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calculation was based on the assumption that no forces were acting except the gravitation of known matter. The discordance amounts to three days at most, showing that any non-gravitational forces acting on the head are of the order of 1/10,000 of gravity.
But in the case of the tail the non-gravitational forces are predominant; further, the gases in the head are frequently almost indistinguishable in the spectroscope from those in the tail. If then the head contained nothing except these gases, it would move in the same manner as the tail does. The inference is plain that the head contains much denser matter, on which the influence of the tail-forming forces is inappreciable, and that this matter emits the gases which form the coma and tail. It seems certain that Halley’s comet will transit the sun’s disc about May 18.6, and it will be of interest to examine whether any trace of the comet on the sun can be detected. If so, it will enable us to form an estimate of the density of the head. The earth will probably pass through the tail at the same time, as happened with the great comet of 1861; it is rather curious to reflect that even passing through the tail does not make it any easier to settle the question of its composition; for in all probability it is far too rarified to have the slightest discernible effect on our atmosphere. A kind of auroral glow over the whole sky was suspected in 1861, and this should be looked for next May.
It only remains now to consider the question of the forces that produce the tail. It does not seem necessary to invoke any other agency than the solar heat to explain the emission of gas from the meteors when the comet approaches perihelion. There are at least three theories to explain the repulsion of the tail from the sun: 1) Light-pressure; 2) electrical repulsion; 3) Mechanical bombardment by electrons, or other tiny particles violently ejected from the sun. It is quite possible that all three act conjointly, as no one of them seems capable of explaining all the facts. The first and second, being central forces, could not alter the rate of description of areas of any particle of the comet about the sun. It is easy to deduce that tails produced by them would always start from the nucleus in the direction of the radius-vector produced. Now the photographs taken during the last