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II. — Bright Clouds on a Dark Night Sky. By C. Piazzi Smyth, Astronomer- 

 Royal for Scotland. (Plates II. to XIV.) 



(Bead 18th June 1883. 



PAQK 

 11 



P.AKT I. Statement of the case, and first reference to Meteorological Observations, . . 



Pakt II. Further reference to Meteorology and its computations, . . ... 15 



Part III. Supposed physical cause of the phenomenon. Diagram on p. 18, 17 



Appendix I. Scottish bi-diurnal Meteorological Observations, April 6-10, 1882, ... 22 



Appendix II. Projections of the same in Plates II. to IX., ..... 29 



Appendix III. Continuous hourly observations and their projections in Plates X. to XIV., . 30 



Appendix IV. Further testimony from Royal Observatory, Greenwich ; Gordon Castle ; and Royal 



Observatory, Edinburgh, touching Aurora, . . . . . . . 34 



Part I. 



On a moonless night, whenever clouds of an ordinary elevation in the 

 atmosphere appear upon, or pass across, the star-spangled sky behind them, 

 they exhibit themselves, as a rule, dark, sometimes even black, in comparison 

 therewith. And no wonder, when everypart of the open sky from visible star to 

 visible star therein must be lit up to some, though doubtless a very small, extent 

 by the faintest general and cumulative radiance of those myriads and myriads 

 of lesser stars, which only a large telescope can show to be individually 

 existent as actual stellar points of light, but in their aggregate more nearly 

 eternal, and still more constant from age to age, than our gigantic Sun itself. 



If the heavens become entirely covered by such clouds, a very dark night is 

 the usual and natural result. That at least is my experience on the Calton 

 Hill through thirty years of observation ; and something very like it is probably 

 so generally recognised as fact over the whole country, that this formal state- 

 ment about it, may appear to many persons a needless truism. 



Yet such darkness of elevated midnight clouds is not without exception. 

 For on April 8, 1882, on looking out Northward about 9.30 p.m. from an upper 

 chamber in the house No. 15 Royal Terrace, to see if there was any Aurora to 

 observe, I saw indeed no Aurora, but in place of it two or three decidedly 

 luminous clouds, on an otherwise dark by comparison, but star -bearing, sky. 



The rounded forms of these clouds were so contrary to the regular arcs and 

 needle-shaped darts of Aurora, and there was so marked an absence of the 

 very dark regions which usually appear on the Northern horizon, low down 

 underneath an Auroral display, that I went next to the South side of the house, 

 and there, to my greater surprise, saw that all the clouds visible from thence 

 were luminous with a white, moon-like radiance. Any portions of sky between 



VOL. XXXII. PART I. C 



