
PROFESSOR PIAZZI SMYTH ON COMETARY PHYSICS. 141 
a straight rigid rod, in defiance of the law of gravitation, nay, even of the received 
laws of motion, extending (as we have seen in the comets of 1680 and 1843)* 
from near the sun’s surface to the earth’s orbit, yet whirled round unbroken; in 
the latter case through an angle of 180° in little more than two hours. It seems 
utterly incredible that, in such a case, it is one and the same material object 
which is thus brandished.” 
This and much more to a similar effect might be quoted from Sir J. Hrr- 
SCHEL, and other authors, as to the difficulties experienced in the usual method 
of viewing a comet, as a planetary body at the nucleus, with an appendage 
attached to, and whirled along with it and by it. No wonder that doubts were 
expressed as to the attractive force of the small nucleus being able to retain 
within its grasp portions of matter thrown out to such distances; and that fears 
were expressed as to the breaking off of the tail; and, because some thought that 
it oughé to bend backwards from the resistance experienced in its course, there- 
fore they said that they did see it bend. Other difficulties also follow from the 
usual mechanical view of the production of the tail at perihelio, as has been 
stated by many, from the heat of the sun causing the nucleus to throw out jets 
of vapour on the side of that luminary, which again has the power to bend them 
back, and sending them streaming past the nucleus once more, forms the tail. 
These jets of vapour ought to drive the nucleus away from the sun; for though 
it may be said that the vapour, being nearly imponderable, should not produce 
any sensible or visible effect on the nucleus,—yet the nucleus itself is, for any- 
thing we know, as imponderable; indeed, if we judge of the masses of the two 
by the quantity of light reflected, which is almost the only indication we have, 
and perhaps not a very bad one, then the mass of the tail in most cases exceeds 
that of the nucleus, 7. ¢., if the quantity of light reflected by the whole envelope 
were to be concentrated into a single point, it would be brighter than the nucleus. 
Hence with the extravagant rapidity and the enormous quantity of vapour rush- 
* This is not stated with perfect correctness, at least with regard to the comet of 1843, 
which might have had a tail of that length some days after the perihelion passage, when it had 
grown with the rapid increase of its radius vector; but the first day after the perihelion passage, 
the tail was observed to be only double the sun’s diameter (excluding inclination), and its distance 
must then have been 100 times greater than at the perihelion; so that if the sesquiplicate ratio holds 
good, we shall have for that epoch a size not very different from planetary bodies. (A curious meet- 
ing this must be of the molecules brought for an instant into such close proximity, after having been 
separated for ages by distances so vast and inconceivable as they must be at the aphelion; and when 
separating for their diverse orbits, what speculations on their next meeting, not in thunder, lightning, 
and in rain, but in light and heat unspeakable. On the last occasion, February 1843, the heat was 
equal (according to Sir J. Herscuer) to 47,000 of our suns, 1900 whereof are sufficient to melt the 
veryrocks. Such, at least, must have been the heat, if the comet travelled at that part of its orbit only 
at the mean rate of the earth; but the velocity was really vastly greater, and the heat much modified 
thereby. The degree to which velocity in the heavenly spaces may modify distance, in respect of heat, 
is one still open for inquiry; and the result, in the case of our own earth, as far as it may have been 
very imperfectly examined into, would lead us to expect that the above proportion would be greatly 
reduced.) 
