“Those clouds are called ‘cirrus’ that look sometimes like light flocks of curly wool, sometimes like drawn-out-filaments of dazzling whiteness, sharply contrasting with the deep blue of the sky. They are the highest of all the clouds. They are often a league high. When cirrus clouds are small and rounded and closely grouped in large numbers, so as to look like the backs of a flock of sheep, the sky thus covered is said to be dappled. It is usually a sign that the weather is going to change. [Illustration: Cumulus] “The name ‘cumulus’ is given to those large white clouds with round outlines which pile up, during the heat of summer, like immense mountains of cotton-wool. Their appearance presages a storm.” “Then the clouds we see over there next to the mountains,” queried Jules, “are cumulus? They look like piles of cotton. Will they bring us a storm?” “I think not. The wind is driving them in another direction. The storm always takes place in their neighborhood. There! Hear that!” A sudden light had just flashed through the flocks of the cumulus. After rather a long wait the noise of the thunder reached them, but greatly weakened by distance. Questions came quickly from Jules’s and Emile’s lips: “Why does it rain over there, and not here? Why does the noise of the thunder come after the lightning? Why—” “We are going to talk about all that,” said Uncle Paul; “but first let us learn the other forms of clouds. ‘Stratus’ is applied to clouds disposed in irregular bands placed in tiers on the horizon at sunrise or sunset. They are clouds that, in the fading daylight, especially in autumn, take the glowing tints of melted metals and of flame. The red stratus of the morning are followed by rain or wind. [Illustration: Stratus] “Finally, we give the name ‘nimbus’ to a mass of dark clouds of a uniform gray, so crowded together that it is impossible to distinguish one cloud from another. These clouds generally dissolve into rain. Seen from a distance, they often look like broad stripes extending in a straight line from heaven to earth. They are trails of rain. “Now Emile may ask his questions.” ------------------------------------------------------------------------ CHAPTER XLII THE VELOCITY OF SOUND “UNDER that big white cloud that you call cumulus,” said Emile, “there is at this very moment a storm. We have just seen the lightning and heard the thunder. Here, on the contrary, the sky is blue. So it does not rain everywhere at the same time. When rain is falling in one country, it is fine in others. And yet, when it rains here the whole sky is covered with clouds.”