« AnteriorContinuar »
mentions the case of a woman near Yeovil in Somersetshire, who remained thus entombed for seven days, and was taken out alive, and recovered. A similar imprisonment in the snow for eight days was endured by another in Cambridgeshire in the year 1799, who heard the bells go two Sundays for church while in the drift, was at last rescued, but died through well-intended, but injudicious treatment. The fertility of the soil is also largely promoted by the nitrogen which the snow takes up from the atmosphere.
Hail is another form under which the aqueous vapours abstracted from the earth are occasionally returned to it. The theory of Volta refers the formation of hail to the play of electricity rapidly abstracting heat from the molecules of vapour in the atmosphere. The common hypothesis is that of the congelation of globules of rain in their fall, by passing through a stratum of dry and cold air. To account for the production of such an intense degree of cold, very partial in its range, is the grand difficulty, for hail generally falls in hot sultry weather. It has been remarked, that hail very rarely falls at night, and is scarcely known at all in latitudes higher than 60°. The course of a hailstorm is commonly very narrow in proportion to its length. In July 1788, a storm, memorable for the havoc it made, passed over France in two parallel lines from south-west to north-east. The one line extended about 500 miles in length, and the other about 600 miles, each having a mean breadth of only 9 miles, an interval of 15 miles occurring between them, in which the rain fell in torrents. Halley describes, in the Philosophical Transactions, a hail-storm of very scanty breadth, in the year 1697, which passed from Snowdon in Wales, through Flintshire, cutting the north-west corner of Cheshire, and extending through Lancashire in a right line from Ormskirk to Blackburn, on the borders of Yorkshire. The hail-stones of this storm were of very considerable dimensions, ploughing up the earth and burying themselves in the ground, killing poultry and sheep, and completely ruining the rising corn. Leslie estimates that hail-stones, an inch in diameter, fall with a velocity of 70 feet per second, or at the rate of about 50 miles
in the hour.
"Striking the ground," he states, "with such impetuous force, it is easy to conceive the extensive injury which a hail shower may occasion in the hotter climates. The destructive power of these missiles in stripping and tearing the fruits and foliage, increases besides in a faster ratio than the momentum, and may be estimated by the square of their velocity multiplied into their mass. This fatal energy is hence as the fourth power of the diameter of the hail-storm." In the narrative of the Jewish wars mention is made of a shower of hail acting with destructive effect upon the routed Canaanites, and in the mountainous districts of Palestine, terrible storms of the kind occur. The cattle have been destroyed in the fields, in the elevated regions of Persia, by the falling stones. Sir Robert Wilson gives the following account of a hail-storm encountered by the British fleet, while at anchor in 1801, in Marmorice bay, in Asiatic Turkey :"On the 8th of February commenced the most violent thunder and hail storm ever remembered, and which continued two days and nights intermittingly. The hail, or rather the ice-stones, were as big as large walnuts. The camps were deluged with a torrent of them two feet deep, which, pouring down from the mountains, swept every thing before it. The scene of confusion on shore, by the horses breaking loose, and the men being unable to face the storm, or remain still in the freezing deluge, surpasses description." Hail-stones exhibit various forms, the spherical, the oval, the pointed, flat, and ragged; and their size occasionally surpasses that of those which have been already mentioned. At Lisle in the Netherlands, in May 1686, the stones that fell during a storm were from a quarter of a pound to a pound in weight. In Hertfordshire, in May 1767, they were from one to fourteen inches in circumference. In the Pyrenees, several of twenty-three ounces avoirdupois, fell in 1784; and a paper by the Abbé Maury read before the Royal Society in 1798, mentions the fall of hail-stones, or masses of ice, in Germany, from half an inch in diameter to the weight of eight pounds.
