Nebula ♅ V. 24 Comæ—after Roberts.
Showing globular structure.
Lastly the spirals are evidently thicker near the centre, thinning out at the edge, and when the central nucleus is pronounced, it seems to have a certain globularity not shared by the arms, and more or less detached from them. This appears in those cases where they are shown us edgewise, and it has been thought perceptible in the great nebula of Andromeda. The difficulty in establishing the phenomenon comes from the impossibility of both features showing at their best together. For the globularity to come out well, the spiral must be presented to us nearly in the plane of sight; for the spirality, in a plane at right angles to it.
Much may be learnt by pondering on these peculiarities. The widespread character of the phenomenon points to some universal law. We are here clearly confronted by the embodiment of a great cosmic principle, causing the helices it is for us to uncoil. It is a problem in mechanics.
In the first place, a spiral structure denotes action on the face of it. It implies a rotation combined with motion out or in. We are familiar with the fact in the sparks of pin-wheel pyrotechnics. Any rotating fluid urged by an outward or an inward impulse must take the spiral form. A common example occurs in the water let out of a basin through a hole in the centre when we draw out the plug. Here the force is inward, and because the bowl and orifice are not perfectly symmetric, a rotation is set up in the water trying to escape, and the two combine to give us a beautiful conchoidal swirl. In this case the particles seek the centre, but the same general shape is assumed when they seek to leave it.
Another point to be noticed is that a spiral nebula could not develop of itself and subsist. To continue it must have outside help. For if it were due to internal explosive action in the pristine body, each ejectum must return to the point it started from, or else depart forever into space, for the orbit it would describe must either be closed or unclosed. If the former, it would revisit its starting-point; if the latter, it would never return. Explosion, therefore, of itself could not have produced the forms we see, unless they be ephemeral apparitions, a supposition their presence throughout the heavens seems effectually to exclude.
