1826.] On the confinement of Dry Gases over Mercury. 217 



beautiful and rare, and the more striking from the absence, to 

 the observer, of the sun or clouds, and the complete insula- 

 tion of the phenomenon, yet by close observation upon other 

 evenings, it was found that partial effects of the same kind 

 were very common, and, from the manner in which these could 

 be observed, the explanation above given was fully confirmed. 

 On several evenings after, when observing the sun- set from the 

 neighbouring hill of St. Catherine, it was found that if the 

 atmosphere was generally clear, but with compact and di- 

 stinctly-formed clouds floating in it, the effect was produced. 

 The usual appearance of rays at sun-set, diverging amongst the 

 clouds in the west, from the sun, is well known ; but even when 

 these were not visible, upon looking to the eastern half of the 

 hemisphere, and especially to the north or south of east towards 

 the horizon, it was rare that some clouds could not be di- 

 stinguished with long shadowy projections behind them, always 

 converging to the spot opposite to the sun. Frequently clouds 

 could be selected moving more immediately in the neighbour- 

 hood of the observer : of those which passed overhead, the 

 shadows could not be observed close. to the clouds; but 

 carrying the eye onwards towards the east, the same shadows 

 became visible, when considerably fore-shortened, and could 

 be observed moving on and changing with the clouds them- 

 selves. All these phenomena, with their variations, were 

 easily referable to their causes, and may be observed at almost 

 any sun-set in fine weather ; but the effect of the first evening, 

 so similar in kind, though so different in appearance, was not 

 again remarked. It is with a view of guarding persons who 

 may observe the same effect, against any mistake as to its 

 origin, that the appearance, with its nature, has been thus 

 particularly described. 



On the confinement of Dry Gases over Mercury *. 

 THE results of an experiment made by myself, and quoted as 

 such, having been deemed of sufficient interest to be doubted, 

 I have been induced to repeat it ; and though the original ex- 

 periment was not published by me, I am inclined to put the 

 latter and more careful one upon record, because of the strong 



* Quarterly Journal of Science, xxii, 220. 



