the head presented the appearance of distinct stratification. The forces brought into action, seemed to be intermittent, and this intermission is marked by the boundaries of the strata. There is no continuous flow in nature, and the play of a fountain gives a very good illustration of this. However steady the pressure of the water may be, the fountain plays by jerks, and not by a steady continuous stream; and the development of the comet illustrates this on a grand scale, the different envelopes being the result. What is the structure that results from this view of the comet's formation? It may be represented by a series of hollow cones packed the one within the other. The grocer's paper cones may serve as a familiar illustration, or a nest of flower-pots, the smallest being in the centre, and the others forming concentric envelopes of increasing size. This will give a striated appearance to the tail, and when there is a space between the envelopes, the successive intervals will appear comparatively dark, so that the comet may seem to have many tails. Almost all the perplexing shapes comets assume, may be explained on this assumption of hollow concentric cones.
There were three distinct stages in the development of Donati's comet. The first was that, immediately previous to its perihelion passage, on the 30th of September; the second at its nearest approach to the earth; and the third, at its nearest approach to Venus. It was ten days before the perihelion pas-
