is by so much the greater as the temperature is higher. In very warm climates the dew is so copious as to assist vegetation essentially, supplying the place of rain during a great part of the year. According to some meteorologists dew is most copious near the sea-board of a country; very little falls in the interior of great continents, and indeed is said only to be apparent in the vicinity of lakes and rivers.* I cannot agree in any statement of this kind made so absolutely. I have never had occasion to see more copious dews than those which occasionally fall in the steppes of San Martin to the east of the eastern Cordilleras, and at a very great distance from the sea; the dew was so copious that for several nights I found it impossible to employ an artificial horizon of black glass in order to take the meridian altitude of the stars; the moment the apparatus was exposed there was such a quantity of water deposited on the surface that it soon gathered into drops and trickled off. I found it necessary to have recourse to mercury to reflect the star under observation. During the clear calm nights the turf of these immense plains receives a considerable quantity of moisture in the form of dew, which materially assists vegetation, and by its evaporation tempers the excessive heat of the ensuing day. In tropical countries the forests contribute to keep down the temperature. In very hot countries it is rare to be out in a cleared spot, when the night is favorable to radiation, without hearing drops of water, produced by the copiousness of the dew, falling continually from the surrounding trees, so that forests contribute further to produce and to maintain springs by acting as condensers of the watery vapor dissolved in the air. I might cite a number of observations upon this point which I made in the forest of Cauca. In the bivouac between the 4th and 5th of July, 1827, the night was magnificent; nevertheless, in the forest which began at the distance of a few yards from our encampment, it rained abundantly; by the light of the unclouded moon we could see the water running from the branches. It is possible that the transpiration from the green parts of the trees might have been added to the dew condensed, and so increased the intensity of the phenomenon which I have described; but I rather incline to believe that the cooling of the leaves by way of radiation had by far the largest share in the production of this dewrain. It It is true that of all the leaves which form the crown of a tree, those whose surface is exposed and radiate freely into space intercept, as would a screen, the radiation of the leaves and branches which are not so exposed; but, as M. de Humboldt has observed, if the leaves and branches which crown a tree cool directly by emission, those which are situated immediately beneath them by radiating towards the lower parts of the leaves which are already cooled a greater quantity of heat than they receive, their temperature will also necessarily fall, and the cooling will thus be propagated from above downward until the whole mass of the tree feels its effects It is thus that the ambient air circulating through the leaves becomes * Kaemtz, Meteorology, translated by W. Walker, London, 1844. cooled during bright nights, and to judge from the influence which forests exert in lowering the temperature of a country, it is enough to recollect with M. de Humboldt that by reason of the vast multiplicity of leaves, a tree, the crown of which does not present a horizontal section of more than about 120 or 130 square feet, actually influences the cooling of the atmosphere by an extent of surface several thousand times more extensive than this section. The proportion of watery vapor which a gas will retain in solution is by so much the greater as the temperature of the air is higher. All the causes which cool air saturated with watery vapor occasion, as we have already seen, the precipitation of a certain quantity of moisture. When this condensation takes place in the midst of a gaseous mass, the precipitated water collects into small floating vesicles, which trouble the transparency of the medium that momentarily holds them in suspension. Mists, fogs, and clouds are collections of these vesicles; a fog, as a celebrated naturalist said, is a cloud in which one is, and a cloud is a fog in which one is not. The vesicles of clouds tend towards the earth, like all heavy bodies, but by reason of their specific lightness the resistance of the air which they displace lessens the rapidity of their descent. When they are of larger size they coalesce and form drops of water which fall with greater celerity. When these drops pass through strata of very dry air they undergo partial evaporation, and this is the reason wherefore there is sometimes less rain upon plains than upon mountains. In opposite circumstances it is the inverse phenomenon that is observed; the drops increase in size in passing through the inferior strata of an atmosphore