

(OBB 
VI.—Some remarks on Theories of Cometary Physics. By C. Piazzi Smytu, Esq., 
E.RSE., Astronomer Royal for Scotland, and Professor of Practical Astro- 
nomy in the University of Edinburgh. 
(Read 1st April, 1850.) 
While the physical appearances of comets have ever excited such intense 
curiosity and interest, all the theories concerning them are generally confessed to 
be insufficient to explain them; and, certainly, if we may judge from the various 
views advocated by different writers, and the anomalous forces gratuitously 
brought in to support the different hypotheses—it is so. 
This unsatisfactory state of things, so different from that in which is the 
theory of the motions of comets,—seems to be owing partly to the difficulty of 
making the necessary observations by reason of the undefinable nature of the bodies 
themselves, and partly from the untoward circumstances under which the obser- 
vations must be made, as well as the rareness of any opportunities offering. 
Hence, theories are built upon accounts handed down from old astrological times, 
when men’s prejudices would have prevented them, even if their means had been 
ample, which they were not, from giving any satisfactory and trustworthy ac- 
counts of the phenomena displayed by the heavens of their day. 
Then, again, the theories appear to have failed from attempting too much, 
attempting things not legitimately within their reach; it would have been enough 
to determine the laws of the changes which the tail undergoes during the orbit 
of a comet; but in place of this, they attempted to shew why the tails were there, 
and how they came into existence. ‘This is as much as in the planetary theory 
to attempt to determine why Saturn has rings; a problem which would have 
eluded the grasp even of Nrewron, and will for ever remain wrapped up in the 
mystery of creation; enough for us that the rings are there; we can measure 
their diameter and thickness, approximate to their weight, and determine the 
laws of their rotation, and alternate appearance and disappearance to the earth, 
and to their own planet; and something of the same sort we may expect to be 
able to do in the case now before us. 
It has been remarked that theory and fact sometimes unite, and that some- 
thing of theory is necessary to enable us to speak correctly of facts. Many in- 
stances of this occur in the history of most of the sciences, but in none have the 
facts been more misinterpreted by the vulgar feeling of the senses, in the absence 
_ of correct theory, than in the case of the physical characteristics of comets. No 
phenomena were so likely to be misinterpreted by reason of the strong pre- 
judices almost innate in men’s minds, as well as the specious and inexplicable 
character of the appearances themselves. Accordingly, because the tails of 
VOL. XX. PART I. 2N 
