Devon Cattle

DEW. The phenomena connected with the formation of dew, and its influence upon vegeta tion and the soil has always been an interesting one in meteorology and, practically, the subject is an important one in agriculture, in all its branches. The subject has been elaborately studied by many eminent scientists and we give, from various sources, the important facts determined by patient investigation. The phenomenon of dew depends upon the cooling of the air by radiation by which it immediately parts with a portion of the mois ture mechanically held. The point of saturation of the air depends upon its temperature. Thus, given the per cent. of water which a stratum of air is capable of holding, at a given temperature, if the temperature be raised by 27°, it will hold double the previous quantity ; if lowered by 27° it parts with half the water held if, previously, it was saturated. Hence we have one of the prin cipal reasons for the phenomena of dew, fog, mist and rain. The condensation of atmospheric mois ture upon bodies, cooled by nightly radiation, causing the deposit of dew, is a phenomenon, says a report on agricultural meteorology for theUn ited States Government, as worthy of study from a. physical point of view as on account of its utility in vegetation. At all times dew has impressed. the minds of the most casual observers; and all have regarded it as destined to furnish plants, by absorption, the means of repairing the losses caused during the day by transpiration. The celebrated Hales, and all succeeding physiologists and agriculturists, have thought that the dew, which wets the leaves, was absorbed by them, and that this water was added to the mass of nourish ing liquids in the plant. By delicate experiments, Duchartre has, on the contrary, proved that dew does not penetrate into the tissues of leaves. There do not appear to have existed, before Duchartre's. researches, any continued experiments which could demonstrate that the leaves of living plants absorbed the dew formed upon their surface, save two incidental passages of Hales in his observa tions on the transpiration of plants. But as these first experiments were defective, his assertions are valueless. Bonnet, having laid upon water detached leaves which retained their freshness for at time, thought that they had absorbed the water in contact with them. Many physiologists have refused to admit this explanation, and Molden hawer and De Candolle, among others, have advanced the opinion that the position of the stomatum upon the water prevents the evapora tion of the juices contained in the leaf and pre serves its freshness. Meyen and Treviranus affirm in the most formal terms that the suppression of transpiration was the sole cause of the facts observed by Bonnet. Duchartre, on the contrary, has established with the aid of the balance that the explanation given by Bonnet was exact, and that detached leaves placed upon water absorb by either of their faces, more rarely by both, a very appreciable quantity of water simply by local imbibition. However, having plunged entirely into the water the foliate capitulum of a Veronica Lindleyana, living and planted in a pot enveloping an exactly tight apparatus, lie saw the plant remain there for forty-eight hours in succession without augmenting in weight, while during the day it transpired sensibly. Thus it is now satisfac torily proved that while detached leaves of plants can absorb by imbibition a certain quantity of water, the same leaves of living plants do not absorb the least particle of the liquid which bathes them. Are we, therefore, to be surprised at see ing leaves in full bloom remain covered with dew during a whole night without absorbing a quan tity appreciable by delicate balances? The fol lowing physiological considerations will dispel what, at the first glance, seems extraordinary in this fact. In order to understand why dew is not absorbed by the leaves, and why, in the same way, it does not exactly wet them, we must con sider the mode in which it is formed upon plants, the nature of the epidermis of leaves, the coating which it presents, and, in fine, the nature of its organs. This has been stated as follows : Air bathes in some way the bodies surrounded by it and it adheres rather strongly to their surface. When we observe the epidermis of leaves under the microscope we recognize this adhesion of the air. Now, dew being condensed on the sur face of the leaves can not displace this stratum of adherent air. The dew as it is deposited forms a great number of small globular drops, distinct and separate, which do not exactly wet the leaves. These little drops augment in volume with the increase of vapor condensation, and com ing soon in contact with each other, they at length form a continuous layer. Under this liquid coat ing we perceive a lamina of air more or less com plete, which is interposed between the deposit of .dew and the epidermis in such a way that con tact is not direct. The epidermis is usually found in a state which renders it more or less difficult to wet. This state is a consequence of the evap oration taking place every day at its surface, or in other words its transpiration. The water alone, says Schleideu, is evaporated at its surface, and is deposited as a thicker or thinner layer of substances which were in solution in the cellular juice, covering the external surface of the epider mic cells. At the same time these substances under the action of atmospheric oxygen undergo a chemical modification, and are changed in a manner which renders it still more difficult for the passage of liquid. It is thus that wax and resin appear finally on this surface. Transpira tion, adds Duchartre, being in direct proportion to the intensity of solar light and heat, it follows that the production