general observation and practical knowledge, needs not the proofs that physical science affords by means of the wet and dry bulb thermometers. The facts are patent and intelligible to all, and can be measured in an uncovered district by the sensible diminution of a mountain stream after a day of intense sunshine. In California, on the eastern side of our great valley, in places where the upper lands have been cleared of trees, the rain water descends impetuously in a torrent, leaving tiny streams, which flow steadily for many days, so long as the sky remains overcast, but cease altogether after a single day of sunshine. In this connection, I would add that the rains are not now either lighter or heavier, or more fitful, than in former times, but there are fewer woods to restrain the drops, which unite to denude the rocks of their soil, and to form the mighty torrents, conveying thousands of tons of detritus to fill up the rivers, as witnessed every Winter season. It cannot be doubted but that an extensive planting of trees in the valleys, at the head of the main ravines, where cachement areas of twenty-two thousand seven hundred and forty-two square miles (1) have been hypothetically plotted out, as seen in the accompanying map in this report, according to the projected irrigation plans of the United States Commission, would superinduce a more humid condition of the atmosphere, and lead to a more constant supply of water, in a region now arid and desolate, for more than six months in the year. The evaporation from such immense reservoirs as are contemplated by these surveys, would be simply enormous, and, if intercepted by the trees before being com (1) From report of the Board of Commissioners on the irrigation of the San Joaquin, Tulare, and Sacramento Valleys of California, eighteen hundred and seventy-four. pletely vaporized, the minute component vesicles of water would coalesce upon the leaves and branches, and fall in drops upon the earth. This I regard as the principal mode by which trees may have a tendency to increase the general humidity of the climate; but, from sanitary considerations, this would be undesirable anywhere, except in the arid and semi-tropical zone of California. In no other respects, perhaps, has the influence of the superinduced dryness of the climate of the Eastern States, already alluded to, been more palpably demonstrated than in its results upon certain diseases; and no stronger evidence can be adduced in support of this conclusion than that afforded in an abstract, compiled from two tables, by Dr. Ham, of Dover, New Hampshire, to whose valuable paper, bearing on the whole subject under discussion, I am largely indebted. From the first table, exhibiting the amount and ratio of sickness and mortality in the United States army from phthisis pulmonalis, during fifteen years, commencing in eighteen hundred and forty and ending in eighteen hundred and fifty-four, it is shown that temperature, considered by itself, does not have that controlling influence upon phthisis which has been attributed to it, but that dryness is the most important atmospheric condition. The lowest ratio of cases of consumption occurs in New Mexico; there 1 per cent per one thousand soldiers; and the highest in the South Atlantic region, where it is 92 per cent per one thousand. The Gulf coast of Florida gives the next highest proportions, being 72 per one thousand of mean strength. New England has 4 per one thousand mean strength. The second table referred to, constructed from the vital statistics of Boston, New York, and Philadelphia, and within the region of the modification of climate in respect to humidity, shows a relative decrease in the number of deaths from phthisis pulmonalis since eighteen hundred and ten. From 1810 to 1820.................................................1 death from phthisis in 6 4-10 deaths. The same authority thinks that this falling off in the relative number of deaths from phthisis, during the last seventy years, obtains, alsó, in all the diseases of the respiratory system, and is largely due to the comparative absence of ozone, which exists in large proportion in a humid atmosphere. Dr. Pfaff gives (1) an account of his observations at Plauer, in Saxony, at one thousand and fifty German feet above the level of the sea. He has not found the direction of the wind influencing the presence of ozone. He has found stormy weather exceedingly favorable to its production; the ozone appearing immediately in large quantity during a storm suddenly coming on, after a succession of fine weather unaccompanied by ozone. Test paper, which had long remained unchanged, would then denote eight degrees of ozone; while as soon as the storm had passed away all reaction on the test paper would cease-the storm seeming to bring and take away with it the ozone. Similar but less rapid increase in the ozone was observed during mere changes of weather, as when fine weather of long duration was followed by rain. As a general rule moisture was favorable to the development of ozone. Little or no influence was exerted by temperature; the proportion of ozone not being greater in Winter than in Summer. The following are Dr. Pfaff's conclusions with respect to the influence of ozone: 1. A large proportion of ozone in the atmosphere acts mischievously on diseases of the respiratory organs. 2. The ozone of the air exerts little or no influence on epidemic diseases, provided that these are not complicated with catarrhal affections. 3. A large amount of ozone in the air, whatever may be the direction of the wind, favors the development of inflammatory affections, and especially of tonsilitis. 