300 Notices respecting New Booh. 



of May, on which day it was still visible, though with great difficulty, 

 in the equatorial. Cloudy nights prevented further operations till 

 the 11th, when the comet was searched for but not found. On the 

 12th another attempt was made, and a faint object was discovered 

 which was supposed to be the comet, and consequently prevented 

 further search ; but on the next night the object was again seen in the 

 same precise situation, and proved to be a small nebula not previously 

 known. " Clouds prevented all further search on that and several 

 subsequent nights, and when at length the weather became favour- 

 able, no trace of the comet could be found after a long and perse- 

 vering search. Thus," says Sir John, " by a most inopportune dis- 

 covery of a nebula, otherwise interesting enough, I found myself 

 deprived of the opportunity I should doubtless have otherwise en- 

 joyed, of accompanying this remote denizen of our system a few 

 steps further on its journey." — P. 398. 



After describing the successive appearances presented by the comet 

 (of which he has given a series of beautiful drawings), Sir John 

 proceeds to make some observations on its physical state as indicated 

 by the changes it underwent. " Four things," he remarks, " in the 

 post-perihelion history of this comet are especially remarkable : — 1st, 

 the astonishingly rapid dilatation of the visible dimensions ; 2nd, the 

 preservation of the same geometrical form of the dilated and dilating 

 envelope ; 3rd, the rapid disappearance of the coma ; and, lastly, the 

 increase in density and relative brightness of the nucleus." With 

 respect to the first of these, a synoptic table is given of the measures 

 of its several parts, taken from January 25 to February 11, upon 

 which he remarks, "The only element in these measurements which 

 is sufficiently definite and comparable throughout the series and at 

 the same time little affected by the unequal lights of the telescope 

 used, is the distance of the nucleus from the vertex of the parabo- 

 loid." Calculating from this element, a series of values was found of 

 " the apparent and real bulk of that definite segment of the parabo- 

 loid which may be conceived cut off at its vertex by a plane at right 

 angles to its axis passing through its nucleus." The real volumes 

 were as follows : — 



Jan. 25 1-000 Jan. 28 5-179 Feb. 1 12-242 



... 26 1-789 ... 30 9-314 ... 2 16*953 



... 27 4-089 ... 31 10-672 ... 4 22-343 



... 12 74-303 



From this it appears that the definite segment in question had 

 actually enlarged to seventy-four times its original bulk in the in- 

 terval of seventeen days between the extreme observations. The 

 total volume, however, of the whole visible paraboloid had increased 

 in a much greater proportion. By comparison of the different mea- 

 surements, it was found that the apparent linear dilatation of the 

 envelope was nearly uniform, and at the rate of 21" a day on the 

 vertical distance. Calculating backwards on this hypothesis, the 

 singular conclusion is obtained that " on the 21st of January at h 

 10 m r.M. the envelope had no magnitude ; that in short at that moment 