of the layer of wax, which coats the epidermic cuticle, takes place in the most energetic manner possible on a fine day, and it is then, too, that dew is ordinarily formed in greatest abundance. This circumstance can not certainly favor the absorption of water as it is deposited. If greasy matter, observes Garreau, is already an obstacle to the absorption of water among plants whose leaves are buried in part in the ground, it becomes henceforth almost certain that those whose leaves float constantly in the air, and exhale, under the influence of summer heat, a large -proportion of fatty substance, ought not to be more endosmotic than the preceding. The existence of this greasy coating on the surface of the epidermis enables us, moreover, to compre hend why the leaves do not absorb the dew deposited upon both their faces. The air found in a more or less considerable quantity between the cells of their parenchyma is a new obstacle to the penetration of water from the exterior to the interior of the organs of leaves, according to their anatomical structure. On the whole, and for the three reasons indicated above, Dutrochet thinks that the non-absorption of dew by the organs which it wets, is a fact of easy explanation. It is not without importance in this place to destroy a popular belief, which has even passed into some scientific works, on the subject of faded plants recovering their turgescence by the direct action of the dew. Seeing that plants wilted by the heat of day, says Duchartre, recover the tur gescence of their tissues and their freshness in the night during which they are covered with dew, it is thought that this circumstance points to the absorption of the water which covered their surface. In this connection scientists, as well as others, have united. Senebier, in common with all physiologists, speaking of dew-drops, said : Plants wilted by the heat of a- parching sun, recover their freshness during the night when they are covered by these drops. Now, in this conclusion, says Dutrochet, there has been attrib uted altogether to local and direct absorption what was due to the simple moistening of the ground by the condensation of the aqueous vapor of the atmosphere Duchartre has been able to elucidate the subject by two modes of observation, which seem to put this confusion into full relief, and according to which it follows that leaves, even in their greatest thirst for water, do not directly introduce into their tissues the dew deposited upon their surface during the night. It is by the moist earth that the absorp tion of dew is effected. To sum up, after the very numerous and delicate observations made by Duchartre during five years, his conclusions relative to the action of dew on vegetation are the following: Plants do not absorb the dew condensed on their surface, and hence ideas of this kind which have prevailed up to our own day are deprived of foundation. Dew does not, therefore, exert any immediate and direct influ ence upon vegetation. Its action upon vegetables is no less important in a great number of cases, but it takes place and is explained differently from what we had always thought. The first effect produced by dew upon vegetables is to suppress their transpiration almost entirely, which is already much enfeebled by the obscurity and descent of the temperature at night, but which will continue in some measure without the deposit of dew. Dew, therefore, causes the plants to pass from a period of activity during the day to a period of repose during the night. Thanks to this suppression of aqueous waste, however little humidity the roots find in the ground, they obtain enough to repair the losses which the diurnal transpiration of the leaves caused. Occasionally, even in the absence of all absorption by the roots, the apparent state of the plant may be notably modified by reason of a single displacement of the nourishing liquids, which, from the stem and root, proceed into the wilted leaves, and restore to them the fullness of their tissues. But it is chiefly through the medium of the ground that dew acts upon vege tation. Its action is exerted in two modes: The dew deposited upon the leaves remains there only in an inconsiderable quantity, for above a certain measure variable with different plants, it commences to fall upon the ground by its own weight, in a sort of local shower. The earth, as a porous and hygroscopic body, then receives the moisture, which is at once sucked up by the plant's roots, in the same way as the moisture in the air. The dew also trickles along the stems, branches, and trunks to the ground. Upon mountains the soil formed of permeable earth is continually humid. Otto Sendtner, who has made observations in Bavaria, asserts that upon these high mountains dew is more abundant than rain. In the forests of warm countries dew-drops continually fall from the trees in the form of a plentiful shower. For the rest the total quantity of dew deposited upon leaves has been much exaggerated. Duchartre has made the following experiments, which prove how trifling this deposit is: A Hortensia bore fourteen large leaves, whose extent was on each face at least a square decimeter, and twenty eight square decimeters for the whole surface of the limb. The dew which covered this Hortensia was in such abundance that it was gathered into pools wherever it found a small cavity. Never theless, all this liquid stratum weighed only 7.2 grams in two nights of observations, and seven grams on the third; it had, therefore, only seven cubic centimeters in volume. We see thence that each leaf had for its part one-half cubic centimeter of water spread out upon two square decimeters of surface. This small quantity of liquid, adds Duchartre, which is sufficient to 'cover entirely both sides of a leaf, so as even to form