The fulminating spark is so fugitive that it is difficult to form an idea of its length. One could easily take it to be a yard or so long, so illusory and deceptive are our impressions. As a matter of fact, it is proved that flashes of lightning cover a distance of several kilometres. There are various methods to which one can have recourse in these scientific researches. The first, which gives the length of horizontal lightning, is based on a minute comparison between the trajectory described by the meteor and the known distance of the terrestrial points between which it travels. In order to gauge the extent of vertical lightning, you must estimate approximately the height of the clouds from which it comes, based on the irregularities of the earth of which the height is known. But there is a still simpler method for approximate measurement within the reach of every one. It consists in multiplying 337 (the number of yards traversed by sound in a second) by the number of seconds during which the thunder lasts. These methods all give the same result, and prove that lightning is often 1, 5, and 10 kilometres in length. The greatest length proved up to the present has been 18 kilometres. When one thinks of the instantaneousness of these flashes, one marvels at their incomparable agility, and we can only be lost in admiration of the magic force of the heavenly sling, which is capable of hurling these whole rivers of fire to roll in their sinuous course right through space, and in a space of time almost inappreciable to our senses.