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The Temperature of Climates at different Periods.

Winter; so that myrtles would not grow, nor olives, and other trees that require a mild air. The laurel, fays he, refifts the cold, and does not perish here oftener than it does at Rome. This defcription is more applicable to Paris at present than to modern Rome, or even to Tuscany. It may be objected, that Pliny's countryfeat was placed on an elevated fituation, the temperature of which correfponded to that of a northern climate. This houfe was near Tifernum, now Citta di Caftello near the Tiber, and in a place, as he fays himfelf, which was at the foot of a hill, and that the ground rofe to wards it with an imperceptible afcent. Thus then Tuscany and the environs of Rome do not feem to have differed in temperature from Paris at prefent; and the teftimony of Pliny juftifies that of Horace, who defcribes the ftreets of Rome in Winter covered with froft and fnow, and, what is ftill more extraordinary, he fpeaks of the rivers as frozen. Neither is Ho race the only one who mentions the rivers of Rome and Italy in this condition. Juvenal, defcribing his fuperftitious woman, reprefents her breaking the ice of the Tiber to perform her ablutions.

Thefe accounts of the cold in ancient times, fhew it to have been much more fevere than it is at prefent. The rivers and the Tiber, which are faid to have been frozen, do not freeze now; and the cold is thought long and rigorous at Rome when fnow lies two days on the ground. Ovid talks of the feverity of the climate and rigour of the cold at Tomos, as we talk at prefent of the climate and cold of Petersburg. Yet the temperature of Tomos is equal to that of the mildeft provinces in France; and the celebrated Tournefort, in his Voyage to the Levant, fays,

139 That

that he never faw a milder.
part of the Danube over which
Trajan threw his bridge, does not
freeze at this day; the fix feet of
fnow is no longer found in the
rout that Cæfar muft have taken
from Languedoc to Auvergne. The
rivers in France, it is true, fome-
times freeze quite over; but in the
years 1709, 1766, and 1768, when
they were frozen,' it cannot be faid
that armies with their equipage at-
tempted to cross them. We ought
therefore to conclude, that the cold
is lefs intense at present than it was
eighteen hundred years ago; and
yet the following facts, which are
not lefs certain, feem to contradict
fuch a conclusion.

Hiftory and traditions inform us, that in ancient times a degree of heat prevailed fuperior to that we now feel. We find that feveral places in the northern parts of France yielded good wine, though now what they afford us is exceedingly bad. Such is the wine of Surenne, which the Emperor Julian thought excellent. There are fome Cantons where we are certain that wine was made, and where now the vine is not cultivated, as the grapes will not ripen. Even in places where much wine was p duced in the year 1561, there is not now heat enough to colour the grapes.

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In the 16th century, we find a great number of vineyards eftablifhed, the ground-rent of which was to be paid in wine, and the term of payment fixed at the feaft of St Michael, the 29th of September: now it is an immemorial cuftom to pay thefe rents in new wine, that is, in wine made in the month of October following, which is the ordinary feafon of the vintage. On this circumftance, which at first fight feems very furprifing, it is to be obferved that thefe eftablishments were antecedent to the re

formation

formation of the calendar by Pope Gregory, fo that the 29th of September old ftile is the fame with 8th of October in the new; and confequently we must confider the term of payment in these establishments as fixed at the 8th of October. But this explanation, which appears to go a good way towards the folution of the difficulty, does not remove it entirely. I obferve that thofe rents payable at Michaelmas might be taken in wine of the first drawing of the yat, or in the tans, at the pleasure of the lord: accordingly these conditions fhew exactly, that the wine was in the tuns on the 8th of October, or at least if it was ftill in the vat, it was in a condition to be drawn off; and thus the vintage must have been finished feven or eight days before, which is the fortelt space they allow the wine to remain in the vat before drawing it off, and confequently that the vintage must have been concluded in the laft days of September new ftile. But as we cannot fuppofe that the grapes were gathered before they were ripe, we muft conclude that the commencement of the vintage then was ear lier than it is with us, which is generally from the 8th to the 20th of October, nor do we ever fee it begun before the 4th. It must follow then, that the Summers, two hundred years ago, were warmer than they are at prefent; that the heat of climates diminishes; and fince the diminution is fo fenfible in two hundred years, it must be very con fiderable in two thousand. Accordingly we read in the 6th chapter of the Gofpel by Luke, that our Saviour's difciples, as they walked near a field of corn about Eafter, that is, in the end of March, or at the lateft in the beginning of April, rubbed the cars of corn in their hands, that they might eat the feed. It muft then have been ripe at this

time, but in our days it is not nearly fo. The Evangelift knew Judea, and cannot be fuppofed to have made fo abfurd an anachronism, if the corn had not been generally ripe about that time.

