Sheet Lightning. 87 



nerely the comparatively very feeble illumination of lightning? 

 i\n answer to this query may readily be supplied. In the year 

 1739, during the course of the experiments which Cassini and 

 Lacaille were making upon the velocity of sound, the light in 

 the atmosphere produced by the discharge of a cannon near the 

 lighthouse of Cette was apparent, when they were situated in 

 stations from which the town and the lighthouse were entirely 

 obscured by intermediate objects, as, for example, the moun- 

 tains of S^t Bauzeli. Again, in the year 1803, M. de Zach used 

 signals on the Broclien mountain in the Harz for determin- 

 ing the differences in the longitude, when it was found that 

 observers placed upon the mountain Kenlenberg, at a distance 

 of more than sixty kagues, saw the flash from the simple explo- 

 sion of six or seven ounces of powder in the open air, and this 

 when the Brocken, on account of the rotundity of the earth, was 

 quite invisible from the Kenlenberg. Finally, I will add, that 

 when the guns of the lower battery at the Hotel des bivalides at 

 Paris are fired, an observer, placed in the walks of the garden of 

 the Lnxeinboiirg, near the rue d'Enfir (in a spot whence 

 neither the body of the building nor even the dome of the hospi- 

 tal can be seen), may still perceive in the sky, the moment of 

 the discharge, a flash which extends to the zenith, and even be- 

 yond it. If, then, the comparatively feeble light resulting from 

 the inflammation of a few ounces of powder is so evidently re- 

 flected in the atmosphere, what may not be anticipated from 

 the reflection from the infinitely more vivid flash of lightning ! 

 In these facts enough certainly may be found to establish 

 the possibility, or the probability, if that idea be preferred, of 

 that explanation of sheet lightning on which we have been 

 dwelling. This, however, is not all : we must attempt to 

 extend to this explanation the character which belongs to the ma- 

 jority of modern scientific theories, — we must endeavour to pass 

 from mere conjecture to real demonstration. All that is re- 

 quired, appears to be supplied in the following observations* 

 The one I have found in the " Vayage *" of Saussure, and the 

 other in carefully perusing the two volumes of Mr Luke 

 Hozcard's Meteorological Observations. In the night between 

 the 10th and 11th of July 1783, the illustrious historian 

 of the Alps was an inmate at the Hospice du Grimsel, when 



