"It follows, as a matter of course, that the _dry season_ commences first, and continues longest in the southern portions of the territory, and that the climate of the northern part is influenced in a much less degree, by the causes which I have mentioned, than any other section of the country. Consequently, we find that, as low down as latitude 39°, rains are sufficiently frequent in summer to render irrigation quite unnecessary to the perfect maturity of any crop which is suited to the soil and climate. "There is an extensive ocean current of cold water, which comes from the northern regions of the Pacific, or, perhaps, from the Arctic, and flows along the coast of California. It comes charged with, and emits in its progress, cold air, which appears in the form of fog when it comes in contact with a higher temperature on the American coast, as the gulf-stream of the Atlantic exhales vapor when it meets, in any part of its progress, a lower temperature. This current has not been surveyed, and, therefore, its source, temperature, velocity, width, and course, have not been accurately ascertained. "It is believed, by Lieutenant Maury, on what he considers sufficient evidence--and no higher authority can be cited--that this current comes from the coasts of China and Japan, flows northwardly to the peninsula of Kamtschatka, and, making a circuit to the eastward, strikes the American coast in about latitude 41° or 42°. It passes thence southwardly, and finally loses itself in the tropics. "Below latitude thirty-nine, and west of the foot-hills of the Sierra Nevada, the forests of California are limited to some scattering groves of oak in the valleys and along the borders of the streams, and of red wood on the ridges and in the gorges of the hills--sometimes extending into the plains. Some of the hills are covered with dwarf shrubs, which may be used as fuel. With these exceptions, the whole territory presents a surface without trees or shrubbery. It is covered, however, with various species of grass, and, for many miles from the coast, with wild oats, which, in the valleys, grow most luxuriantly. These grasses and oats mature and ripen early in the dry season, and soon cease to protect the soil from the scorching rays of the sun. As the summer advances, the moisture in the atmosphere and the earth to a considerable depth, soon becomes exhausted; and the radiation of heat, from the extensive naked plains and hill-sides, is very great.