"As the earth, in its path round the sun, gradually brings the _line_ of attraction north, in summer, these currents of air are carried _with_ it; so that about the middle of May the current from the north-east has extended as far as the 38th or 39th degree of north latitude, and by the twentieth of June, the period of the sun's greatest northern inclination, to the northern portions of California and the southern section of Oregon.
"These north-east winds, in their progress across the continent, towards the Pacific Ocean, pass over the snow-capped ridges of the Rocky Mountains and the Sierra Nevada, and are, of course, deprived of all the moisture which can be extracted from them by the low temperature of those regions of eternal snow, and consequently no moisture can be precipitated from them, in the form of dew or rain, in a higher temperature than that to which they have been subjected. They, therefore, pass over the hills and plains of California, where the temperature is very high in summer, in a very dry state; and, so far from being charged with moisture, they absorb, like a sponge, all that the atmosphere and surface of the earth can yield, until both become, apparently, perfectly dry.
"This process commences, as I have said, when the line of the sun's greatest attraction comes north in summer, bringing with it these vast atmospheric movements, and, on their approach, produce the dry season in California; which, governed by these laws, continues until some time after the sun repasses the Equator in September, when, about the middle of November, the climate being relieved from these north-east currents of air, the south-west winds set in from the ocean charged with moisture--the rains commence and continue to fall, not constantly, as some persons have represented, but with sufficient frequency to designate the period of their continuance, from about the middle of November until the middle of May, in the latitude of San Francisco, as the _wet season_.
"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.