saturated with moisture, condensing vapor in their course. This is what happens most generally. In taking a survey of a large amount of observations, meteorologists have inferred that the annual quantity of rain varies with the latitude; that, greatest at the equator, it gradually lessens as higher northern and southern latitudes are attained; this is as much as saying that the quantity of rain is greater as the temperature of the climate is higher. But to this rule there are numerous exceptions; for instance, under the line at Payta on the sea-coast it only rains very rarely; a shower of rain is an event, and when I visited the country eighteen years had elapsed since they had had any thing of a fall of rain. Local causes have the greatest influence upon the fall of rain, so that countries on the same parallel of latitude are far from being equally distinguished by dryness or humidity. It is believed that in Europe it rains more heavily and more frequently in the day than in the night. In the equinoctial regions, at least in those parts that I have visited, it would seem that the opposite rule held good. Every one in South America allows that it rains principally during the night, and the observations which I made in the neighborhood of Marmato enable me to state that of 7.874 inches of rain which fell in the month of October, 1.336 inches fell in the day, 5.638 inches in the night; of 8.881 inches which ll in the month of November, 0.707 inches came down in the day, 8.174 inches in the night; of 5.934 inches which fell in December, 0.786 inches fell in the day, 5.148 inches in the night. Two series of observations taken in the same country at two stations not far from one another, but situated at very different elevations, seem to confirm, in reference to the equatorial regions, the conclusions of European meteorologists as to the fact that the annual quantity of rain which falls diminishes as the height above the level of the sea increases. They also show that in latitudes which do not differ materially, more rain falls where the mean temperature is 68° F. than where it is 58° F. Marmato lies in N. lat. 5° 27", and 75° 11" (?) W. long., at a height of 4676 feet above the level of the sea; Santa Fé in N. lat. 4° 36", W. long. 75° 6", at a height of 8692 feet above the level of the sea. And while the quantity of rain at the former place amounted, according to my own observations for 1833, to 60 inches, according to Caldas, in 1807, at the latter there fell but 39.4 inches. In temperate climates the quantity of rain that falls varies with the seasons. Near the equator, where the temperature remains constant throughout the year, the rainy season commences precisely at the period when the sun approaches the zenith; and whenever the latitude of a place in the torrid zone where it rains is of the same denomination and equal to the declination of the sun, storms occur. In such circumstances the sky in the morning is of remarkable purity, the air is calm, the heat of the sun insupportable. Towards noon clouds begin to show themselves upon the horizon, the hygrometer does not advance towards dryness as it usually does, it remains stationary, or even falls towards extreme humidity. It is always after the sun has passed the meridian that the thunder is heard, which being preceded by a light wind is soon followed by a deluge of rain. In my opinion the permanence or incessant renovation of storms in the bosom of the atmosphere is a capital fact, and is connected with one of the most important questions in the physics of our globe, that of the fixation of the azote of the air by organized beings. The most recent inquiries show dry atmospherical air to consist in volume of: The air contains in addition from 2 to 5 10,000ths of carbonic acid gas, and quantities perhaps still smaller of carbureted combustible gas. The experiments of M. Theodore de Saussure, as well as those of Professor Liebig, have further demonstrated in it traces of ammoniacal vapor. I have already shown that animals do not directly assimilate the azote of the atmosphere. Azote is nevertheless an element essential to the constitution of every living being, and is met with indifferently in either kingdom of nature. If we inquire into the source of this principle in connection with the herbivorous tribes of animals, we find it as an element in the food which sustains them. If we next inquire into the immediate origin of the azote which enters into the constitution of vegetables, it is discovered in the manure which proceeds more specially from animal remains; for vegetables, to thrive, must receive azotized aliment by their roots. We thus come to apprehend that plants supply animals with their azote, and that these restore it to plants when the term of their existence is accomplished; we are led to discover, in a word, that living organic matter derives its azote from dead organic matter. This view leads us to conclude that the amount of living matter on the surface of the globe is restricted; that its limits are in some sort determined by the quantity of azote in circulation among organized beings; but the question must be viewed from a loftier eminence, and we must ask what is the origin of the azote which enters into the constitution of organic matter considered as a whole? If we now turn to the possible sources or magazines of azote, we shall find, setting aside organized beings and their remains, that there is in truth but one, the atmosphere. It is therefore extremely probable that all living beings have previously obtained their azote from the atmosphere, just as it seems very certain that they have thence derived their carbon.