Most dry climates are subject to periodical droughts. In Australia, they return after every ten or twelve years, and are then followed by excessive rains, which gradually become less and less till another drought is the consequence. When Mr. Darwin was in South America, he passed through a district which had long been suffering from dry weather. The first rain that had fallen during that year was on the 17th of May, when it rained lightly for about five hours. “With this shower,” he says, “the farmers, who plant corn near the sea-coast, where the atmosphere is more humid, would break up the ground; with a second, put the seed in; and, if a third should fall, they would reap in the spring a good harvest. It was interesting to watch the effect of this trifling amount of moisture. Twelve hours afterwards the ground appeared as dry as ever; yet, after an interval of ten days, all the hills were faintly tinged with green patches; the grass being sparingly scattered in hair-like fibres a full inch in length. Before this shower every part of the surface was bare as on a high road.” A fortnight after this shower had fallen, Mr. Darwin took an excursion to a part of the country to which the shower had not extended. “We had, therefore,” he says, “in the first part of our journey a most faint tinge of green, which soon faded away. Even where brightest, it was scarcely sufficient to remind one of the fresh turf and budding flowers during the spring of other countries. While travelling through these deserts, one feels like a prisoner, shut up in a gloomy courtyard, longing to see something green, and to smell a moist atmosphere.”