effect of extremes in the growth of vegetation is apparently similar to that on animal life-to produce the maximum activity. Central Asia has been regarded as the point of departure for most of the cultivated fruits and grains, domestic animals, the race of man, and the germ of civilization and letters. Valleys, with arid districts and plains intervening, and a climate possessing the greatest curve of changes, both of a constant and of a non-periodic character, make up as large a proportion of the surface of California and Western America as of Asia; and we accordingly find the fruits and plants of the zones of transition from tropical to temperate climates eminently vigorous in the transition district here. Murray mentions the stone pine, and a "cypress of prodigious hight," as characteristic forest trees of Upper Persia, and these are characteristic of California also; whether or not specifically the same with our mammoth cypress, the Sequoia Gigantea, or the large coned pines of our valleys, is unimportant to the analogy, or to the similarity of climate they establish. Besides the effect of the extremes of heat and cold which have thus been considered in a general manner in reference to the climatological distinctions of California, a knowledge of the mean annual temperature, and especially of the distribution of temperature among the different seasons of the year, more particularly with regard to the heat and duration of the summer months, is of the utmost practical importance in an agricultural point of view. Turning to our table of monthly and annual means, we find the respective mean temperature of the seasons to be as follows: for the spring months, mean, 550 31, the mean maximum being 71° 20, and the mean minimum 420 13; for the summer, mean 700 19, and the mean maximum and minimum 920 50 and 550 11 respectively; for the autumn, mean 58° 47, and mean maximum and minimum 780 20 and 440 00 respectively. In the two winter months the mean is 45° 94, the mean maximum 600 90, and the mean minimum 290 70. Thus it is demonstrated that there is a mean difference between winter and spring of 90 35, between spring and summer, of 14° 88, between summer and autumn, of 110 72, and between autumn and winter of 120 53. The difference of the means of the hottest and coldest months, between summer and winter, is also shown to be 24° 25, and the extreme variation, or the difference between the mean maxima of the former and the mean minima of the latter, 41° 50. It will be noticed that in our division of the seasons we have, in accordance with the phenomena observed, defined February as the first of the three spring months, and appropriated five months to summer and only two to autumn and two to winter. Indeed, the dormant season is of so short duration, that the tropical division into the wet and dry seasons would perhaps be more appropriate. The whole period of sensible winter is far from being a complete season of suspension of vegetation. During the period we have assigned to it, many forms of vegetable life are still active, particularly the roots of grasses and winter grains. The lowest mean daily temperature belonging to this period is seldom below 40°. Although the thermometer has been known to fall as low as 330 as late as the 10th February, still the leafing process generally commences during the first week of February, and is completed at a temperature not much exceeding that of the mean annual. The measure of heat increases very gradually from month to month; indeed the same uniformity of temperature obtains throughout all the meteorological seasons. In summer the greatest vicissitudes of temperature are found to occur. The commencement of autumn is quite similar to the beginning of spring in its mean of daily temperature. The earth remaining warmer than the atmosphere under the decline of temperature, activity is partially renewed, after the drought of summer, by the influence of the light early showers of October. The first frosts occur about the middle of November, and the decline into winter is prolonged until the latter part of December. Ice is sel dom formed before the beginning of January, and then rarely remains unthawed for twenty-four consecutive hours. We have already alluded to the arid state of the atmosphere, during the summer months, revealed by the psychrometer. The principal agent in this hygrometric peculiarity of the climate, is to be found in the direct effect of northerly winds. In the winter and spring the north winds are the coldest, and serve, as the land is then cooler than the sea, to condense the moisture wafted with the atmospherical current from the southern hemisphere, and to precipitate it in the form of rain. During this season the south-east trades, charged to their utmost capacity with moisture, commence descending as their temperature decreases, and precipitate more and more rain as they become chilled by the north winds. During the summer, owing to the fact of these northerly winds passing over a highly heated and arid surface, their temperature is raised, thereby increasing their capacity for moisture, which not being able to obtain from the surface passed over, they appear as dry winds, reminding one of the reputed