Page:Proceedings of the Royal Society of London Vol 1.djvu/439

Rh tion in the form of a very long cone of light. called the tail of the comet.

The planetary body Was at no time perceptiny otherwise than circular. Its apparent magnitude was about three quarters of a second, and its real diameter is estimated at 428 miles. The position of this body never appeared to be in the centre of the head, but to be more or less eccentric at different periods of observation, but always more remote than the centre from the sun.

Nevertheless, the greatest illumination of the surrounding- head is represented by Dr. Herschel as greatest in the centre, and in its decrease from thence to be somewhat brighter on the side toward the sun than at the part occupied by the planetary body. The apparent magnitude of the head was found to measure 3% minutes; so that its real magnitude is estimated to have been 127,000 miles.

Between the head and the surrounding envelope there was a space comparatively dark, which Dr. Herschel imagines to be ﬁlled with an elastic atmosphere, and estimates its actual extent to be at least 507,000 miles, since its apparent diameter was nearly 15 minutes of a degree.

The train of light, to which Dr. Herschel give the name of envelope, from its surrounding the head on one side in a semicircular form, was found to measure 19 minutes of a degree in diameter, and was thence inferred to be 648,000 mil in real extent.

The two extremities’ of this curve being continued beyond the head in two streams of light, rather divergent from each other, form the appearance which is called the tail. The distance to which this appears to reach from the head varied on different nights, according to the state of the atmosphere, as well as from other circumstances which affected its actual length. The greatest length observed by Dr. Herschel was on the 6th of October, when he measured it 25°; but he thinks the measure of (235°, taken on the 15th of October, more to be depended upon; and he thence computes the actual length to have been at that time 100,000,000 of miles.

With respect to the curvature of the tail, Dr. Herschel remarks, that it varied not only in degree, but in direction; for on the 2nd of December he observed that it appeared convex on the following side, as if the extremity of the tail preceded the head instead of being left behind.

The author also notices many other irregular appearances of the tail, the branches on each side occasionally dividing into two or three parts, and sometimes one branch, sometimes the other, seemed longest.

From the appearances observed, Dr. Herschel next infers what is the real construction of the various parts. And, ﬁrst, the planetary body seems to be spherical, as might be expected from the common laws of gravitation, and to shine by light of its own; for if it were not so, it must have appeared to change its ﬁgure in moving as it did through more than a quadrant while it remained visible. The head also must, for the same reason, he spherical: and so likewise that portion of the envelope which is on the side towards the sun