Many kinds of vapours are unfriendly to animal life; efpecially fuch as arise from metallic folutions. In some places the earth exhales vapours which prove inftantly fatal to fuch animals as breathe them; thus the Grotto del Cani, near Naples, fo called because the experiment is commonly made with dogs (canes); fee page 154.

CLOUD S.

FROM the aqueous vapours raised by the heat of the fun, from the fea and furface of the earth, are formed the clouds, which being condenfed by cold, defcend in rains, fogs, and dews; rain in drops of a confiderable fize, which is found to increase as they approach the earth; fogs in fmall fphericles, very little heavier than air; and dews fo fmall as to be invifible. Fogs are fuppofed to be produced by vapours, which being condenfed by cold are brought down to the earth before they have afcended far in the atmosphere. So that fogs are only clouds in the lowest region of the air, as clouds are nothing but fogs raised on high. Dews are formed either by the defcent of fuch vapours as have been raised during the day-time, or from the vapours afcending from the earth during the night.

The ancients fuppofed the clouds to be formed not merely from evaporation, (liquore egreffo in fublime,) but also by the conversion of air into water, (ex aere coacto in liquorem,) Plin.

ii. 42..

When the aqueous particles are frozen in the atmosphere, they defcend in fnow, hail, and hoar froft, Plin. ii. 60.

To the rain and melting of the fnows which fall on the tops of mountains, fome afcribe the origin of SPRINGS. But this is not fufficient to account for them, as the depth of rain which falls one year with another in different parts of Europe amounts only from between 19 or 20 inches, to between 40 and 50 inches perpendicular, not nearly equal to the quantity raised by evaporation. Dr. Halley therefore more justly fuppofes the rife of fprings to be chiefly owing to the dews that fall on the tops of mountains, by which they are attracted.

By experiment he found, that in a certain degree of heat, the 60th part of an inch of water is exhaled every 2 hours, and confequently one 10th of an inch in 12 hours. Hence allowing the Mediterranean sea to be 40 degrees long, and 4 de. grees broad, at a medium, and its whole furface to be 160

square

fquare degrees, it will in 12 hours, or a day's time, yield 5,280,000,000 tuns of water; which defcending in rains and dews, &c. is more than fufficient to supply all the springs, rivulets, and rivers which run into that sea.

Some have thought the origin of fprings owing to waters brought from the fea by abforption, or by fubterraneous ducts or canals, which lose their faltnefs by percolation or filtration, as they pafs through the earth. This was the opinion of Seneca, 2. Nat. iii. 5. & 15.; Lucretius, v. 269.; and, as it is thought, of Solomon, Ecclefiaft. i. 7. It is fuppofed to be confirmed by the quantity of water iffuing from fprings always remaining the fame; being neither diminished by drought, nor increased by rain. Befides, it is certain that in most parts of the earth, water is found at a fmall depth from the furface, which may be fuppofed to be derived from the fea below ground, as well as from the rains and dews which fall from the atmosphere. Ariftotle imputed the origin of springs to the air contained in the caverns of the earth, condenfed by cold near its furface, and thereby changed into water, which issued forth where it could find a paffage.

In Egypt, Sindy, and Peru, it feldom or ever rains; and in fome places under the equator it is faid to rain for one half of the year, and to be fair the other. Varen. i. vi. 19. prop. 42.

Sometimes clouds are highly electrified, and in fouthern regions produce the most fatal effects. A cloud of this kind in the island of Java in the Eaft Indies, on the 11th of Auguft 1772, defcended on a mountain in the night-time, and destroyed every thing near the top of it; about 2140 people, and a vast number of cattle were killed. Another cloud of the fame kind at Malta, 29th October 1757, deftroyed many houfes and fhips, and about 200 people.

The height of clouds is commonly not very great. The fummits of very high mountains are ufually free from them; as Etna, the Alps, &c. and hence, from the top of thefe, a perfon may, in perfect fecurity, hear the thunder roll, and fee the lightning flash from the clouds far below him, which is one of the moft fublime and awful scenes in nature.

But the most dreadful effects of fire and heat are exhibited in earthquakes or volcanos or burning mountains.

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. Quæft. N. vi. 11, 12; Plin. ii. 79, 80, 81, &c. Hence they are moft frequent in the neighbourhood of volcanos.

But although earthquakes produce the moft dreadful effects of any thing in nature, the hiftory 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 ufually preceded by a general ftillness 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 fhock comes on with a rumbling noife, like that of carriages or of thunder; fometimes the ground heaves perpendi'cularly 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 fhort intervals, for a confiderable length of time. During the fhocks, chafms 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. Sonitimes perfons have been fwallowed up in one chafm and thrown out alive by another. Sometimes houses are fhuffled from their places, and yet continue ftanding; and farms have been moved half a mile from their places, without any confiderable alteration. Sometimes whole islands are funk, and new ones raifed; the courfe of rivers is changed; feas break into the land, forming gulfs, bays, and ftraits, tearing islands from the land, or joining them to the continent, &c.

Thefe and various other circumftances are enumerated in the descriptions we have of earthquakes in ancient times, Plin. ii. 79. f. 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 deftroyed; 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 almoft the whole city was laid in ruins; which earthquake was felt alfo in various other places, and in fome of them with equal destruction. 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 luante, 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 quisquam, quàm diffufe per omne natura fubinde aliter atque aliter numen erum pens? Plin. ii. 93. . 95. The force of volcanos is supposed to be the greatest of any thing yet known in nature. In the great eruption of Vesuvius in 1779, a ftream of lava, of an immenfe magnitude, is faid to have been projected to the height of at leaft 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 furface of the ground.

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

the readieft conductors of it moft eafily 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 best non-conductor, very readily transmits heat.

CONGELATION.

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

The inftrument made ufe of for measuring the different degrees of heat and cold in the atmosphere by means of the elaftic 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 purpose. Spirit of wine was first used by Ferdinand II. Grand Duke of Tuscany, 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 Fahrenheit, a celebrated artist of Amfterdam. But thofe points were not ascertained without great difficulty, and after much labour bestowed by the most eminent philofophers.

Thermometers are not uniformly marked in the fame manner in the different countries of Europe. Some perfons 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 Fahrenheit's thermometer is at 32 degrees above o; vinegar at 28°, wine at 209, brandy at 7o below o, light fpirit of wine not till it has funk to 34° below 0, and mercury, as is thought, at 48° below o, a degree 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 rest, 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 has been lately found to be owing to an innumerable quantity

of