the deepest layer it can hold, explains very well the shower of dew which the ground receives every time that the condensation of atmospheric humidity is effected energetically. Mists must be dense enough and humid enough to deposit a film of water upon plants. The action of mists upon vegetation has been studied by Duchartre after the same method adopted in the case of dew, and in the conden sation of vapor upon vegetation he reached the same conclusions. Mist condensing on the surface of plants, even like a heavy dew, has never increased their weight by any appreciable quantity. The leaves and different parts of the plant act under mist just as under the deposit of dew. Where the mist has not wet the plants, the transpiration of the leaves has been only weakened, not suppressed, particularly during the day. But the suppression of transpiration was complete, or at least nearly so, when the deposit of mist had clothed these organs with an entire coat of moisture. These facts establish conclusively, as Duchartre believes, that mists exert upon vegetation only a secondary influ ence, since they supply plants with nothing, and only diminish, or at most ffiomentarily suppress their waste. Their function becomes much more important in certain localities, par ticularly in the zone of mean altitude upon inter-tropical mountains, where the epiphyte plants abound, and where prevails, chiefly through this cause, an excessive humidity. In every case it is from the trickling of water to the ground that the action of heavy mists is as advantageous to vegetation as that of dew. For the action of mists upon vegetation, as for that of dew, we are forced to confine ourselves to the experiments and conclusions of Dnehartre; because all that has been said on this subject has either no scientific value or presents no fact of importance. On the other side, after the hygroscopic action just pointed out, mists appear to have no other influence than that of certain electric mists still doubtful and unknown, or those called dry mists because of the atmospheric dryness which universally accompanies them. The late Prof. Henry, some years ago, in a report to the government on Meteorology in relation to dew and hoar frost, gave these instructions for meteorological observations, published by the Smithsonian Institution ; When a mass of moist air is brought in contact with a cold body, its vapor is condensed into water, and deposited in minute globules on the cooled sur face, which constitute dew. If the temperature of the surface is below the freezing point, the globules of water will he frozen into minute crys tals of ice, which constitute hoar frost. For a long time the nature of these phenomena was entirely misconceived ; the effect was put for the cause, the dew being regarded as producing the chill which accompanies its formation, instead of the reverse. Dr. Wells, of London, born in South Carolina, was the first who gave the sub 4ect a scientific investigation, and, by a series of ingenious. accurate and conclusive experiments,

furnished a definite explanation of all the phe nomena. They are simply due to the cold pro duced in different bodies by radiation. The earth is constantly radiating heat into celestial space, and is constantly receiving it from the sun during the continuance of that body above the 'horizon. As long as the heat from the sun exceeds that radiated into space, the temperature of the surface of the earth, and that of the air, in contact with it continues to increase; but when the two are equal, the temperature remains stationary for a short time, and then begins to decline as the heat of the sun, on account of the obliquity of the rays, becomes less than the radi ation into space. The maximum of heat gener ally takes place between two and three o'clock in the afternoon, and the cooling from this point goes on until near sunrise of the next morning. As soon as the sun descends below the horizon, the cooling of the surface of the earth takes place more rapidly if the sky be clear; the air in con tact with grass and other substances which are cooled by this radiation, will deposit its moisture in a manner analogous to that of the deposition of water on a surface of a metallic vessel con taining a cold liquid. Although the atmosphere may contain the same amount of vapor, yet the quantity of dew deposited during different nights, in different places, and on different sub stances, is very different. It is evident that, all things being equal, it must depend upon the quantity of moisture, since if the air were dry no deposition could take place. It must also depend upon the clearness of the sky; for if the heavens be covered with a cloud, the radiant heat .from the earth will not pass off into space, but will be partly absorbed by the cloud and radiated back to the earth. This has been proved by direct experiment. The experiment is detailed, but will not be necessary here, but in relation to cloudi ness and haziness of the atmosphere, obstruct ing the deposition of dew, Prof. Henry says, it is probable that the lower surface of the cloud is really a little warmer than the air in which it is floating from the. radiation of heat by the earth, while the upper surface is probably colder on account of the uncompensated radiation into space. But be this as it may, the counter radia tion of the clouds prevents the cooling down of the bodies at the surface of the earth sufficient for the deposition of dew, or at least to allow of the formation of a copious quantity. A haziness of the atmosphere, and it is probable a large amount of invisible vapor, will retard the radiation, and hence a still, cloudless night, without a deposi tion of dew, is considered a sign of rain. The amount of deposition of dew will also depend upon the stillness of the