THE MYSTERIES OF LIGHTNING

New Zealand Mail, 15 February 1900, Page 25

THE MYSTERIES OF LIGHTNING

The various phenomena of an electric ? arc ’ says /‘Engineering/’ susceptive ot explanation m general terms bv considering the two charged bodies—say, a pair ot clouds, on a cloud and the underlying surface of the earth—as the two coatings of a large condenser, the intervening mass of air constituting the dielectric. When the strain to which the air is subjected exceeds its “dielectric strength it suddenly gives way along a line of minimum resistence. The displacement t.-f electricity constituting tlie discharge <r ives rise to violent surgings forward and backward m the jiath thus opened, which surgings continue until the potential difference that caused them has been wiped out. The energy ot this oscillatory discharge is communicated to the surrounding air, which becomes heated accordingly. We are made aware of this-only when the released energy iy sufficient TO HEAT THE AIR UP luminescence. It then affects our sense ot sight, producing on the retina an effect depending oil the form and intensity of the flash. So lar, then, as the eye is concernea, every nasli of which, it is cognisant is necessarily accompanied by some" degree ot luimnosily. But, according to “Engineering, we have the testimony of persons whose veracity is as unimpeachable as then accuracy of observation is beyond dispute, that they several times saw in the sky tracks of discharges which were unmistakably dark. Such observations naturally demanded some consideration; the possibility of dark flashes wets, therefore, entertaii eel fer a while, ancl theories put -or ward to explain their occurrence. It was not long, however, before the astronomical idea of a "personal equation” suggested a retinal effect as a possible explanation. To many it appeared to be not only a possible, but a plausible explanation; and, indeed, the only one. So, too, thought Lord: Kelvin, when, one evening, last August, at Aix-les-Bains, he wa a favoured with a vivid dsplay of lightning. The opportunity was too good to be neglected, and he forthwith proceeded to put the retinal theory to a test; but this has noiv, as fully explained by “Engineering, been

NEGATIVED BY PHOTOGRAPHIC TESTS,

where the plate is far more sensitive to the ether ripples that consitute light than is the network of filaments which form the retina. It is, moreover, neither nervous nor impatient. Mr Clayden has established that if the plate be exposed after a flash has acted upon, it, either to the. continued action. of a feebly diffused light, or to the powerful, glare arising from one or more subsequent flashes, then on development the imago of the original flash will probably come out black, and by these and other photographic- reasearches described in “Engineering,” we know the exact meaning of the black zig-zags and their lateral ramifications which may be found in Ithe photograph,c- of lightning, and we know also how the dazzling flash of our skies may be made to yield a dark retinal impression.