Page:The story of the comets.djvu/51

III. gravity overcomes the pressure upwards of the water and the water begins to curl over, umbrella fashion, and to fall to the ground.

Several modern comets exhibited what may be termed jet features. The accompanying illustrations [Plate III, Figs. 11-16] bring out this idea without the necessity of any verbal description.

Closely akin to the jets of light just alluded to are the "envelopes" of which observers of large comets nearly always make mention. The best idea of what these envelopes are, and of the way in which they are given off, is to be had by comparing the nucleus of a comet to the core of an onion, and the successive envelopes of the comet to the successive skins of the onion as they come off, one by one. The existence and developement of these envelopes will usually become known and proceed somewhat in the following manner. If the comet, already possessing a bright nucleus, is approaching the Sun (and the Earth) and day by day becomes brighter, there will sooner or later be noticed something in the form of an arc or a semi-circle of light half encompassing the nucleus on the side away from the Sun. This arc of light (the outline of which will be not truly semi-circular but parabolic) will gradually stretch out and become the commencement of a tail, or if a tail already exists will become lost in it. Other arcs of light will one by one manifest themselves and spread as the first one did, so that eventually there may be half a dozen or more of these envelopes, concentric with the first, and remaining with a certain amount of permanency grouped around the nucleus. It will sometimes be seen that the innermost arc is linked with the nucleus by one or more bridges, as they may be called (often fan-shaped), the innermost end of the bridge joining on to and forming part of the nucleus, whilst the outermost extremity is lost in the nearest arc which forms a temporary boundary to it. It must be pointed out that the arcs and fans spoken of are not truly such, but only appear to be such by the unavoidable effect of perspective. The true form of an arc under such circumstances is what is called a "paraboloid of revolution"