atmosphere; for if a brisk wind be blowing at the time, the different strata of air will be mingled together, and that which rests upon the surface of the ground will be so quickly displaced as not to have time to cool down sufficiently to produce the deposition. Again, the deposition will be more copious on bodies the surfaces of which are most cooled by the radiation. It is well known that different substances have different radiating powers. The following table from Becquerel exhibits the pro portional tendency of different substances to promote the deposition of dew. The figures do not represent the relative emissive power, but the combined effects of emission and conduction : 1. Lamp black 100 2. Grasses 103 3. Silicious sand 103 4. Leaves of the elm and the poplar 101 5. Poplar sawdust 99 6. Varnish 97 7. Glass 93 8. Vegetable earth 92 Polished metals are, of all substances, the worst radiators; they reflect the rays of heat as they do those of light, and it would appear that by internal reflection the escape of heat is prevented from the capacity of the metal. In order that the surface of a body should cool down to the lowest degree, it is necessary that it should be a good radiator and a bad conductor, particularly if it be in a large mass and uninsulated. Thus a surface of a mass of metal eoated with lamp black, though it radiates heat freely, will not be as much cooled under a clear sky as a surface of glass, since the heat lost at the surface is almost immediately supplied by conduction from within. If, however, a very small quantityof metal, such as gold leaf, be suspended by fine threads, the dew will be deposited, because the heat which is radiated is not supplied by conduction from any other source, and hence the temperature will sink to a low degree. H. Melloni, repeating the experiment of Wells, established the correctness of his conclusions, and added some particulars of interest. He found that the apparent tempera ture of the grass, which in some eases was 8° or 10' lower than that of the air at the height of three or four feet, was not entirely due to the actual cooling of the air to that degree, but to the radiation and cooling of the thermometer itself, the glass bulb of which is a powerful radiator. To obviate this source of error in estimating the temperature he placed the bulbs of his thermometer in a small conical envelope of polished metal of about the size of an ordinary sewing thimble. This prevented a radiation, and,, by contact with the air, indicated its true temperature. He found, with thermometers thus guarded, that the solid body was in no case cooled down more than 2° below the tempera ture of the surrounding air, and that the amount of radiation was nearly the same at all tempera tures. The explanation, therefore, of the great cold of the air between the blades of grass is as follows: By'the radiation of the heat, the grass is at first cooled two degrees lower than the air at the surface of the earth, and next the thin stratum of air which immediately surrounds the grass is cooled by contact to the same degree. It then sinks down and another portion of air comes in contact with the blade of grass and is, in its turn, cooled to the same extent, and so on until all the air between the blades is 2° lower than that of the air farther up. The radiation, however, continues, and a stratum of air from the mass already cooled is cooled 2° more, which sinks down as before, and so on until the air between the blades is cooled to 4° below its normal condition ; and in this way the process may be continued until the temperature descends to 8° or 10° below that of the stratum of air a few feet above. In this way we can readily explain the small amount of dew deposited on the tops of trees, since the air as soon as it is cooled sinks down toward the ground, and its place is continuously supplied by new portions of the atmosphere. To the same cause, we may attribute copious deposition of dew on wool and other fibrous materials which, though they do not radiate heat more freely into space, they entangle and retain the air between their fibres, and thus allow the cooling process we have described to go on. It would appear that spider webs radiate heat freely into space, since they are generally covered with a large amount of dew ; their insulated position prevents them from renewing their heat, but, according to the above principle, a much larger amount of deposition ought to be produced by the same material, were it loosely gathered up into a fibrous mass. The fact of the screening influence of the clouds teaches us that a thin cloth or even a slight gauze supported horizontally over tender plants is sufficient to neutralize the radiation, and to prevent injury from frost during the Blear nights of spring or autumn. The same effect is produced by artificial clouds of smoke. Radia tion from the surface of the earth is most intense on clear nights, when the moon is visible, so many effects which are due to this cause. have been referred to lunar influence; for example, a piece of fresh meat exposed to the moonlight, is said to become tainted in a few hours; this may arise from the deposition of moisture on the surface of the meat due to tile cooling from radiation. The moon itself, however, acts as a cloud, and radiates back to the earth a portion of the heat which it received from the earth, as well as a portion of that which it received from the sun; and hence Sir John Herschel has referred to this cause, with apparent probablity, the origin of an assertion of the sailors, that the moon eats the clouds. He supposes that they may be dissi pated by the radiant heat from that body, which, being of low intensity and but feebly the lower stratum of the atmosphere, may serve to dissipate the clouds. Though a, wrong explanation