The preceding statements receive confirmation from the experience of Mr. Darwin in South America, as recorded in the intensely interesting journal of that eminent naturalist. Referring to a posta at the foot of the Sierra Tapalguen, in the state of Buenos Ayres, he observes: "We were here told a fact which I would not have credited, if I had not partly ocular proof of it; namely, that during the previous night hail as large as small apples, and extremely hard, had fallen with such violence, as to kill the greater number of the wild animals. One of the men had already found thirteen deer (Cervus Campestris) lying dead, and I saw their fresh hides; another of the party, a few minutes after my arrival, brought in seven more. Now I well know that one man without dogs could hardly have killed seven deer in a week. The men believed they had seen about fifteen dead ostriches, part of one of which we had for dinner; and they said that several were running about evidently blind in one eye. Numbers of smaller birds, as ducks, hawks, and partridges, were killed. I saw one of the latter with a black mark on its back, as if it had been struck with a paving-stone. A fence of thistle-stalks round the hovel was nearly broken down, and my informer, putting his head out to see what was the matter, received a severe cut, and now wore a bandage. The storm was said to have been of limited extent; we certainly saw from our last night's bivouac a dense cloud and lightning in this direction. It is marvellous how such strong animals as deer could thus have been killed; but I have no doubt from the evidence I have given, that the story is not in the least exaggerated. I am glad, however, to have its credibility supported by the Jesuit Dobrizhoffer, who, speaking of a country much to the northward, says, hail fell of an enormous size, and killed vast numbers of cattle: the Indians hence called the place Lalegraicavalca, meaning the 'little white things.' Dr. Malcolmson also informs me that he witnessed in 1831, in India, a hail-storm which killed numbers of large birds, and much injured the cattle. These hail-stones were flat, and one was ten inches in
circumference, and another weighed two ounces. They ploughed up a gravel-walk like musket balls, and passed through glass windows, making round holes, but not cracking them."
The aqueous vapour in the atmosphere assumes another form, that of dew, under circumstances favourable to its production. These occur when the sun is absent, the sky clear and nearly serene, and when the air, replete with moisture, is chilled by contact with some surface or substance colder than itself. It was once supposed that the dew, fringing the blades of grass and the leaves of the trees with silvery beads, sparkling in the morning sunlight, dropped
"Like the gentle rain from heaven."
Hence our phrase, the drops of dew, alludes to its presumed descent from the upper strata of the atmosphere. It is surprising that this should have been the popular belief down to a very recent period, for dew may be seen upon an under surface which nothing falling can touch, and upon a side surface, which nothing, by either rising or falling, can reach. "The dew-drop is familiar to every one from earliest infancy. Resting in luminous beads upon the down of leaves, or pendant from the finest blades of grass, or threaded upon the floating lines of the gossamer, its orient pearl varies in size, from the diameter of a small pea to the most minute atom that can be imagined to exist. Each of these, like the raindrops, have the properties of reflecting and refracting light; and hence, as from so many minute prisms, the unfolded rays of the sun are sent up to the eye in similar brilliant colours to those of the rainbow. When the sunbeams traverse horizontally a very thickly bedewed grass-plot, these colours arrange themselves so as to form an iris or dew-bow; and if we select any one of the drops for observation, and steadily regard it while we change our position, we shall find the prismatic colours follow each other in their regular order." The poets of nature in all ages and countries have seized upon the clear, tremulous, pendant, and sparkling drops of dew, as images of purity, gentleness, and beauty; and on account of their service to mankind in the economy of the vegetable kingdom, and the interesting mode of their formation, they claim the attention of the scientific inquirer. With us, the dew is most copious in spring and autumn; and it has been produced, during a single night, in a quantity sufficient to be measured by the rain-gauge. This occurs chiefly in the autumn. Mr. Howard found by his instrument a deposition of dew equal to one-tenth of an inch, on the morning of September 1st, 1818, the production of the preceding night. Dr. Dalton estimates the annual deposit in this country to be five inches, or about 22,161,237,355 tons, reckoning the ton to be equal to 252 imperial gallons.
