“Yes,” resumed Marcus, “the winds are the great water-carriers, that distribute the rain over the earth. Here, then, we have the whole list of forces employed in this wonderful rain power, viz.:
“1. The sun, to draw the water by its heat.
“2. The atmosphere, to hold it.
“3. The winds, to transport it over the continents.
“4. The cold, to discharge it from the clouds when it has reached its destination.”[[5]]
[5]. Strictly speaking, these agents may be reduced to two; for the wind is only air in motion, and cold is not a substance, but merely the absence of heat, as darkness is the absence of light.
“How does the cold make the rain fall?” inquired Ronald.
“It contracts the air,” replied Marcus, “and the vapor is consequently condensed, or crowded together, so that its particles unite and form drops of rain, which are heavier than the air, and fall to the earth. As I said before, the atmosphere may be compared to a sponge, which holds a certain quantity of water, in minute particles. When the air sponge is contracted, these particles mingle together and run out, and then it rains.”
“What a squeezing the sponge must have had to-day!” exclaimed Ronald.
“When the vapor freezes before it falls to the earth,” said Kate, “it becomes snow; and when very cold and very hot and moist air come together, they make hail, or ice.”