EARTHQUAKES and VOLCANOS.

Earthquakes were fuppofed by the ancients to be produced by immenfe quantities of inflammable air contained in the caverns of the earth, which being greatly rarified by internal fires, and finding no outlet, forces a paffage through whatever oppofes it; Senec. Quaft. N. vi. 11. 12.; Plin. ii. 79. 80. 81. &c. Hence they are most frequent in the neighbourhood of volcanos.

But although earthquakes produce the most dreadful effects of any thing in nature, the history of them is very incomplete. The deftruction they occafion engroffes the attention of people too much to permit them to examine accurately the appearances which occur.

Earthquakes are usually preceded by a general stillness in the air; the fea fwells and makes a great noife, the fountains are troubled and fend forth muddy water; the birds feem frightened, as if fenfible of the approaching calamity, &c.

The shock comes on with a rumbling noife, like that of carriages or of thunder; fometimes the ground heaves perpendicularly upwards, and fometimes rolls from fide to fide. A fingle fhock is but of very fhort duration, feldom lafting a minute; but the fhocks frequently fucceed each other at short intervals, for a confiderable length of time. During the fhocks, chasms are made in the ground, from which flames, but oftener great quantities of water, are difcharged. The chafms are fometimes fo wide, as to overwhelm whole cities at once. Often the earth opens and clofes again, fwallowing up fome people entirely, and fqueezing others to death caught by the middle. Sometimes perfons have been swallowed up in one chasm and thrown out alive by another. Sometimes houfes are fhuffled from their places, and yet continue ftanding; and farms have been removed half a mile from their places, without any confiderable alteration. Sometimes whole iflands are funk, and new ones raised; the course of rivers is changed; feas break into the land, forming gulfs, bays, and straits, tearing islands from the land, or joining them to the continent, &c.

These and various other circumstances are enumerated in the defcriptions we have of earthquakes in ancient times, Plin. ii. 79. S. 81.—94. f. 96.; Senec. Nat. 2. vi. 1. &c.; Marcellin.

xvii. 7. and in modern times, of that which happened in Jamaica a. 1692, when Port-Royal was destroyed; in Calabria a. 1638, when the town Euphemia was totally funk, and nothing but a difmal and putrid lake to be feen where it ftood; in Sicily, a. 1693, when the city Catania was deftroyed; and of 18,900 inhabitants fcarcely 900 furvived; at Lisbon, ift November 1755, when almost the whole city was laid in ruins; which earthquake was felt alfo at various other places, and at some of them with equal deftruction. Its effects are supposed to have extended over a confiderable part of the globe.

Earthquakes have been accounted for from the power of electric matter contained in the bowels of the earth; which is also supposed to be the cause of volcanos. Pliny afcribes earthquakes to the fame caufe which produces thunder; Neque aliud eft in terra tremor, quàm in nube tonitruum: nec hiatus aliud, quàm cum fulmen erumpit: inclufo fpiritu luctante, et ad libertatem exire nitente, ii. 79. f. 81. and concludes his defcription of fubterraneous effufions with this beautiful remark; Quibus in rebus quid poffit aliud caufa afferre mortalium quifquam, quàm diffufa per omne naturæ fubinde aliter atque aliter numen erumpens ? Plin. ii. 93. f. 95. The force of volcanos is fuppofed to be the greatest of any thing yet known in nature. In the great eruption of Vefuvius in 1779, a stream of lava, of an immenfe magnitude, is faid to have been projected to the height of at least 10,000 feet above the top of the mountain.

COLD.

THE Cause of cold is as uncertain as the nature of fire. Some maintain that it is only the abfence of heat; but others, that it is a real fubftance. At a certain diftance below ground, where there is a free circulation of air, there is an uniform temperature; whence it is thought that the atmosphere is the fource of cold as the fun is of heat. For the rays of the fun heat the atmosphere only by reflection; and where that cannot reach, an intense degree of cold is always found to take place. When the cold is most intenfe, it is found only to affect the surface of the ground.

Some fuppofe cold to confift in certain faline or nitrous particles; because a mixture of water with faline fubftances is confiderably colder than either the water or the falt unmixed. Others attribute cold to the action of the electric fluid, because

the

the readieft conductors of it most cafily tranfmit heat and cold ; thus, metals: whereas wool, hair, filk, &c. which will not conduct this fluid, are found to be the beft prefervatives against both heat and cold; but glafs, which is the beft non-conductor, very readily tranfmits heat.

CONGELATIO N.

WHEN fluids are changed by cold into a folid ftate, it is called congelation, or freezing.

The inftrument made ufe of for meafuring the different degrees of heat and cold in the atmofphere by means of the claftic and expanfive power of fluids, is called a THERMOMETER ; the invention of which is attributed to different perfons. Air was the fluid at firft made ufe of for this purpofe. Spirit of wine was firit ufed by Ferdinand II. Grand Duke of Tufcany, or by. the members of the academy Del Cimento, under his protection. Boyle first introduced the thermometer into England.

The fluid now univerfally preferred is quickfilver or mercury, as being more uniformly heated or cooled than any other with which we are acquainted, and which, till lately, it was thought could not be congealed.

