21st. This could not have 

 been done, had he not been able to direct his telescope point blank on 

 the spot, by calculation, since it would have been impossible in any 

 other way to have known it from a star. And, in fact, it was this 

 very thing which caused Mr. Maclear and myself to miss procuring 

 earlier observations. I am sure that I must often have swept, with a 

 night-glass, over the very spot whore it stood in the mornings before 



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