We are very imperfectly acquainted with the present mean and extreme temperature, or the precipitation and the evaporation of any extensive region, even in countries most densely peopled and best supplied with instruments and observers. The progress of science is constantly detecting errors of method in older observations, and many laboriously constructed tables of meteorological phenomena are now thrown aside as fallacious, and therefore worse than useless, because some condition necessary to secure accuracy of result was neglected, in obtaining and recording the data on which they were founded.
To take a familiar instance: it is but recently that attention has been drawn to the great influence of slight differences in station upon the results of observations of temperature and precipitation. Two thermometers hung but a few hundred yards from each other differ not unfrequently five, sometimes even ten degrees in their readings; [Footnote: Tyndall, in a lecture on Radiation, expresses the opinion that from ten to fifteen per cent. of the heat radiated from the earth is absorbed by aqueous vapor within ten feet of the earth's surface.—Fragments of Science, 3d edition, London, 1871, p. 203. Thermometers at most meteorological stations, when not suspended at points regulated by the mere personal convenience of the observer, are hung from 20 to 40 feet above the ground. In such positions they are less exposed to disturbance from the action of surrounding bodies than at a lower level, and their indications are consequently more uniform; but according to Tyndall's views they do not mark the temperature of the atmospheric stratum in which nearly all the vegetables useful to man, except forest trees, bud and blossom and ripen, and in which a vast majority of the ordinary operations of material life are performed. They give the rise and fall of the mercury at heights arbitrarily taken, without reference to the relations of temperature to human interests, or to any other scientific consideration than a somewhat less liability to accidental disturbance.] and when we are told that the annual fall of rain on the roof of the observatory at Paris is two inches less than on the ground by the side of it, we may see that the height of the rain-gauge above the earth is a point of much consequence in making estimates from its measurements. [Footnote: Careful observations by the late lamented Dallas Bache appeared to show that there is no such difference in the quantity of precipitation falling at slightly different levels as has been generally supposed. The apparent difference was ascribed by Prof. Bache to the irregular distribution of the drops of rain and flakes of snow, exposed, as they are, to local disturbances by the currents of air around the corners of buildings or other accidents of the surface. This consideration much increases the importance of great care in the selection of positions for rain-gauges. But Mr. Bache's conclusions seem not to be accepted by late experimenters in England. See Quarterly Journal of Science for January, 1871, p. 123.]
The data from which results have been deduced with respect to the hygrometrical and thermometrical conditions, to the climate in short, of different countries, have very often been derived from observations at single points in cities or districts separated by considerable distances. The tendency of errors and accidents to balance each other authorizes us, indeed, to entertain greater confidence than we could otherwise feel in the conclusions drawn from such tables; but it is in the highest degree probable that they would be much modified by more numerous series of observations, at different stations within narrow limits. [Footnote: The nomenclature of meteorology is vague and sometimes equivocal. Not long since, it was suspected that the observers reporting to a scientific institution did not agree in their understanding of the mode of expressing the direction of the wind prescribed by their instructions. It was found, upon inquiry, that very many of them used the names of the compass-points to indicate the quarter FROM which the wind blew, while others employed them to signify the quarter TOWARDS which the atmospheric currents were moving. In some instances, the observers were no longer within the reach of inquiry, and of course their tables of the wind were of no value. "Winds," says Mrs. Somerville, "are named from the points whence they blow, currents exactly the reverse. An easterly wind comes from the east; whereas on easterly current comes from the west, and flows towards the east."—Physical Geography, p. 229.
There is no philological ground for this distinction, and it probably originated in a confusion of the terminations -WARDLY and -ERLY, both of which are modern. The root of the former ending implies the direction TO or TO-WARDS which motion is supposed. It corresponds to, and is probably allied with, the Latin VERSUS. The termination -ERLY is a corruption or softening of -ERNLY, easterly for easternly, and many authors of the nineteenth century so write it. In Haklnyt (i., p. 2), EASTERLY is applied to place, "EASTERLY bounds," and means EASTERN. In a passage in Drayton, "EASTERLY winds" must mean winds FROM the east; but the same author, in speaking of nations, uses NORTHERLY for NORTHERN. Lakewell says: "The sonne cannot goe more SOUTHERNLY from us, nor come more NORTHERNLY towards us." Holland, in his translation of Pliny, referring to the moon, has: "When shee is NORTHERLY," and "shee is gone SOUTHERLY." Richardson, to whom I am indebted for the above citations, quotes a passage from Dampier where WESTERLY is applied to the wind, but the context does not determine the direction. The only example of the termination -WARDLY given by this lexicographer is from Donne, where it means TOWARDS the west.
Shakespeare, in Hamlet(v., ii.), uses NORTHERLY wind for wind FROM the north. Milton does not employ either of these terminations, nor were they known to the Anglo-Saxons, who, however, had adjectives of direction in -AN or -EN, -ern and -weard, the last always meaning the point TOWARDS which motion in supposed, the others that FROM which it proceeds. The vocabulary of science has no specific name for one of the most important phenomena in meteorology—I mean for watery vapor condensed and rendered visible by cold. The Latins expressed this condition of water by the word vapor. For INVISIBLE vapor they had no name, because they did not know that it existed, and Van Helmont was obliged to invent a word, gas, as a generic name for watery and other fluids in the invisible state. The moderns have perverted the meaning of the word vapor, and in science its use is confined to express water in the gaseous and invisible state. When vapor in rendered visible by condensation, we call it fog or mist—between which two words there is no clearly established distinction—if it is lying on or near the surface of the earth or of water; when it floats in the air we call it cloud. But these words express the form and position of the humid aggregation, not the condition of the water-globules which compose it. The breath from our mouths, the steam from an engine, thrown out into cold air, become visible, and consist of water in the same state as in fog or cloud; but we do not apply those terms to these phenomena. It would be an improvement in meteorological nomenclature to restore vapor to its original meaning, and to employ a new word, such for example as hydrogas, to explain the new scientific idea of water in the invisible state.]
