50
THE
ERUPTION
OF
PELÉE
15,1905, pp. 230 et seq.) A solar corona or Bishop's ring was also observed (by M. R. Fink) on the St. Gotthard on various days between Oct., 1904, and J u n e 30, 1905; on the Tödi (Bosshardt), on Oct. 5 and 15, 1904; Great St. Bernard (Mercanton and F. C. Forel), on Nov. 5, 1904; Col de la Faucille (Sarrasin), Nov. 29, 1904; Zurich (Maurer), Dec. 4, 1904; and Morges (Forel), Dec. 8 and 12, 1904. These later appearances of this interesting phenomenon are, how ever, associated b y Forel rather with the volcanic outbreak in Iceland of April 16, 1904, t h a n with any floating ash of the West Indian volcanoes. Forel concludes t h a t the virtual continuity in appearance of the Bishop's ring is, at least, presumptive proof t h a t the ash-belt in the higher regions of the atmosphere was a continuous one. This is thought not to have been the case with the lower belt or zone of ashes which originated the crepuscular glows, for these appeared only a t irregular intervals, differing in this respect from the glows following the K r a k a t a o eruption.* Diminution in the Intensity of Solar Radiation.—A most interesting obser vation has latterly been made touching the distribution and retention of the volcanic particles in the atmosphere, namely, t h a t they have served as a cushion or screen to reduce the intensity of solar radiation. According to Henri Dufour,t such a diminution in radiation was noted, among other places, at Clarens, Lausanne, Heidelberg, Warsaw, Washington, etc., beginning in December, 1902, and continuing, b u t steadily diminishing, to March, 1903. This opacity of the atmosphere, which is attributed to the Antillean outpourings, and m a y be due directly to easier condensation of vapor under the influence of ash-nuclei, is evidenced: 1, b y diminution in the intensity of the solar radiation; 2, dimi nution of the optical transparency of the atmosphere; 3, diminution in the sky's polarization; and 4, displacement of the neutral point of Arago and Babinet. These several conditions had been clearly noted in the atmospheric disturbances following the eruption of Krakatao. Ladislas Gorczynski, who has been follow ing u p Dufour's observations and is inclined to accept Dufour's interpretation of the phenomena, notes t h a t the diminution was observed in Warsaw, Poland, as early as May, 1902, and t h a t from t h a t time it increased progressively until the spring of 1903; it had practically disappeared before the close of t h a t year.‡ The same diminution in radiation was noted at Asheville, N. C , b y Kim ball (Monthly Weather Review, May, 1903) ; at Heidelberg, b y Max Wolf (Astronom. Gesell. year 38, p a r t 2); a t Fribourg, Switzerland, by Gockel (Meteor. Zeitung, July, 1903, p. 328), who states t h a t on m a n y days the ultra violet radiation was reduced to one-half its normal value; and a t other points in Europe and America. E . Marchand, from observations made a t the Pic du Midi de Bigorre in the Pyrenees, notes a diminution in solar radiation beginning * Archives des Sciences Physiques et Naturelles, F e b . 15, 1903. †Comptes Rendus, C X X X V I , p p . 713-715; Archives des Sciences Physiques et Oct. 15, 1903, pp. 459, 460. ‡" Sur la D i m i n u t i o n de l ' I n t e n s i t é du R a y o n n e m e n t solaire en 1902 et 1903." Rendus, Feb. 1, 1904, C X X X V I , p. 255.
Naturelles, Comptes