The same appears to be true of cotnets. The longest tails belong to those cotnets that ap proach closest to the sun's surface, and accord ingly approach and recede from it with the greatest rapidity. Of the comets that do not approach remarkably close to the stm's surface, those which come during years of great solar activity are more likely to be provided with long tails than those coming when the solar surface is quiescent. Of the past 62 years dur ing which sun spots have been recorded, 31 have been classed as of greater, and 31 as of less, solar activity. If the sun's activity had no 'influence upon a comet's tail, we should expect that an equal number of comets provided with long tails would have appeared in each of these intervals of 31 years. During the 62 years 10 comets have appeared that were furnished with long tails, and yet did not pass within 10,000,000 miles of the sun's surface. Of these, seven came during the years of great solar activity, and only three during those of less. From this we see that comets' tails, like our auroras, other tlungs being equal, appear to be more pro nounced dunng years of great solar activity.
Since the electnc current spreads out from the nucleus on all sides as it recedes from the sun, there must, by the well-known laws of currents, be a rotation produced by the mag netic force of the sun acting upon the current ((Annals of Harvard Observatory,' XXXII, 275). The tendency would be for the tail to revolve about its axis, in one direction, if located north of the sun's magnetic equator, and in the other direction if located south of it. Such a rotation has in fact been observed in the case of the bright comets of 1825 IV, 1835 III (Halley's) and 1892 I (Swift's).
Quite recently another explanation of comets' tails has been given by Arrhenius. He attributes .the recession of the particles form ing the tail, however, to the repulsion of light. It was first shown mathematically by Maxwell that light must exert a slight mechanical pres sure upon all bodies above a certain size that are exposed to it. Gaseous molecules are too small to be affected by it. In the case of large bodies it would be concealed by the over whelming attraction of gravitation. In order to be repelled from the sun, particles having the density of water must have diameters lying between 1-1000 and 1-14000 of a millimeter, or between 1-25000 and 1-350000 of an inch. For particles of greater specific gravity the dimen sions will have narrower limits.
It is most likely that several causes com bined produce the observed repulsion of the tail. Several of these are suggested by Nichols and Hull in their article describing their at tempt to produce an artificial cometary tail (Astrophysical Journal, 1903, XVII, 352). It was shown in the (Harvard Annals,' XXXII, 288, that the gaseous spectrum persisted in the tail of Swift's comet to a distance of 3,000,000 miles from the head. The presence of dust would be indicated by reflected light giving the solar spectrum. On account of the faintness of the tail we should hardly expect to distinguish the solar lines, but a distribution of photo graphic intensity in the spectnim similar to that which we find in the sun might be accepted as evidence of the presence of dust in the tail.
Such a distribution was found in the case of the bright comet of 1:.:1 III.
We may therefore say that the most prob able explanation of a comet is that the head consists of a more or less concentrated swarm of meteors enveloped in gas and dust and that the tail is a current of gas and sometimes dust proceeding from the head, being most pro nounced when the comet crosses the greatest niunber of electro-magnetic equipotential sur faces.
The shape and size of the tail enable us to compute the intensity of the repulsive force causing the tail to recede from the sun. This was done by Bredichin, who divided all comets' tails into three classes according to the sharp ness of their curvature, and suggested that the sharpness of the curve indicated the atomic weight and therefore the general chemical con stitution of the matter forming the tail. This last suggestion is open to doubt More recently, by means of photography, we have been enabled to detect condensed areas in the tails of certain comets, and by compar ing the photographs upon successive nights we have measured directly the speed of recession of the particles forming the tail from the head of the comet. Two comets so far have been measured in this manner that of lt392 I (Swift's), by the writer, and that of 1893 II (Rordame's), by Hussey. For the comet of 1892 the repulsive force was 39.5 times that of gravity. In the case of the comet of 1893, the repulsive force was 36 times as great ((Pub. Astron. Soc. Pac.,) VII, 185), portions of the tail at times receding with a velocity exceeding 150,000 niiles an hour. A remarlcable be havior of a part of the material composing the tail was observed in the bright comet, Moorehouse, during the month of October 1908. Two great cloudlike knots appeared, an approximately normal, fan-shaped tail e3otend ing outward from the first knot, and the second being merely connected to the comet's head by a narrow band of luminous matter. It was evident that for some cause the great repulsion of material had almost ceased for a short time; the matter already expelled and forming the first condensation had receded some distance when a second active repulsion took place, thus pushing away the second condensation. The latter was found to be receding with a speed of 99,000 miles an hour, which was 10,000 miles greater than the rate of recession of the first. It was this fact, combined with the forward motion of the comet itself in its orbit, that caused the two clouds of material to be seen later side by side in the interrupted tail. It has become evident from this and from several other anomalous cases that comets' tails are not due to simple light pressure, or to the comets passing steadily into regions of higher or lower electric potential, but that the full causes must be of a very complicated nature.