4. Other diseases besides those mentioned do not seem to be influenced by the amount of ozone. In the presence of such facts and deductions, the planting of trees in California may appear inconsistent. It must be remembered, however, that the extreme aridity of the climate is very peculiar-in fact, it is too dry; and it need only be remarked here, that this extreme does not belong to the Summer season alone. The mean relative humidity of the five rainy months (October to March) in Sacramento is 72°, and at no time ever reached complete saturation. During the dry season the moisture generally amounts to less than fifty per cent; the temperature of evaporation during the hottest part of the day not unfrequently reaching 25° to 30°. Taking the mean of the whole year, this percentage is 66. Now, as the most agreeable and salutary amount of humidity (2) is between seventy and eighty per cent, such a great deviation from this healthy standard as is here met with, cannot but be fraught with more or less danger to the imprudent. The equability of any climate is largely dependent upon the presence of aqueous vapor. The most potent of the sun's heating rays are largely intercepted in an atmosphere which is, to any extent, charged with watery vapor; and hence it is that the entire solar force is unfelt in our coast region, where the evaporation from the sea perpetually supplies an effectual screen. The intensity of the sun's direct rays, as measured by a blackened-bulb thermometer, in vacuo, fluctuates from 120° to 135°. The variations appear to coincide distinctly with the amount of atmos pheric humidity, the thermometer rising to 148° in our great valley (1) Vol. 46 of Braithwaite's Retrospect. (2) Parks' Practical Hygiene. 13-(1) during the arid northwest winds, and seldom attaining more than 125° during our humid southeast winds. The dangerous difference between sunshine and shade is, therefore, due to the absence of aqueous vapor. There is no cold shade in an atmosphere reasonably humid, inasmuch as the contained aqueous vapor intercepts and diffuses the excess of solar heat, and renders the shade safe and temperate. But when the air is too dry to intercept any great quantity of solar heat, the direct rays of the sun become oppressively hot, whilst the shade is dangerously cold. The vapor of water has also another use. When the atmosphere is dry and the sun is gone, the earth rapidly loses heat by radiation into space. A moist atmosphere, on the contrary, is to a certain extent impervious to the passage of the rays of heat, and a moderate temperature is maintained throughout the night. In the Summer climate of the interior valleys of California this shield of vapor is absent, and hence we experience great extremes of night and day, and of Summer and Winter. While referring to the accompanying tables to sustain what is advanced, we would here remind the general reader of the nature and value of mean temperatures. It must be understood that they are merely averages founded upon columns and pages of individual observations. Mean temperatures merely give the amount of heat observed in given periods, without mention of the manner in which it is distributed, and the consequent variations to which a climate may be subject. A moderate mean annual temperature may, for instance, represent a climate like that of Santa Barbara, wherein night and day, Winter and Summer, closely approximate in the quantity of apportioned heat; and also a climate such as that of Nice and Mentone, in France, regularly or irregularly subject to heat too intense in Summer to be encountered by invalids, and also too frosty cold in Winter. Nothing is more common than for those who consult meteorology to seize upon the annual mean temperature as a solitary point of comparison whereupon to ground their judgment, not discerning that therein the excess of Summer heat is made to compensate for deficient Winter warmth. Now the great reduction, by rapid radiation of heat after the maximum is reached, is the most striking as well as the most important feature, from a sanitary point of view, of the interior climate of California. The extreme monthly ranges prove that the greatest transitions occur from May to October, inclusive, which is the rainless period. The mean maximum for these six months is 89.73°, and the mean minimum 40.63°. Consequently, the mean extreme Summer range is 49.10°. But this does not exhibit the extreme monthly ranges, which sometimes reach beyond 50° during our arid north winds, when the thermometrograph leaves its mark in the neighborhood of 100°. However high the wave of temperature may tower up, under the influence of a vertical sun and almost vaporless atmosphere, it sinks proportionately low at night, rendering it, by contrast, so cold and chilling that blankets become indispensable for comfort. This Asiatic feature of the climate, while it imparts a resiliency or elasticity to animal life, is at the same time treacherous to the health, especially of the feeble and delicate, and often acts as an exciting cause of disease. We thus are enabled to understand why an attack of intermittent is sometimes brought about by the removal of an inhabitant of the interior valleys to San Francisco, or to a cool mountain region. In fact, every sort of cooling down, dry as well as moist, especially if the body has been particularly heated, may give rise to the development of malarial affections. Paradoxical as it may seem, after what has been advanced in the preceding pages of this report respecting the well-recognized agency of humidity as one of the factors of malaria, to recommend measures calculated to promote this very humidity, still it must be remembered that excesses of heat and aridity are the great exigencies in our present sanitary forecastings. Do what we will and all we can, our labors would prove but pigmy efforts towards transforming those grand climatic features which are due to influences far beyond our control. To provide against extremes, and to temper the burning aridity of our treeless plains, which, during a northwest wind in Summer, compares almost at times with that of the Desert of Sahara, is a very different thing from attempting to induce that excess of moisture due to cosmic causes, and which sometimes imparts to the climate of our Atlantic cities the deadly characteristics of the Terras Calientes of the Mexican coast. All we can hope to effect is to equalize, to a certain extent, the temperature in sunshine and shade, and, through the instrumentality of arboriculture, to retard, if not prevent, the action of the sun in quickening into activity noxious fermentation. It has been my purpose to show that our climate possesses inherent capacity for sanitary modifications; and knowing that it may be ultratropical at times, during our dry Summer, it is incumbent on me to prepare for all the contingencies of such a condition, by suggesting every possible safeguard against the dangers to which the people may be subjected at such periods. The following tables, of the results of meteorological observations for a series of years at Sacramento and San Francisco, being representative types of the interior valley and coast climates of California, are worthy of close study and attention: |