H. M. Parkhurst—Tails of Comets. 37 
Art. V.—On the Tails of Comets; by HENRY M. PARKHURST. 
[Read before the Amer. Assoc. for the Adv. of Science, Aug. 18, 1874] 
IN predicting the form and position of a comet’s tail, I adopt 
the theory of Professor Pierce : 
“Each particle of the matter which composes the tail is sup- 
posed to move in a hyperbolic orbit, with the sun in the focus 
of the opposite branch, under the influence of a repulsive force 
emanating from the sun, and decreasing by the law of the in- 
verse square of the distance.” (Gould's Astronomical Journal, 
vol. v, page 186. 
The application of the formule gives for any is ae time 
one line only, commencing at the comet and extending indefi- 
nitely. Professor Pierceadopted, in his computations upon the 
tail of Donati’s comet (Gould’s Astr. J., vol. vi, page 51), 
an excess of repulsive force 14 times that of gravitation, mak- 
ing his comparisons with the front edge of the tail, and arbi- 
trarily adopting such a repulsive force as best to explain the 
observations. For the purpose of comparing with the cen- 
ter of the tail, I adopted in my computations a repulsive 
force exactly equal to and replacing gravitation as the more 
probable law. On comparison with the observations, how- 
ever, I find that the computed line closely agrees in nearly 
every instance with the observed right hand edge of the tail; 
and I shall, therefore, adopt that as the standard of comparison. 
Although the comet apparently moved in the other direction, 
yet in fact that was the front edge of the tail. (% 
Most of the earlier observations were furnished to me by W. 
S. Gilman, Jr., of New York City, and all the later ones by 
Lewis Swift of Rochester, whose observations continued several 
anys after the comet had ceased to be visible in New York 
It 
y. 
_ From June 12 to June 30, I have only the record of the direc- 
tion of the tail, without reference to stars (G), agreeing with com- 
putation. On July 1, the tail “pointed to and reached 55 of 
Camelus.” (P.) The computed line passes nearly through that 
star. On July 7, the front edge was seen a little to the left of 
63 Arg. 749. (Bonn Catalogue.) (P.) The computed line 
passes a small fraction of a degree to the left of the star. On 
curvature on this evening was commonly noticed. Computa- 
tion indicates an arc of about 15°. On July 14, “A U. Maj. 
was in the middle of the tail.” (S.) Allowing for the recorded 
