THUNDER-STORMS. 



515 



SILENT LIGHTNING. 



When the heavens are perfectly serene in hot weather, lightnings ars f.c- 

 quently observed to continue flashing in the atmosphere for many hours unac- 

 companied by thunder. These have been called heat lightnings. Such appear- 

 ances are not confined, as has been supposed, to those parts of the atmosphere 

 which are near the horizon ; on the contrary, their light extends frequently over 

 the whole visible firmament. 



Lightning, unaccompanied by thunder, appears much more rarely when the 

 heavens are clouded. Sufficient evidence, however, of this phenomenon in dif- 

 ferent parts of the globe has been collected by M. Arago. 



Thibalt de Chanvalon, in his meteorological observations, records its occur- 

 rence on two days in July, 1751, at Martinique. Such lightning is very com- 

 mon at the Antilles. Dorta mentions the same phenomena at Rio Janeiro, in 

 a paper published in the memoirs of the Academy of Sciences of Lisbon, in 

 the years 1783, 1784, 1785, and 1787, during which time he witnessed one 

 hundred and seventy days on which lightnings were seen unaccompanied by 

 thunder. 



Lind witnessed at Patna, in India, latitude N. 25 37', in the year 1826, on 

 seventy-three days lightning without thunder ; but neither Lind nor Dorta state 

 whether the heavens were clear or clouded. The probability is, that where 

 the occurrence of the phenomenon was so frequent, they were sometimes 

 clouded. 



De Luc, the younger, mentions a great storm which took place at Geneva on 

 the 1st of August, 1791, daring which very vivid lightnings were seen without 

 any audible thunder. Some of the flashes on this occasion were so strong that 

 the loudest claps of thunder would have been expected to follow them. In the 

 same storm, however, other flashes were accompanied by loud thunder. 



Dalton states that, in Kendal, on the 15th of August, 1791, at nine o'clock in 

 the evening, he witnessed in a storm vivid and continual flashes of lightning, 

 but heard only some thunder which was distant. 



At Philadelphia, in the month of July, in the year 1841, and in New York, 

 in the following month, I witnessed frequent thunder-bursts (as they are there 

 called), in which in a clouded sky I saw a constant succession of flashes of 

 lightning, which sometimes continued for several hours, accompanied by very 

 short, occasional showers of rain. On these occasions thunder was sometimes 

 not heard at all, and sometimes it was only heard after long intervals of silence, 

 and seemed from its sound to be distant. The lightnings, nevertheless, were 

 vivid, and illuminated the heavens to the zenith. They appeared generally like 

 a light behind the clouds, the edges of which were strongly illuminated, ths 

 centres more faintly. These lightnings sometimes succeeded each oth',r so 

 rapidly that they had a fluttering appearance, like the motion of the wings of a 

 small bird ; and this fluttering of light would be often continued for three or 

 four seconds. These trembling lightnings would succeed each other '-'. inter- 

 vals of some minutes. 



OF LUMINOUS CLOUDS. 



In the darkest nights of winter, at the hour of midnight, when the influence 

 of the solar light is altogether withdrawn from the atmosphere, and in the ab- 

 sence of moonlight, a sufficient quantity of light is always uffused to render 

 objects around us faintly visible, and to enable us to walk without hesitation in 

 any open country. If the firmament be serene and cloudlese : this light is as- 



