308 L. Boss—Tail of Comet 6, 1881. 
The tail of the comet of 1807 presents most striking resem- 
blance to this under discussion. On October 22, 1807, the 
comet of that year had, generally speaking, the same position 
in space as the present comet had on July 22. On that occa- 
sion (Astr. Nachr., vol. xiil, p. 228), Bessel found two tails. 
The first he considered to be nearly straight and in length 
about 45°. The other was strongly curved, broader than the 
first, and in length about 8°. Dr. Bredichin (Mosc. Ann., vol. v, 
pt. 2, p. 56), has computed the value of ¢g’ for the end of each 
tail. This enables us to compare the two descriptions in a very 
satisfactory manner. We have— 
Comet of 1807. Comet of 1881. 
A fi 8 A i 8 
For the right-line tail, - ae ey Sika aay ad 082° 6°25" -0 
-For the curved tail, - - a LOG. 24°23 30 ‘O67. J49 va 9 
Allowing for the difference in values of 4 and 7, the agree- 
ment is quite within the probable errors of observation. It is, 
thus seen that there is great similarity in the physical appear- 
ance of the two comets, as well as between the elements of their 
respective orbits. Since, in general, we have the greatest pos- 
sible variety in the appearance of the tails of the comets, and 
especially in the combination of tails of different types, we may 
confidently say, that the very remarkable similarity above 
' shown furnishes another important fact, in addition to those 
which already tend to indicate a common origin for the comets 
of 1807 and 1881. 
Sir Isaac Newton and others after him have shown that the 
tail might be produced by a repulsive force emanating from the 
sun, and acting on detached particles, which are continually 
thrown out from the nucleus of all great comets. Bessel has 
investigated formule (Astr. Nachr., vol. xiii) which enabled 
him to compute the repulsive force necessary to produce a tail 
of the form actually observed in the case of Halley’s comet. 
The repulsive force in these formule is, of course, an implicit 
function. Bessel’s formule are shown (Mosc. Ann., vol. v, pt- 
2) to give results which are but roughly approximate for large 
distances from the nucleus. Professor Norton, Dr. Bredichin 
and others have published formulz which are more rigorously 
exact. In all these investigations it is supposed that a particle 
projected from the nucleus is repelled by a force (1—y) the re- 
verse of the Newtonian. e effective force acting on the par- 
ticle will be #, and when combined with the tangential velocity 
of the nucleus will cause it to describe a hyperbolic orbit. This 
hyperbola will be convex or concave to the sun, according as 
(1—,) is greater or less than unity. In the volumes of the Mos- 
cow Annals, Dr. Bredichin presents a variety of reasearches 
concerning the consequences to be deduced from this assump- 
tion of repelling forces. 
