517 



this light was equally spread in all directions. It resembled the light of the 

 moon seen through clouds. 



De Luc states that, returning home to his lodgings in the neighborhood of 



London, on a winter night, when the atmospherewas clear, and not cold, he 



saw a band of clouds intersecting the southern meridian, about thirty or forty 



degrees from the zenith, and extending on either side nearly to the eastern and 



( western horizons. The brightness of this cloud resembled that of a thin cloutj 



I concealing the moon, and was sufficient to render the stars in its neighborhood 



( invisible. 



Dr. Robinson, professor of Astronomy at Armagh, states, in a letter to M. 

 Arago, that, during the voyage of Major Sabine in Scotland, undertaken to ob- 

 serve the lines of equal magnetic intensity, that officer, being at anchor in 

 Lough Scarig, in the Isle of Sky, observed a cloud which constantly enveloped 

 the summit of one of the naked and lofty mountains which surround that island. 

 This cloud, which resulted from the precipitation of the vapor brought by the 

 constant west winds from the Atlantic, was self-luminous at night, not occa- 

 sionally, but permanently. Major Sabine saw frequently issue from it jets of 

 light resembling those of the aurora. He rejects, however, the supposition that 

 these jets were produced by real auroras .near the horizon, and which were 

 concealed from direct observation by the mountain. He regarded all these 

 phenomena of continued and intermitting light as originating in some physical 

 property of the cloud itself. 



OF THUNDER. 



Thunder, as every one knows, is a certain noise, proceeding apparently from 

 the clouds, which usually follows, after a greater or less interval, the appear- 

 ance of a flash of lightning. Of all natural phenomena, those which occupy 

 the meteorologist present the greatest difficulties, when it is necessary to con- 

 vey a precise notion of them to those who may not immediately have witnessed 

 them. It is, doubtless, to this difficulty that we must ascribe the practice of 

 meteorological writers of resorting to similes and other like illustrations in their 

 descriptions. 



Thunder is described by some as a sound resembling the acute noise pro- 

 duced when stiff paper is torn, or when a strong silk cloth is suddenly torn, or 

 \vhen a heavy wagon is rolled rapidly over a rough, stony road. It is imitated 

 with much effect in theatres by shaking a piece of sheet-iron about four feet 

 long and two feet broad. This is held in the hand at one of its corners, and 

 the varieties of thunder may be imitated by skilfully varying the movement of 

 the hand. 



Thunder is sometimes heard as a clear, single, distinct sound, like the report 

 of a gun, unattended by any reverberation. More frequently the sound is deep, 

 or, in a musical sense, grave, and consists, not of a single sound, but of that 

 rapid succession of sounds, first increasing and afterward diminishing in inten- 

 sity, which has been expressed by the term rolling. 



The difficulty of expressing and recording in words the exact nature of such 

 phenomena has limited to a small number the observations on which any safe 

 reasoning can be based. 



The duration of the rolling of thunder was observed and recorded by De 

 L'Isle, in Paris, in the year 1712. On one occasion it was observed to endure J 

 for forty-five seconds. On other occasions, during the same storm (17th June), 

 the roll continued from thirty-four to forty -one seconds. On the 3d, 8th, and 

 28th of July, the roll continued on different occasions from thirty-five to thirty- 

 nine seconds. 