Thermometers are adjusted to the boiling and freezing points, according to the method propofed by Fahreinheit, a celebrated artift of Amfterdam. But thofe points were not afcertained. without great difficulty, and after much labour bestowed by the moft eminent philofophers.

Thermometers are not uniformly marked in the fame manner in the different countries of Europe. Some pcrfons and focieties mark them in one way, and fome in another.

Different degrees of cold are requifite for the congelation of different fluids. Water congeals when Fahreinheit's thermometer is at 32 degrees above o; vinegar at 28, wine at 20°, brandy at 7 below o, light fpirit of wine, not till it has funk to 34 below o, and mercury, as is thought, at 48° below 0, a decree of cold, of which, in this country, we have no conception.

The action of congelation is always inftantaneous. Although all known fubftances, and water among the reit, are confiderably diminished in bulk by cold; yet after water is arrived at the freezing point, the congelation which then inftantly takes place, makes it fuddenly expand itself about of its bulk, which as been lately found to be owing to an innumerable quantity

of

of fmall bubbles, with which the ice is filled. lighter than its bulk of water, being as 8 to 9, floats on the furface.

Hence ice is

and therefore

Water is most eafily frozen when it has been boiled; and more so when it is a little moved, than when quite at reft. What is ftrange, water in the act of freezing becomes a little warmer; the thermometer when immerfed in it finks below but immediately returns to its former ftate.

32,

Water mixed with falts, and melted fnow or ice, will not freeze till the thermometer is confiderably below 32°; but if a glafs of pure water be immerfed in this mixture, it will immediately freeze.

Various caufes for froft have been affligned, but all of them liable to fome objections.

The force with which water expands itfelf in the act of congelation is prodigious; it will burft the strongest cannon. Hence the reafon why the ftones of a pavement or of a building are loofened after froft.

The ice in northern countries is harder than in more fouthern climates. In 1740, a palace was built of ice at Petersburgh, 52 feet long, and 20 feet high. Even cannons were made of it, from one of which a ball went through an oak plank 2 inches thick, at the diftance of 60 paces; and the piece did not burft with the explosion.

Froft proceeds from the upper part of bodies downwards; but how deep it will reach in earth or water is uncertain; feldom above 2 feet in the ground, and 6 in water.

Artificial ice may be made by pounded ice or fnow mixed with any falt, particularly with fal-ammoniac. In the East Indies ice is produced without the affiftance of fnow or ice of any kind, between 254 and 234 degrees of north latitude, where natural ice is feldom or never feen, fimply by the effect of the air on water placed in pits funk a little below the furface of the ground in a particular pofition; and by means of a folution of nitre in water.

Among the various effects of heat and cold on the atmofphere, one of the most important is wind.

WINDS.

WINDS are produced by an agitation of the air, occafioned chy by the variations of heat and cold, by which it is cither rarefied or condenfed. Thus Pliny, Ventus nibil aliud quam

fluxus aeris, &c. ii. 44. So Seneca, Ventus eft aer fluens; Nat. Q.v. i. and Cicero, de nat. D. ii. 39.

As the air is fubject to the laws of gravitation, like other fluids, it has a constant tendency to preserve its equilibrium; so that if it is by any means more rarified or rendered lighter in one place than another, the weightier air will rush in from all parts to that place; which currents of air, if strong, are called winds; if gentle, breezes or gales. Thus the air is conftantly carried from the polar regions towards the torrid zone, where it is also affected by the diurnal motion of the fun from east to west. The winds, therefore, for a confiderable space north of the equator, about 30 degrees in the open fea, blow from the north-east, and as far fouth of the equator, from the south-east. These are called TRADE-WINDS, from their facilitating trading voyages.

In the day time the air above the land is much hotter than above the fea, whofe furface being conftantly evaporated keeps the air cooler. Hence in the day-time a breeze always blows from the fea, more or lefs ftrong in proportion to the heat. But at night, when the influence of the fun's rays is withdrawn, the falling of the dews renders the air at land colder than at fea; whence a land breeze, or a current of air from the land, fucceeds, increafing gradually like the fea breeze, `but never fo ftrong. Thefe land and fea breezes are not confined to the torrid zone. The fea breeze in particular, during the fummer season, is as fenfibly felt along the coafts of the Mediterranean as within the tropics.

The currents of air from the north and fouth meeting where the fun is vertical, by their oppofition darken the atmosphere, and occafion heavy rains; hence in the torrid zone they have then the coldest and most inconftant weather, which they call winter. For they make fummer to consist in a clear sky; and winter in wet weather and a little cold; fo that under the equator they have two winters and two fummers in the year.

In the Indian ocean, from its particular fituation and that of the lands which furround it, the trade-winds blow one half of the year in one direction, and another half in an oppofite direction: these are called MONSOONS. From April or May, to October or November, the wind blows from the fouth-eaft or north-east; and during the rest of the year from the oppofite quarters. The changing or fhifting of the monfoons is generally attended with terrible ftorms of rain, thunder, and lightning; in fome places with calms and variable winds.

As the trade-winds always blow from the east, it is easy to fail weftwards in the Atlantic and Pacific oceans; but to fail

eastwards