is generally given by the observer of natural phenomena, and though effects and causes are frequently made to change places in his explanations. yet it is true, as Biot has properly said, that the scientist who devotes himself assiduously to investigate the subject of popular errors, will find in them a sufficient amount of truth to fully repay him for his labor. The difference between a fog and a cloud relates principally to the conditions under which they are severally formed. A fog has been aptly called a cloud resting on the earth, and a cloud, a fog suspended in the atmosphere. The circum stances under which a fog is usually produced are the following : Either the surface of the earth or water is warmer than the air, or it is cooler. If the temperature of a river or of a damp portion of ground is higher than that of the atmosphere which rests upon it, the warmer surface will give off vapor of an elastic force due to its tempera ture. Should the superincumbent air be ex tremely dry, the vapor will diffuse itself up through it in an invisible form, without conden sation, and no fog will be formed, until, by the continuation of the process, the air becomes com pletely saturated; and then if an excess of heat remain in the evaporating surface, the fog will be produced, and will increase in density and height so long as a difference of temperature continues. If, however, a wind be blowing at the time, so that successive portions of unsatu rated air are brought over the place, no fog will be produced. A still atmosphere, therefore, is a necessary condition to the accumulation of fog. The foregoing is the usual method in which fog is produced, for it is well known that in cold weather the surfaces of lakes and rivers are much warmer than the strata of air which rest upon them. It is, however, frequently observed that fogs are formed during still nights, in low places, when the surface of the ground is colder than the stratum of the atmosphere which rests upon it, and, indeed, we have shown that the tempera ture of the surface of the earth on a still and clear night, is always lower than that of the air which is immediately in contact with it ; and it is not easy, without further explanation, to see the reason why fogs should not always be produced in this case as well as dew. When the atmos phere is still, the condensation of the vapor by the coldness of the surface is so gradual that the air is not disturbed, and the strata immediately above the grass has relatively less moisture in it than that a few yards higher ; hence, no fog ought to be produced in this case, since all the precipitation produced is that which has settled -dIrectly upon the grass in the form of dew. In this case, we may define the dew to be a fog entirely condensed into drops of water. The question -still arises, how, under these conditions, can a fog really be produced. The answer is, that another condition is required, namely, that the surface, cooled by radiation, should slope to a lower level, as in the side of a hill or the concave surface of the sides of a hollow. In this case, the superincumbent stratum of air of which the temperature has been lowered by contact with the cold earth, flows down the declivity, by its greater weight, into the valley below, and there, mingling with the damp air which generally -exists in such places, precipitates a part of its transparent vapor into visible fog. In the way we have described, large hollows are sometimes seen in the morning, filled with a mass of fog, exhibiting a definite and level surface, presenting the appearance of a lake of which the shores are bounded by the surrounding eminences; and if a depression of sufficient depth occurs in any part of the circumference of the basin, through this the fog is seen to flow like a river from the out let of a lake. The explanation we have here given of the formation of fog in low places, is also applicable to the phenomenon, frequently observed, of early frost in the same localities. As rapidly as the air is cooled on the sides of sloping ground, it sinks into the valley below, and its place is supplied by the warmer air above, which has not been subjected to the cooling influence. Thus, hollows are sometimes found several degrees colder than the more elevated parts of the surrounding surface. Fogs are pro duced on the ocean when a gentle wind, charged with moisture, mingles with another of a lower temperature. The wind from the Gulf Stream, mixing with the cold air which rests upon the water from the arctic regions, which flows along close to the eastern shores of our continent, gives rise to the prevalence of fog over the Banks of Newfoundland and along the Atlantic Coast. From the extracts given, the reader will form a good idea of some of the principal phenomena connected with the formation and action of dew, fog and mist. Rain is produced in a great degree from the same causes. In relation to some of these phenomena, they may be summed up as follows. As much as twenty grains of dew may be deposited on a surface of four square inches in one night. The electrical conditions of a body has no iufluence on its capability of receiving dew. Objects a few inches above ground collect more dew than those lying directly on the ground. The leaves of plants receive more dew than the earth, and for the reason that they cool more quickly. Fog differs from dew in the fact that it moistens all bodies indif ferently, while dew attaches itself more freely to some bodies than others. Dew is more abun dant near water, as the shores of oceans and lakes, and the banks of streams, ponds, etc.; and as a rule, the first clear night succeeding a con tinued rain will cause a more copious deposit of dew, and generally it increases with the humid ity of the air. (See also article Dew Point.)