« السابقةمتابعة »
most vicious reasoning in a circle, if we maintained that ice first cooled the air, and that this cold air next increased the fields of ice.
But, whatever may be the vicissitudes of the Polar ice, they cannot, in any sensible manner, affect the climates of the lower latitudes. The whole circumjacent space where frost holds its reign, bears a very small proportion to the surface of the northern hemisphere. Reckoning even from the parallel of 60 degrees, it would not exceed the eighth part; but, since the gelid region hardly extends below the latitude of 75 degrees, it may be stated at the thirty-second part of the hemisphere. On the supposition, therefore, that the Arctic cold were all transferred and infused into the atmosphere of the South, it could yet produce no visible alteration of climate.
Even if we imagined, with Mr Scoresby, that during the years 1816 and 1817, two thousand square leagues of ice have disappeared in the Greenland seas, between the parallels of 74 and 80 degrees, this extent would still scarcely exceed half the surface of Ireland. It may be calculated, that the loss of heat on our globe, occasioned by a total eclipse of the sun, reckoning this only equivalent to a complete obscuration for the space of a single hour, is as much as would be absorbed by the thawing of a circle of ice 500 miles in diameter, and 150 feet thick. This quantity surpasses at least sixty times the ice-fields dispersed from Greenland, allowing them the mean thickness of 30 feet; and yet the temperature of the air is never depressed more than a degree or two during the continuance of any solar eclipse. · But the idea is quite chimerical, that any winds could ever transport the Polar influence to our shores. It may be shown, from the results of accurate experiment, that a current of air flowing over a warmer surface, whether of land or water, becomes, in the space of an hour, penetrated with the same temperature through a stratum of 80 feet; though the limit of actual contact, or of mutual attrition, is confined to a surface not exceeding the 500dth part of an inch in thickness. If we assign to it the height of a mile, which is a most ample allowance, it would lose all its sharpness, and acquire the standard heat in the course of 66 hours. Admitting this wind to travel at the rate even of 20 miles each hour, it would consequently spend all its frigorific action in a tract of 1320 miles. The gales from the remotest north must thus discharge their store of cold into the German sea or the Atlantic ocean. Nor could such impressions, though continued through a course of ages, have the smallest power to chill the superficial water; for the moment any portion of this was cooled, it would, from its increased density, sink down inte the vast abyss. The surface would not be affected till after the cooling had, in its progress, pervaded the whole mass from the bottom upwards. According to the calculations of Laplace, founded on a comparison of the theory of tides with actual observation, the mean depth of the ocean exceeds ten English miles. Supposing, therefore, a wind blowing from some northerly point, and ten degrees colder than the water, were to sweep over the Atlantic six months every year, at the rate of fifteen miles an hour, it would take 220 years to cool that vast body of water only a single degree.
Some persons have imagined, that the mountains or islands of ice which are occasionally drifted into the Atlantic ocean must be sufficient, by their frigorific influence, to modify the character of our climate. One of the first who advanced that opinion, was the ingenious Richard Bradley, Fellow of the Royal Society, and Professor of Botany in the University of Cambridge. In • A Survey of the Ancient Husbandry and Gardening, collected from the Greek and Roman writers,' printed in octavo at London in 1725, he introduces the following remarkable passage.
* I the rather mention the Case of Winds becoming Cold, by mixing with the Efluvia of Snow or Ice; because I have made some Remarks upon the tempestuous Weather, which often happens about the End of May, or in June, which has in all my Observations been brought in by Westerly Winds; and again, I as surely find, that at such Times, large Islands of Ice and Snow are passing to the Southward in the Western Ocean, as I have been inform’d by several Captains of Ships that were then coming from our Plantations to England: Some of these Islands are so large, as to measure 60 Miles in Length, and yielding so great a Vapour, that for a Day's Voyage on one side of them, the Weather has been so hazy, that the Ma. riners could not discover what they were, and this was accompany'd with so much Cold, that they imagin'd they had mistaken in their Accounts, and got several Degrees too far towards the North ; but a Day or two explain'd the Matter, and gave them an Opportunity of surveying what they had been so much surpriz'd at. Now considering the extraordinary Heat of the Sun, at the Season these appear, the Vapour must be very considerable that rises from them, and 'tis no Wonder then, that as it expands itself, it presses the Air with Violence enough to cause Tempests, and carry Cold along with it."
But a little reflection will convince us, that such remote influence on our climate must be quite insignificant. At a very wide estimation, the surface of ice exposed to the winds could aerer exceed the thousandth part of the whole expanse of the Atlantic ocean. Consequently, the general temperature of the air would not be altered the fortieth part of a degree. Nor could this minute impression be wafted to our shores, being invariably spent in the length of the voyage. The opinion which Mr Bradley entertained near an hundred years ago, might have bren tolerated in the infancy of physical science; but that the same notion should be revived and proclaimed with confidence at this day, may well excite surprise.