M. Bufching, in his Geography, fays, that, according to ancient accounts, Greenland in fome places produced excellent corn, which is no longer the cafe: that in Iceland grain cannot be brought to perfection now, but that many circumftances fhew that the ancient inhabitants cultivated corn, and that it was not till the 14th century that they defifted.

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Having thus examples that heat has diminished, in the cold, the temperate, and the warm climates, we have reafon to conclude that it univerfally and continually diminishes ; while, on the other hand, from the inftances mentioned above, we might conclude that cold diminishes likewife. Let us endeavour to reconcile thefe oppofite conclufions.M

A learned Mathematician, the Abbé Boffut, of the Academy of Sciences, has fhewn that, in the folution of the problems relating to the motion of the planets, it is neceffary, after a certain time, to make allowance for a fmall change in their mean places, that the obfer vations may quadrate with the ta bles. It has therefore been doubted, whether these flight alterations in the mean motion be owing folely to flight errors in the calculation, or if they are to be attributed in part to the refiftance of a medium through which the planets move. The Abbé fhews by obfervations, that the mean motion of the moon is accelerated, and that this acceleration, as all aftronomers allow, is very fenfible: and he adduces feveral reafons to prove that it really is owing to the refiftance of the Ether.

This opinion has been likewife maintained

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On the Formation and Phenomena of Glaciers.

maintained by other learned men. In the Phil. Tranf. there is a letter from Euler, in which he proves from obfervations, that the earth infenfibly approaches the fun. In the Memoirs of the Academy of Sciences, M. de la Caille has afcertained the greatest equation of the fun; and thus he determines the length of the year, which he imagines is infenfibly diminishing. He found it in the year 1750 to confift of 365 days, 5 hours, 48' 40"; but we find it longer, when we confult ancient records, By the obfervations of de la Caille, the fun's apogee is 10 or 12' more advanced than in the tables of Caffini and Halley; and the epoch of the mean longitude of the fun, by his calculation, gives the fun's place 11" more advanced than the tables of Caffini, 25" more than those of Flamstead, and 36" more than those of Halley. After these remarks, we may perhaps pay fome attention to those of Plutarch, or rather to thofe of the Priefts of Jupiter Ammon, who affirmed that their lamp, which was never extinguished, confumed less and less oil every

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141

year; and therefore concluded, that the years grew gradually fhorter. Whether this opinion of theirs was founded on aftronomical observation, or on the diminished confumption of their oil, it still fhews that a decrease of the folar year was at that time fufpected.

It is from this obfervation that I would account for a difference in the degree of heat and cold at different periods. The earth being anciently at a greater distance from the fum, there was lefs specific heat in the ancient winters, and hence the great cold of which old historians make mention. But as to the Summer, in which for the fame reafon the heat ought, at present to be fpecifically greater than formerly, and confequently ought to ripen the fruits of the earth better; it must be confidered, that maturity does not depend folely on the intenfity of the heat, but alfo on its duration. Our year being abridged in its length, makes the Summers fhorter, and therefore the heat does not last long enough now to ripen the grapes in those places that formerly produced them.

In a Letter ad

Remarks on the Formation and Phenomena of Glaciers,
dreffed to the Queen of Great Britain, by Monf. de Luc*.

MADAM,

T

HE Glacieres are Mountains of Ice; and if your Majesty will confider that this is by no means a figurative expreffion, you will have a juft idea of them.

Every chain of mountains will affift us in conceiving their formation: we have only to let our imagination figure them elevated into that region of the atmosphere where the heat feldom reaches the degree that is neceflary for keeping water

in folution. Instead of rain, therefore, the clouds will generally produce nothing but fnow, and this too will never melt but in the warmest feafons, and even then only in the middle of the day: freezing again during the night, from fnow it will turn into folid ice. The accumulated maffes, formed on the more fteep declivities, becoming too heavy to fupport themfelves, will fall into the vallies and fill them up.

Lettres fur quelques parties de la Suiffe. Juft published.

The

The more gentle declivities will retain a cruft, which, growing thicker and thicker, will bear the fame proportion to the ice which we commonly fee around us, that centuries bear to a few of our Winter days.

It is thus that the Glacieres of the Alps have been formed. The lapse of ages has covered with ice the higher vallies over the whole extent of the chain. The fummits that are not very steep, are likewife covered with ice, and the hollows between thofe that are more pointed are filled with it. Thus the whole chain, in its amazing extent from Nice to Tirol, when viewed at a distance, wears the afpect of Winter in the middle of Summer, for all the tops feem covered with fnow.

