Page:Popular Science Monthly Volume 58.djvu/561

Rh outline, the reader will gather, first, that the book possesses a certain originality of its own, it stands for solid work by its author and affords one the pleasure that such work gives. Second, it is attractive, because it marks a stage of transition. Ten years hence, these clean-cut distinctions within experience will have become impossible. The work is, therefore, to be commended as a faithful and forthright representation of that type of thinking which, though well aware of the futilities of eighteenth century dualism, has not yet awakened to the demands of twentieth century system. Being thus a type, it is well worth taking into consideration.

radiant of a shower of shooting stars is the point or area from which all the stars appear to move when perspectively projected on the celestial vault. If the tracks of a shower of meteors are laid down on a star map, and if these tracks are prolonged, all of them will intersect in a point, or, at least, within a small area—the radiant. The meteors are really moving in parallel straight lines in space. Their paths are perspectively projected into great circles of the celestial sphere, and have a common vanishing point. The case is easily understood by that of the 'sun drawing water,' which is often seen about sunset. The rays of the sun are really parallel, but they seem to radiate in all directions from the sun's disc in great circles that have a common vanishing point.

This perspective theory demands that the radiant point of a shower of meteors should rise, culminate and set by the earth's diurnal motion, precisely as the sun, or a star, rises, culminates and sets. The meteors on any night do, in fact, radiate from spots which remain fixed among the stars, and which rise, culminate and set as do the stars themselves. If the shower continues for many nights (like the Perseid shower, for instance) the place of the radiant usually shifts among the stars, as it ought to do, since its position is due to a geometric configuration which changes as the earth moves. The perspective appearances change as the place of the spectator is altered by the earth's motion in its orbit. Mr. W. F. Denning, of Bristol, England, an experienced and assiduous observer of meteors, reports that he has found cases where the appearances differ from these normal conditions. For certain showers of meteors, the radiant does not change its place among the stars as the earth moves in its orbit, but, on the contrary, the radiant remains stationary for weeks. A typical case of the sort is the shower of the Orionids. This shower persists for about two weeks (October 10-24), and the radiant remains stationary near the star ν Orionis, instead of shifting with the earth's motion as the laws of celestial perspective demand.

No satisfactory explanation of such stationary radiants has been forthcoming; and many astronomers have doubted the correctness of Mr. Denning's observations on that account. Granting that the observations are correct, an explanation of the phenomenon has been given by Professor von Niessl, of Brünn, and this explanation was briefly reported by Prof. Alexander Herschel at a recent meeting of the Astronomical Society of France. From a rather meager account of the report it appears that M. von Niessl has sought for a path of a meteor stream so situated in space and so curved that the observed phenomena would necessarily follow. Given the phenomena and the fact that they are produced by the perspective projection of the actual paths of meteors in space, he has inquired what the paths must be to satisfy all the conditions. If we assume swarms of meteors, moving with small velocities in space, in hyperbolic orbits nearly parallel, the orbits being asymptotic to the sun, meteors proceeding from such swarms would seem to have a stationary