There is one branch of research which is of the utmost importance in reference to these questions, but which, from the great difficulty of direct observation upon it, has been less successfully studied than almost any other problem of physical science. I refer to the proportions between precipitation, superficial drainage, absorption, and evaporation. Precise actual measurement of these quantities upon even a single acre of ground is impossible; and in all cabinet experiments on the subject, the conditions of the surface observed are so different from those which occur in nature, that we cannot safely reason from one case to the other. In nature, the inclination and exposure of the ground, the degree of freedom or obstruction of the flow of water over the surface, the composition and density of the soil, the presence or absence of perforations by worms and small burrowing quadrupeds—upon which the permeability of the ground by water and its power of absorbing and retaining or transmitting moisture depend—its temperature, the dryness or saturation of the subsoil, vary at comparatively short distances; and though the precipitation upon very small geographical basins and the superficial flow from them may be estimated with an approach to precision, yet even here we have no present means of knowing how much of the water absorbed by the earth is restored to the atmosphere by evaporation, and how much carried off by infiltration or other modes of underground discharge. When, therefore, we attempt to use the phenomena observed on a few square or cubic yards of earth, as a basis of reasoning upon the meteorology of a province, it is evident that our data must be insufficient to warrant positive general conclusions. In discussing the climatology of whole countries, or even of comparatively small local divisions, we may safely say that none can tell what percentage of the water they receive from the atmosphere is evaporated; what absorbed by the ground and conveyed off by subterranean conduits; what carried down to the sea by superficial channels; what drawn from the earth or the air by a given extent of forest, of short pasture vegetation, or of tall meadow-grass; what given out again by surfaces so covered, or by bare ground of various textures and composition, under different conditions of atmospheric temperature, pressure, and humidity; or what is the amount of evaporation from water, ice, or snow, under the varying exposures to which, in actual nature, they are constantly subjected. If, then, we are so ignorant of all these climatic phenomena in the best-known regions inhabited by man, it is evident that we can rely little upon theoretical deductions applied to the former more natural state of the same regions—less still to such as are adopted with respect to distant, strange, and primitive countries.
STABILITY OF NATURE.
Nature, left undisturbed, so fashions her territory as to give it almost unchanging permanence of form, outline, and proportion, except when shattered by geologic convulsions; and in these comparatively rare cases of derangement, she sets herself at once to repair the superficial damage, and to restore, as nearly as practicable, the former aspect of her dominion. In new countries, the natural inclination of the ground, the self-formed slopes and levels, are generally such as best secure the stability of the soil. They have been graded and lowered or elevated by frost and chemical forces and gravitation and the flow of water and vegetable deposit and the action of the winds, until, by a general compensation of conflicting forces, a condition of equilibrium has been readied which, without the action of main, would remain, with little fluctuation, for countless ages. We need not go far back to reach a period when, in all that portion of the North American continent which has been occupied by British colonization, the geographical elements very nearly balanced and compensated each other. At the commencement of the seventeenth century, the soil, with insignificant exceptions, was covered with forests; [Footnote: I do not here speak of the vast prairie region of the Mississippi valley, which cannot properly said ever to have been a field of British colonization; but of the original colonies, and their dependencies in the territory of the present United States, and in Canada. It is, however, equally true of the Western prairies as of the Eastern forest land, that they had arrived at a state of equilibrium, though under very different conditions.] and whenever the Indian, in consequence of war or the exhaustion of the beasts of the chase, abandoned the narrow fields he had planted and the woods he had burned over, they speedily returned, by a succession of herbaceous, arborescent, and arboreal growths, to their original state. Even a single generation sufficed to restore them almost to their primitive luxuriance of forest vegetation. [Footnote: The great fire of Miramichi in 1825, probably the most extensive and terrific conflagration recorded in authentic history, spread its ravages over nearly six thousand square miles, chiefly of woodland, and was of such intensity that it seemed to consume the very soil itself. But so great are the recuperative powers of nature, that, in twenty-five years, the ground was thickly covered again with tree of fair dimensions, except where cultivation and pasturage kept down the forest growth.]
The unbroken forests had attained to their maximum density and strength of growth, and, as the older trees decayed and fell, they were succeeded by new shoots or seedlings, so that from century to century no perceptible change seems to have occurred in the wood, except the slow, spontaneous succession of crops. This succession involved no interruption of growth, and but little break in the "boundless contiguity of shade;" for, in the husbandry of nature, there are no fallows. Trees fall singly, not by square roods, and the tall pine is hardly prostrate, before the light and heat, admitted to the ground by the removal of the dense crown of foliage which had shut them out, stimulate the germination of the seeds of broad-leaved trees that had lain, waiting this kindly influence, perhaps for centuries.