sirocco of Italy. Nevertheless, dry as these winds apparently are, on coming in contact with the westerly winds, chilled by the oceanic polar current along the coast, and their temperature being again reduced, the vapor they contain is rapidly condensed; hence the heavy mists that are precipitated during the afternoon at San Francisco, and at the gaps along the coast. In the valley, as a general rule, the direction of the wind is from north by west to south-east. It seldom blows from the east or north-east with any appreciable force. Doubtless the prevailing wind off the coast, where no causes of local deflection exist, is west, as stated by Lieut. Maury. This wind, rushing into the heated valleys through the gap at San Francisco and Benicia, reaches us at Sacramento and the northern part of the valley as a south-west wind, while at Stockton and the San Joaquin valley it is a westerly and north-westerly wind. To this wind, together with that descending from the slopes of the Sierras, may be attributed our cool summer nights. The influence of the winds on the temperature, as we have seen respecting the hygrometric condition of the air, varies according to the season of the year. It is during the occurrence of northerly winds in the summer that we experience the hottest weather, which seldom lasts long, however, before the temperature becomes equalized by a change of wind to the southward. Upon an examination of our daily and hourly records we find it to be a common occurrence during the summer months for the wind to commence blowing from the north at or shortly after the morning observation, and to remain in this quarter until afternoon, when it would change_round to the south, freighted with moisture and invigorating freshness. It is the prevalence of these cool winds which temper our summer climate so delightfully, the greater or less predominance of which renders the mean temperature plus or minus. As regards the force of the wind, it is generally but slight. The observations in this respect having been registered for the last two years only, it is impossible to make full deductions therefrom with any degree of completeness. As a general rule it very rarely rains with the wind from the northern half of the octant, which may be attributed to its coming to a warmer from a colder region. As is well known, the rains of California are confined to a particular period; that period being the summer of the southern hemisphere, when the greatest amount of vapor is being taken up and borne by the south-east trades to form the rains of the northern hemisphere. The south-east trades, on approaching the latitude of California, descend to the surface, and meeting the colder northerly winds, precipitate during the wet season almost the whole rain we experience. In the more elevated regions a greater amount of precipitation occurs, while towards the center of the valley the rain-fall is least. At Stockton, according to a register kept during 1853, 54, '55 and '56, by Dr. R. K. Reid, it is only 15 inches, while the average of the same years at Sacramento gives 18.163 inches. Like the results of the thermometer, the barometer also reveals some features of a tropical rather than of the temperate climate to which latitudinally Sacramento appertains. The elastic force of vapor, which was not applied at the times of the readings, from which the results in the following table are computed, increases directly with the temperature. In 1857 it amounted to nearly half an inch during midsummer, or one sixty-seventh of the entire atmospheric weight. The absence, also, of either abrupt or great changes in the readings of the mercurial column, gives further indications of the tropical features which the climate possesses. As a general rule, the atmospheric pressure varies but little, and that through slow and long continued movements, rather than in the sudden manner characteristic of the latitude on the Atlantic coast and elsewhere. Nevertheless, although the mercurial column rises and falls within very restricted limits, yet there are changes, represented it is true by small measurements, which occur with wonderful regularity and certainty-diurnal movements at fixed hours, as well as annual ones, having reference to the sun in the ecliptic. The former, or horary oscillations, as ascertained by a series of hourly observations, present in a marked degree the two diurnal maxima and minima observed within the tropics-the ante-meridian maximum at about 9 to 10 A. M. being more constant than at the same period post meridian. Without a single exception the pressure is always less at 3 P. M., and this has no reference to whether the column stands high as in the cold, or low as in the hot season. In conclusion, and while referring to the following tables for confirmation of all that has been herewith advanced, we would remark that the mean of the means of the barometer for July is apparently higher than that of June, in consequence of some peculiar disturbing causes in June, 1853, which month should be regarded as exceptional. Monthly and Annual Averages of Direction and Force of Wind at Sacramento. The Direction is calculated for the years 1853-7, and the Force for the years 1856-'7 only. |