REVIEWS—DONATIS COMET. 67 
improbable that the structure previously mentioned as described by 
Bessel may be only an imperfect observation of those detailed by 
Prof. Bond, as we notice that the descriptions and even the figures of 
many of the observers tally very closely with those of Bessel, while at 
the very same time Prof. Bond’s figures so plainly shew the envelope- 
formation. Indeed so very unlike are the drawings of the comet, 
made by different observers to represent its condition at the same 
time, that it is hard, while looking at the Plate in which they are put 
side by side, to credit that they ave intended to represent the same 
object. Of course the blame of this must be laid to atmospheric 
causes, and the inferiority of the telescopes to the great Equatorial at 
Harvard. 
We may note that Prof. Bond has calculated the nature of the 
curve followed by the outline of the head and envelopes towards the 
sun, and finds it to be a catenary, and the enveloping surface would be 
thus generated by the revolution of a catenary (not necessarily of con- 
stant directrix) about its axis. Prof. Bond could not decide whether 
the sections perpendicular to the axis were circles, and observes that 
he finds no evidence to show that they are not. We should rathen 
remark on the extreme @ priori improbability that they should 
be so. 
Before proceeding to the physical hypotheses which have been set 
forth, there are two points worth consideration. First, can a comet be 
said to be in any sense a solid or opaque body? Second, is its light 
self-derived, or merely reflected like that of the planets? With re- 
gard to the first of these, Newton remarks—( we quote the quaint lan- 
guage of his first translator): “ Now if one reflects upon the orbit 
describ’d, and duly considers the appearance of this comet, he will be 
easily satisfied that the bodies of comets are solid, compact, fixt and 
durable, like the bodies of the planets. For if they were nothing else 
but the vapours or exhalations of the earth, of the sun, and other. 
planets, [rather :—vapours or earthy exhalations of the sun and 
planets, ] the comet in its passage by the neighbourhood of the sun, 
would have been immediately dissipated. For...... the heat, which 
dry earth on the comet while in its perihelion, might have conceived 
from the rays of the sun, was about 2000 times greater than the heat 
motion of the nature of that seen in Halley’s Comet, as described by Bessel, 5th. The 
repetition of spots and rays, and other similar peculiarities of structure in successive 
envelopes, in nearly the same direction, strougly indicates that the nucleus itself constantly 
maintained the same aspect towards the sun, without sensible rotation other than is ime. 
plied in this condition, and without oscillation.” 
