Professor Tait was once looking at a part of the sky which seemed clear. As he looked, a long streak rapidly formed, which presently disappeared (if I remember his original description aright) almost as rapidly as it had formed. At any rate, the appearance of the streak was rapid enough to remind him of what astronomers said about the rapid (apparent) development of comets’ tails. The phenomenon itself was easily explained. There had been a flight of seabirds, traveling after their wont in a widely extended layer, which when he began his observations had been looked at somewhat aslant, so that—the distance being too great for the birds to be seen individually—nothing of the flight could be discerned at all. But it is evident that in such a case a very slight movement on the part of each bird would suffice so to shift the position of the layer in which they were traveling, that it would be seen edgewise, and then the birds, being so situated that the range of sight toward any part of the layer passed athwart a great number of them, would of course be seen, not individually but as a cloud, or long straight streak, a side view in fact of the layer in which they were traveling. _Eureka!_ shouted Professor Tait; and presently announced to the world the marvelous theory that the rapid formation of comets’ tails may be accounted for on the same general principle. Astronomers have found that along the tracks of some comets (where the tails never lie, by the way, but that is a detail) are countless millions of meteoric bodies separately undiscernable (and never yet discerned as a cloud—another detail); therefore it follows that the tails of all comets are formed by movements of “brickbats and paving stones” in them (Professor Tait’s own description of meteors), after the manner of the seabirds he saw from Arthur’s Seat. Professor Thomson at the Edinburgh meeting of the British Association endorsed this theory with special reference to the value of the “seabird analogy” in explaining the phenomena of Newton’s comet. Dr. Huggins, who, as he does not claim to be a mathematician (or to speak more correctly, as his labors in physical research have not given him time for profound mathematical research), may be more readily excused, also speaks of the seabird theory as if it had some legitimate standing. “The tail, he conceives,” he says, referring to Dr. Tait, “to be a portion of the less dense part of the train illuminated by sunlight, and visible or invisible to us, according, not only to circumstances of density, illumination, and nearness, but also of tactic arrangement, as of a flock of birds under different conditions of perspective.” Of course, the theory is utterly untenable—by astronomers who know something of the actual facts, and have enough mathematics to consider simple geometrical relations. Bodies moving in a plane surface like birds, if they individually travel in the same plane, keep its position unchanged.