1874.] Curved Appearance of Comets’ Tails. 513 
and these not straight but curved like a scimitar.’—From 
Popular Astronomy. By O. M. Mitchell, LL.D. 
It is interesting to find how much attention has thus been 
given to the fact that comets’ tails are usually curved ; but 
it seems that the geometrical laws which cause this curving 
have escaped observation. There is, however, one advantage 
derived from the problem, viz., that it appears it has pre- 
vented some hypotheses from being framed to account for 
what was supposed but did not occur. 
It is most essential that before we invent a theory to ac- 
count for what we see, we should be certain that we are not 
misled by mere appearances. We have demonstrated the 
cause of a comet’s tail appearing in the heavens as a curve 
to one observer, whilst it appears at the same instant as a 
straight line to another observer at a different part of the 
earth; and we have shown that this appearance is due to 
the geometry of the sphere. 
It seems singular to find that so many writers whose 
popular reputation in connection with astronomy is great 
have by their remarks on this subject shown they were quite 
unacquainted with what we may term the practical geometry 
of the sphere, and with those elementary laws of apparent 
curves and apparent straight lines in the heavens which we 
have demonstrated in connection with the pole star and 
pointers and the curvature of comets’ tails. This singular 
neglect of so important a study must not be passed over 
without some further remarks, which we bring forward for 
the purpose of eliciting truth, and with the objet of 
checking if possible an evil in conne¢tion with science which 
seems to be spreading greatly and rapidly. This evil is, 
that there are a very large number of persons who pass for 
and call themselves astronomers, who consider that this 
science, based as it is on observation, and regulated as it is 
by geometrical laws, is to be considered solely as a subject 
to be treated by physical theories, and to be explained by a 
sort of dogmatic assertion or sentence, which if it contain 
some such words as “‘ gravity,” “attraction,” or “repulsion,” 
is supposed to be unanswerable. 
When we find that even in the newspapers men write 
and state that the principal object of expeditions connected 
with the transit of Venus is to enable navigators to better 
find their latitude and longitude at sea, we at once know 
the writer to be ignorant of his subjeét. When, again, we 
find critics speaking of the rate of rotation of a planet 
being dependent on the laws of gravitation, we again know 
them to have but little knowledge of what these laws can 