* The most reasonable supposition in the actual state of science, is to consider the ammoniacal vapors diffused through the atmosphere as the prime source of the azotized principles of vegetables, and then through them of animals; a consequence of which hypothesis would be to assume with Liebig, that carbonate of ammonia existed in the atmosphere before the appearance of living things upon the face of the earth. The phenomena and effects of thunder-storins appear to me calculated to support this opinion. It is known, in fact, that so often as a succession of electrical sparks passes through moist air, there is formation and combination of nitric acid and ammonia. Now nitrate of ammonia is one of the constant ingredients in the rain of thunder-storms. But nitrate of ammonia, being a fixed salt, cannot exist in the atmosphere in the state of gas or vapor; and then it is not the nitrate, but the carbonate of ammonia that has been signalized in the air. In bringing to mind the series of reactions of which I have spoken, it is not difficult to perceive how the nitrate of ammonia, precipitated in thunder-showers, and thus brought into contact with calcareous rocks, should suffer decomposition, pass into the state of carbonate on the return of fair weather, and become fitted to undergo diffusion in the state of vapor through the atmosphere. We should in this way be led to regard the electrical agency, the flash of lightning, as the means by which the azote of the atmosphere is made fit for assimilation by organized beings. In Europe, where thunder-storms are rare, an office of so much importance will perhaps be accorded reluctantly to the electricity of the clouds; but in tropical countries no difficulty would probably be felt on the matter. In the torrid zone, thunder-storms happen in one place or another ɔt only every day, but every hour, and even every minute of every * Boussingault, Annales de Chimie, t lxxi. 1839. hour throughout the year; so that an observer, placed at the equator, were he endowed with organs of sufficient delicacy, would never lose the roll of the thunder. As the equator is quitted, the times at which rain falls become less specific or periodical. Under the tropics, the rains of thunderstorms, which are always the most copious, fall while the sun is in the neighborhood of the zenith. In the northern hemisphere, the greatest quantity of rain falls during winter; and at places somewhat far south on the temperate zone, the summer rain is altogether insignificant. In assuming the number 100 to express the whole annual quantity of rain, we should have in Less rain falls in the eastern parts of Europe than in the western. The annual rain, too, is distributed very unequally over the different seasons, as has been shown by M. Gasparin in a remarkable paper. If we express by 100 the quantity of rain gauged in a year, we should have for each season: The quantity of rain which falls in the course of a year varies considerably according to the climate; to form an idea of the extent of these variations, it is enough to notice the results obtained at different observatories; but it is less the annual quantity of rain that falls, than the way or quantities in which it is distributed over the different months of the year, which interests the farmer; upon this distribution, in fact, in many districts, depend the productiveness and fertility of the soil. I add a table of the mean quantities of rain. in inches and 10ths, that fall at London in the different months of the year: Jan. Feb. March. April. May. June. July. Aug. Sept. Oct. Nov. Dec. in. in. in. in. V. ON THE INFLUENCE OF AGRICULTURAL LABORS ON THE CLIMATE OF A COUNTRY IN LESsening strEAMS, ETC. A question of great importance, and that is frequently agitated at this time, is, as to whether the agricultural labors of man are influential in modifying the climate of a country or not? Do extensive clearings of woods, the draining and drying up of great swamps, which certainly influence the distribution of heat during the different seasons of the year, also exert an influence on the quantity of running water of a country, whether by lessening the quantity of rain which falls, or by promoting the more speedy evaporation of that which has fallen? In some districts it has been held, that the streams which had |