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Prof. Piazzi Smyth’s typical Auroræ.

Prof. Piazzi Smyth was kind enough lately to send me the fourteenth volume of the ‘Astronomical Observations made at the Royal Observatory, Edinburgh, during the years 1870–1877.’ This volume, amongst its other interesting matter, affords some valuable information on the subject of the Aurora Borealis. The Aurora plates are five in number, three comprising some well-executed chromo-lithographs of typical Auroræ, from sketches made by Prof. Smyth, the other two plates being of the Aurora spectrum. The Auroræ delineated are thus described:—

Aug. 6, 1871, quiescent arc. August 21, 1871, active arc.

Plate 5. (August 6, 1871.) An example of a mild quiescent kind of auroral arc, with dark cavernous substratum. (August 21, 1871.) An example of a bright large active arc darting out rays.

Sept. 7, 1871, arc streamers and clouds. May 8, 1871, double arc (longitudinal).

Plate 6. (September 7, 1871.) An auroral arc, with streamers and dark clouds, and maintaining a bright appearance though in proximity to the moon. (May 8, 1871.) A double-arched auroral arc (the arches are longitudinally arranged).

April 28, 1871, multiple arc. Oct. 25, 1870, coloured Aurora.

Plate 7. (April 28, 1871.) A multiple-arched arc of Aurora with moonlight. (October 25, 1870.) A case of grandest coloured Auroræ, or Aurora superb and almost universal.

All the foregoing drawings are very vivid and striking, and form a most interesting set of typical forms of Auroræ.

According to my own experience, the Aurora with arches arranged longitudinally, thus, , is the rarest of all the forms. I have not met with it myself, nor do I recollect an illustration of one other than Prof. Smyth’s.