Those entire vallies that are invefted with one coat of ice are properly named the Glacieres in the Alps but there is a particular modification of them called Glaciers, though the words are fometimes confounded. Thefe Glaciers are the particular objects of the curious, for they are within their reach; few are acquainted with the true Glacieres. The hunters of the chamois and of the rock goats, and thofe that fearch for rock cryftal, are almost the only human beings that venture into these regions. But the Glaciers, properly fo called, are found even in the vallies that are cultivated, and that are fometimes very warm. The Glacier exhibits a fort of continual flowing of ice, which is maintained only by its prodigious quantity. I fhall endeavour to explain how it is formed:The interior temperature of our globe, in almost every part of it to which we can have accefs, is generally warmer, by 22 or 23 degrees of Fahrenheit's thermometer, than that where the ice can preferve itfelf from melting. The rocks, and

the bottom of the high Alpine val lies, although covered with ice, always participate more or lefs of this temperature; for which reason the ice here is perpetually melting below, however cold it may be above. In the depth of Winter, when during. the day as well as in the night their furface freezes, the rivers continue to flow from under the ice by channels which remain always open. This continual melting below, joined with the general melting which takes place in Summer, does not however equal the quantity which is accumulated in Winter, for the ice every where increases; and though this were not plain from obfervation, to know that it exifts would be fufficient to fatisfy us.

When the vallies are horizontal, the melting of the ice produces on-) ly gaps, which are formed from time to time with a noile like thunder. But if there is a declivity, there happens an infenfible progress of the whole ice, which produces the moft fingular phenomena. The ice, wherever it melts, ceafes to reft on the ground; this indeed it could not do except the melting was every where equal, which is almost im poffible. Vaft caverns, therefore, are formed, and the mafs is fupported only by certain parts that have melted more flowly. These parts, in the form of columns, being at laft themfelves undermined, the whole mafs finks, and while it is in motion the declivity gives it a tendency downwards: this, it is true, is not very great, for the inequalities of the ground on which it refted at first foon stop its progress; but as thefe commotions are renewed from time to time by the same caufe, the fuperior vallies at length difcharge immenfe maffes of ice by all their openings into the inferior vallies. And if there openings are likewife vallies which have not de

clivity

On the Formation and Phenomena of Gliciers.

clivity enough to allow the ice to tumble down in pieces, they are themselves choaked up, and fpread the ice over the fineft paftures, and in places where, if it did not come thus in fuch quantity, there would be little more than in the neighbouring plains.

It is this accumulated ice which, like immenfe lavas, defcends from the fuperior to the inferior vallies, that they properly call Glaciers: But the name is likewife extended to all the steep fides of the Glacieres by which they have fallen down, and which discover vertical precipices. For this reafon, that heap of ice which covers the Buet is called a Glacier, though it be properly a Glaciere. It is a mafs formed upon a vast rock precipitous on all fides; but as it every where fhews thofe vertical precipices by which the ice falls, it is called a Glacier. This is what I could collect in general, with regard to the different denominations given, by the inhabitants of thofe places, to fuch maffes of ice.

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There are two magnificent Glaciers of the firft kind on the confines of Grindelwald, that is, of those lavas of ice that defcend from the high valley. They are acceffible without much labour, and therefore this place is more vifited than many others where the fame object is to be seen, and on a larger fcale: for example, the valley of Chamouni, in Savoy, is in this refpect much more remarkable than that of Grindelwald.

143

gation of these effects, either on the principles of phyfies or mechanics They fee the Glaciers advance and retire; they fee them elevated and depreffed; they fee the immense gaps enlarge and contract; they obferve thefe gaps engulph or repel the rocks that fall into them: but not perceiving the cause of fuch wonderful phenomena, they perfonify the Glaciers, and attribute to them a particular principle of action, like the Indians to the fun and the moon.

The greater part of these effects may be very well accounted for from the neceffary progress of the ice on the declivities, where undoubtedly, as it inceffantly melts below, it must glide down. But the whole mafs cannot move at once; its bottom or its fides being involved in the finuofities of the valley. However, when the pillars that fupport any part of it give way, that part which is, perhaps, half a fquare mile in extent, must fink; and if there is a declivity, it muft tend towards the bottom, tho' it fhould be only for a few feet: it approaches the piece that preceded it, and clofes the gap between them, while it enlarges another as it recedes from the piece that is to follow. When the lowest piece moves, it forces the ground before it; then the Glacier advances. But as it is often very warm in thefe low val lies, the ice melts rapidly; then the Glacier retires, till, by another fall, it again advances.

It must be confeffed, however, Till one has acquired an idea of that there are other phenomena the formation of thefe Glaciers, they very difficult to be explained; paraftonish even those who are born in ticularly the ejection of ftones, and their neighbourhood, and who fee even very large rocks, which seem to them every day. The Glaciers afcend from the bottom of the gaps; certainly have a progreflive mo- and certain heaps of rubbish and tion, though it is not obferved but, flones which are found in the midby its effects. It is this that afto- dle of the Glaciers apparently out nishes the inhabitants. They do not of the reach of the nearest rocks. puzzle themselves with an invefti- I am not furprised that these phe

VOL. VI. N° 33.

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nomena

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