But if they move individually at an angle to that plane (as they occasionally do), they change its position—the surface, however, in which they collectively are at any moment, still remaining plane. In such a case only could such a phenomenon as was observed by Professor Tait be seen. But in such a case the visibility of the streak formed by the flight of birds would last but a few minutes, for the same motion which had in a few minutes brought the streak into view would in the next few minutes take it out of view. During the short time that a flight is visible in this way, it has an unchanging position, or a scarcely changing one. If the tail of Newton’s comet had rapidly formed and as rapidly vanished, remaining, while visible, in an almost unchanging position, the “seabird analogy” might explain that particular phenomenon, however inadequate to explain multitudes of others. But the phenomena to be explained are entirely different. Leaving out of the question the varying position and length of the tail as it approached the sun, and after it left the sun’s neighborhood, all of which were entirely inconsistent with the seabird analogy, what we are called upon to explain is that a visible tail ninety millions of miles in length, seen in position 1A on one day, was seen three days later in position 3A (having manifestly in the meanwhile passed through all the intermediate positions, including 2A). If Professor Tait, profound mathematician though he be, though he may “differentiate and integrate like Harlequin,” can show how any flight of bodies, like or unlike seabirds, can accomplish such a feat as the above, appearing first to form a thin streak A1, and in less than four days a thin streak A3, each ninety millions of miles long, without _some_ of them having had to travel a distance nearly equal to the line 1 to 3—or some one hundred and fifty millions of miles long, instead of the trifling journeys he assigned them, he should take a rank above Newton and Laplace as a mathematician. But there is another feat, apparently equally difficult to him, which he might achieve very readily with great advantage to those non-mathematicians among astronomers whom his name—well deserved, too—as a mathematician has hitherto misled, and with not less advantage to his own reputation: he might frankly admit that the idea which occurred to him while watching those unfortunate seabirds, had not quite the value which at the moment he mistakenly attached to it, and has since _seemed_ to do. [Illustration] 1 2 3 \ | / \ | / \ | / \ | / . A