IN NATURE of 16th December, Prof. Tait advances the opinion that the tail of a comet consists of nothing but meteorites; mentioning in proof of this that the orbits of the August and November meteors have been determined, and found to be identical with those of two known comets. I do not question the importance of this most remarkable fact, but I think the older opinion, that the tail of a comet is gaseous, is demonstrably true. Sir John Herschel, in his “Elements of Astronomy,” remarks with wonder how the tail, in the comet's perihelion passage, is whisked round in apparent defiance of the law of inertia, so as always to keep pointing away from the sun. Were the comet an assemblage of meteorites this would be impossible; the tail would, in that case, always lie parallel to the direction of the comet's orbit. The fact just mentioned as to the perihelion motion of the tail is, to my mind, a conclusive proof that the tail is not formed once for all, but is a cloud which is constantly in process of formation, and as constantly evaporated. This view is supported by the fact that Halley's Comet was seen to increase in apparent magnitude as it receded from the sun, in consequence, as was suggested, of the conversion of invisible vapour into visible cloud as the heat grew less intense.