Claire made the calculation. The result was 4080 meters. “The flash of lightning was 4080 meters away; we are more than a league from the storm-cloud,” said her uncle. “How easy that is!” exclaimed Emile. “You count one, two, three, four, and without moving you know how far away the thunderbolt has just fallen.” “The longer the time between the flash and the noise, the farther away is the cloud. When the report comes at the same time as the flash, the explosion is quite near. Jules knows that well since the day of the storm in the pine woods.” “I have heard that there is no longer any danger after the lightning is seen,” said Claire. “A thunderbolt is as rapid as light. An electric explosion is, therefore, ended as soon as the flash appears, and all danger is then passed; for the thunder, however loud it may be, can do no harm.” ------------------------------------------------------------------------ CHAPTER XLIII THE EXPERIMENT WITH THE BOTTLE OF COLD WATER UNCLE PAUL had rightly said, the evening before, that clouds are nothing but fog floating high in the air instead of spreading over the earth; but he had not said what fogs are composed of and how formed. So the next day he continued his talk on clouds. “When Mother Ambroisine hangs the clothes she has just washed on the line, what does she do it for? To dry the linen, to free it from the water with which it is saturated. Well, what becomes of this water, if you please?” “It disappears, I know,” answered Jules, “but I should find it very hard to tell what becomes of it.” “This water is dissipated in the air, where it dissolves and becomes as invisible as the air itself. When you wet a heap of dry sand, the water permeates it throughout and disappears. It is true that the sand then takes a different appearance: it was dry before, it is wet afterward. The sand drinks the water that comes into contact with it. Air does the same: it drinks the moisture from the linen and becomes damp itself; and it drinks it so completely that all—air and water—remain as invisible as if the air held no foreign substance. Vapor is the name given to water thus made invisible, or in some sort aërial, that is to say resembling the air; and the reduction of water to this new state is called evaporation. The moisture of the linen we wish to dry evaporates; the water is dissipated in the air and thus becomes invisible vapor, which spreads in every direction at the will of the wind. The warmer it is, the quicker and more abundant the evaporation. Have you not noticed that a wet handkerchief dries very quickly in a hot sun, and loses its moisture only very slowly if the weather is cloudy and cold?” “Mother Ambroisine is always very glad when she has a fine day for her washing,” Claire remarked.