Page:Popular Science Monthly Volume 30.djvu/38

26 higher refrangibility. Langley finds these rays also abundant in the radiation from the electric arc, so that we can hardly suppose them originally absent from the solar energy. Unless there is some unsuspected error in the observations, it looks as if we must admit that they have been suppressed either in the atmosphere of the sun itself, or in interplanetary space.

Arrangements are now made by the English Solar-Physics Committee by which it is expected to secure at least one solar photograph, on a scale of eight inches to the sun's diameter, for every day of the year. These photographs are to be taken at Greenwich, at Dehra-Doon, in India; on the Island of Mauritius; and at some station in Australia: their comparison, measurement, and reduction are undertaken by the astronomer royal, at Greenwich. Much solar photographic work is also done at Potsdam and Meudon, but as yet nothing of the kind has been undertaken in the United States. Janssen has recently obtained some sun-spot photographs on a very large scale, but, so far as we know, they do not reveal anything new.

With the great twenty-three-inch telescope at Princeton, and on a few occasions, when the seeing has been fine enough to permit the use of powers of from six hundred and upward, the writer has found that, in many cases at least, the apparently club-like, almost bulbous, ends of the penumbral filaments are really fine, sharp-pointed hooks, reminding one of the curling tips of flames, or grass-blades bending over. Ordinarily they are seen as club-like, simply because of their brightness, and the irradiation and diffraction effects of moderate-sized object-glasses.

Some recent investigations upon the rotation of fluid masses, by Jukowsky, of Moscow, as applied to solar conditions by his colleague Belopolsky, seem to warrant a hope that the phenomena of surface-drift in longitude, and even the periodicity of the spots, may soon find a rational explanation as necessary results of the slow contraction of a non-homogeneous and mainly gaseous globe. The subject is difficult and obscure; but if it can be proved, as seems likely, that, on mechanical principles, the time of rotation of the central portions of such a whirling mass must be shorter than that of the exterior, then there will be, of necessity, an interchange of matter between the inside and outside of the sphere, a slow surface-drift from equator toward the poles, a more rapid internal current along and near the axis, from the poles toward the equator, a continual "boiling up" of internal matter on each side of the equator, and, finally, just such an eastward drift near the equator as is actually observed. Moreover, the form of the mass, and the intensity of the drift and consequent "boiling up" from underneath, might, and probably would, be subject to great periodical variations. Belopolsky's paper is given in the "Astronomische Nachrichten," No.2722, and there is an English notice of it in "Nature" for May 20, 1886.