On the hypothesis that the quantities of ice which encumber the Arctic seas have been accumulating for a long succession of years, it is assumed as a fact, that throughout Europe a milder and more genial climate had formerly prevailed. A closer inspection of the details, however, will show this supposition to be destitute of any solid support. We hear continual complaints, indeed, of the altered condition of the seasons, especially from elderly persons, whose bodily frame has become more susceptible of the impressions of cold. But similar lamentations have been repeated by the poets and the vulgar from the earliest tiines. If we listened implicitly to such querulous declaimers, we should believe that Nature has at length spent her fires, and is hastening fast into decay. Immense forests anciently clothed the highest parts of this island, and of other northern countries, where scarcely a tree can now be made to grow; the period of vintage was in former ages several weeks earlier, in France, than at present; vineyards were planted, during the time of the Romans, in various parts of the south of England, where at this day even hops are raised with difficulty; and the sides of many hills in Scotland bear evident traces of the plough, which have been long since irretrievably abandoned to the dusky heath.
But, in answer to such allegations, we may observe, that a patch of wood will not thrive in cold situations, merely for want of the shelter which is afforded by extensive plantations. In Sweden and Norway, which are mostly covered with natural forests, it has become an object of police to prevent their indiscriminate destruction. The timber in those sylvan countries is cut at stated periods of its growth, and in detached portions ; the vacant spaces being left as nurseries, embosomed amidst an expanse of tall trees. Some places in Sweden, where the forests have been accidentally destroyed by fire, present the image of sterility, and of wide desolation
It is probable, that the vines grown in ancient times were coarser and hardier plants than those which are now cultivated. A similar observation extends to all the products of gardening. A succession of diligent culture softens the character of the vegetable tribes, and renders them more delicate, while it heightens the flavour of their fruit. The Roman soldiers stationed in Britain would naturally prefer wine, their accustomed beverage, however harsh and poor, to the cervisia, or unpalatable ale brewed by the rude arts of the natives. The marks of tillage left on our northern hills evince only the wretched state of agriculture at a remote period. For want of a proper system of rotation, and the due application of manure, the starving tenantry were then tempted to tear up with the plough every virgin spot they could find, and, after extracting from it a pitiful crop or two of oats, to abandon it to a lasting sterility. The cattle in those days, having no sort of provender through the winter but dry straw, were quite feeble and exhausted in the spring. The soil, too, was very stiff, from want of repeated and seasonable tillage. Under such circumstances, it affords no proof of any great heat, that the slothful peasants, oppressed with a load of clothes, usually then began their operations in the field before sunrise, in preparing the ground for the reception of the barley seed.
It is very difficult to ascertain the precise condition of the weather in distant ages. The thermometer was not invented till 1590, by the celebrated Sanctorio; nor was that valuable instrument reduced to a correct standard before the year 1724, by the skill of Fahrenheit. We have hence no observations of temperature which go further back than a century. Prior to this period, we must glean our information from the loose and scanty notices which are scattered through the old chronicles, relative to the state of the harvest, the quality of the vintage, or the endurance of frost and snow in the winter. Great allowance, however, should be made for the spirit of exaggeration, and the love of the marvellous which infect all those rude historical monuments. Toaldo and Pilgram have, with incredible industry, prosecuted this research; and, from a bulky work of the latter printed in the German language at Vienna in 1788, we shall select the most remarkable passages concerning the state of the weather for more than a thousand years back, and combine with them the observations made by Professor Pfaff of Kiel. The following years are noted for the severity of the winter.
In A. D. 401, the Black Sea was entirely frozen over.
over the ice to avenge his brother's death in Swabia. In 545, the cold was so intense in winter that the birds allow
ed themselves to be caught by the hand. 10 763, not only the Black Sea, but the Strait of the Dar:
danelles was frozen over. The snow in some places rose 50 feet high, and the ice was so heaped in the cities 'as to
push down the walls. In 800, the winter was intensely cold. In 822, the great rivers of Europe, such as the Danube, the
Elbe and the Seine were so hard frozen as to bear heavy waggons for a month. In 860, the Adriatic was frozen. In 874, the winter was very long and severe. The snow con
tinued to fall from the beginning of November to the end of March, and incumbered the ground so much, that the fo
rests were inaccessible for the supply of fuel. In 891, and again in 893, the vines were killed by the frost,
and the cattle perished in their stalls. In 991, the winter lasted very long, with extreme severity.
Every thing was frozen ; the crops totally failed; and famine and pestilence closed the year. In 1044, great quantities of snow lay on the ground. The
vines and fruit-trees were destroyed, and famine ensued. In 1067, the cold was so intense, that most of the travellers
in Germany were frozen to death on the roads. In 1124, the winter was uncommonly severe, and the snow
lay very long. In 1133, it was extremely cold in Italy; the Po was frozen
from Cremona to the sea ; the heaps of snow rendered the roads impassable; the wine casks were burst, and even the
trees split, by the action of the frost, with immense noise. In 1179, the snow was eight feet deep in Austria, and lay
till Easter. The crops and rintage failed; and a great murrain consumed the cattle. The winters of 1909 and 1210, were both of them very se
vere; insomuch that the cattle died for want of fodder. In 1916, the Po frose 15 ells deep, and wine burst the casks. In 1934 the Po was again froren: and loaded waggons
crossed the Adriatic to Venice. A pine forest was killed
br the frost at Ravenna. In 1996 the Danube was frozen to the bottom, and remain
ei long in that state. In 1963, the frost was most intense in Scotland, and the
ground bound up. The Categat was frozen between Nor
way and Jutland. In 1981, such quantities of snow fell in Austria as to bury
the very houses In 1999, the Rhine was frozen erer at Breisach, and bore
loaded wasons. One sheet of ice eltended between Nor.