The deposition of dew was first satisfactorily explained by Dr. Wells, in the year 1814, in an Essay which has been pronounced by a high authority-Dr. Thomson-"one of the most beautiful examples of inductive reasoning in the English language." When the sun is below the horizon, and for a short period before his setting, bodies upon the surface of the earth, exposed to the aspect of a clear sky, cool by the radiation of the particles of heat absorbed, and at a more rapid rate than the atmosphere. The air in immediate contact with these bodies, replete with humidity in the form of transparent aqueous is chilled by their cold embrace; and, owing to the increase of its density, it becomes incapable of holding in suspension the moisture with which it is charged in the same quantity as before. The surplus is therefore disengaged, and appears upon the surface of the refrigerating object in globules of dew. It is essential to this process, that the night should not be a cloudy one; because when the sky is overcast, the radiant caloric proceeding from the surface of the earth, and which otherwise would go off into free space, is intercepted by the clouds, and returned by them in sufficient quantity to prevent the decrease of temperature necessary to compel the atmosphere to surrender a
portion of its hoard of aqueous particles. On nights that are perfectly cloudless, therefore, the deposition of dew is greater than when the sky is partially screened; on those that are both cloudy and windy there is none whatever formed; but a gentle motion in the air on a clear night is favourable to its production in the greatest copiousness, by bringing fresh portions of the atmosphere, laden with moisture, into contact with the colder bodies at the surface. The theory of the dewing process at once explains the rationale of the practice adopted by gardeners, to protect tender plants and fruit trees in blossom on clear nights from cold, by spreading over them a mat or awning. The covering performs the same office as the clouds. It returns the heat radiated from the plants to them, and thus a temperature is preserved which prevents refrigeration. It is often observable that substances exposed at night to the action of the same circumstances, differ greatly in the amount of dew deposited on them, some being thickly coated with its pearls, while others are without a single globule. This arises from the varying capacity of bodies for the radiation of heat. Light downy substances part with it more freely than the solids; so that the former cool down to the dew-point, or that degree of the thermometer at which its disengagement from the atmosphere commences, while the latter, remaining above it, receive no deposition. The following enumeration has been given of the substances which dew, in its formation, has been remarked more particularly to cling to:
As the temperature of substances must be reduced below that of the atmosphere in order to the formation of dew, it is never observed, in temperate climates, upon the naked parts of a living and healthy human body.
The substances that show a marked inaptitude to receive dew, are
Considerable masses of water.
The metals, in the following order:
Platina, least inaptitude.
Iron, steel, zinc, and lead, the next.
Dr. Stocke recorded the following results as to the relative quantity of dew deposited upon a variety of bodies:
In opposition to the moisture of dew, that of mists is deposited upon all substances exposed to it alike; and another distinction is, that the moisture of mists exists previous to deposition in a visible state, and is produced quite independent of the bodies that receive it.
A preceding statement, that in temperate climates dew is never observed upon the naked parts of a living and healthy human body, is not true of tropical countries, where, after the high temperature of the day, under a perfectly clear sky, the earth radiates its
heat with great rapidity. This is the probable solution of some cases of physical injury sustained by persons sleeping in the open air with the face exposed, commonly supposed to be the effect of the moonlight. Messrs. Bennet and Tyerman, in their travels, state: "Lunar influence seems to occasion phenomena of a very curious nature. It is confidently affirmed, that it is not unusual for men on board a ship, while lying in the moonlight with their faces exposed to the beams, to have their muscles spasmodically distorted, and their mouths drawn awry - affections from which some have never recovered. Others have been so injured in their sight, as to lose it for several months. Fish, when taken from the sea-water, and hung up in the light of the moon during a night, have acquired such deleterious qualities, that, when eaten the next day, the infected food has produced violent sickness and excruciating pains. We have conversed with people who have been themselves disordered after having partaken of such fish. It is hazardous to touch on this subject; we repeat what we heard from those who ought to be believed, and who would not affirm that of which they were not themselves persuaded." Now the circumstances under which these effects transpired-a clear tropical moonlight night-are precisely those favourable to the production of dew, which promotes the putrefaction of animal matter, and renders it deleterious; and the injury sustained by the parties sleeping exposed to the moonbeams—not a solitary example of such an occurrence— was far more probably caused by the cold and moisture produced by the immense radiation of heat, consequent upon a cloudless night sky after a hot day, than by the lunar light, which all scientific examination shows to be innocuous and uninfluential.
Anacreon — inhabiting the southern part of the north temperate zone known ode pictures Cupid wet with the dews:
A bard, still more tropically situated than he of Teos, has written in a similar vein: "I sleep, but my heart waketh; it is the voice of my beloved that knocketh, saying, Open to me; for my head is filled with dew, and my locks with the drops of night." The abundance of this deposition from the atmosphere in Palestine-in certain specified localities, chiefly the hilly districts-is frequently alluded to in the sacred writings. "We were sufficiently instructed," says Maundrell, "by experience, what the Psalmist means by the dew of Hermon,' our tents being as wet with it as if it had rained all night." Its value is fully appreciated there, and throughout Western Asia, where it seldom rains from April to September-the season of the greatest heat-the vegetation consequently then mainly depending upon its copious supply. "God give thee of the dew of heaven" -a patriarchal blessing at the close of life-illustrates its importance in the estimation of a pastoral chieftain; nor could the imagination conceive of a direr calamity than that expressed in the Hebrew elegiac:-"Ye mountains of Gilboa! let there be no dew, neither let there be rain upon you!"