Obs. In the process of evaporation the salt of the sea 18 not taken up: the water from the clouds is therefore fresh and pure.

601. The quantity of rain which falls in Great Britain is about 24 inches per annum in the eastern counties, and 36 in the western; because these receive the first clouds as they are brought from the Atlantic by the westerly winds.

In the West-Indies, 120 inches fall annually; and in the East-Indies, from 80 to 100.

As mountains are conductors of heat and electricity, and precipitate clouds, so it constantly rains on the Andes, and seldom rains in Siberia and Tartary; the clouds generally falling before they reach those countries in their passage from the oceans.

Ob. This principle of the effect of high conductors on clouds, led to a public proposal, by Sir Richard Phillips, in 1793, for the erection of artificial conductors in Great Britain, and other civilized countries, by which the descent of rain might in various degrees be regulated.

2.-Of course, as much rain falls as is evaporated; and it may be supposed, from the causes above named, that at least 75,000 solid miles of water fall every year on the land only, and the rest in the sea.

602. Springs, rivers, &c., are attributed to rain. Rain oozes down by the crannies of the stones, and enters the caverns of the hills.

These being filled, the overplus water runs over by the lowest place, and breaking out by the sides of the hills, forms springs.

These running down the vallies, between the ridges of the hills, and uniting, form little rivulets or brooks; and these, meeting in one com

mon valley, become a river, which runs into the sea, the common level.

I see the rivers in their infant-beds!
Deep, deep I hear them, lab'ring to get free;
I see the leaning strata, artful rang'd:
The fissures gaping to receive the rains,
The melting snows, and ever-dripping fogs.
Strew'd bibulous above, I see the sands;
The pebbly gravel next, the layers then
Of mingled moulds, of more retentive earths
The gutter'd rocks, and mazy-running clefts,
In pure effusion flow. United, thus
Th' exhaling sun, the vapour-burden'd air,
The gelid mountains, that to rain condens'd
These vapours in continual current draw,
And send them, o'er the fair-divided earth,
In bounteous rivers to the deep again;
A'social commerce holding arm support.

THOMSON.

603. The instruments for making observations on the atmosphere, are the barometer; which ascertains the weight of the air, and varies in height between 30 and 28 inches.

The thermometer; which ascertains the degree of heat by the expansion or contraction of a fluid in the bulb, which sensibly affects the quantity in a small connected tube.

Thirty-two degrees are called the freezing point; and 212 is the heat of boiling water. It is hot weather at 70; but it has been in England as high as 95; and is, in winter, sometimes 50 degrees below the freezing point, or 18 degrees below O, or zero. At 40 degrees below O, mercury freezes.

Obs. Mr. Wedgewood's clay-Pyrmometer measures variations of heat, as high as 32,000 degrees!

THE THERMOMETER,

142

At A, there is a bulb or ball filled with quicksilver or spirits; which, distending or shrinking with heat or cold, raises or falls the thread of the same liquid, contained in the connected tube; and the scale and figures at the side, indicate the relative degrees of heat and cold,

For the purpose of acquiring a scale, the bulb is first plunged into melting ice, and the place where the liquid stands is marked; the bulb is afterwards plunged into boiling water, and the same operation repeated, On Fahreinheit's scale, this space is divided into 180 equal parts; and similar parts are taken above and below, for extending the scale'; and the freezing point of water is placed at 92°, and the boiling point at 219a,-1,8 degrees of Fahrenheit are equal to one degree of the Centigrade thermometer; and 2,95 to one degree of Reaumur,

THE BAROMETER,

This consists of a basin of mercury at the bottom, lato which a tube, closed at top, and open at bottom, deprived of its air, is plunged; the external atmosphere pressing then by its elasticity on the surface of the basin, raises the mercury in the tube to a height equal to the elastic force of the air; varying between 28 and $1 inches, and indicated by an accurate scale at the top.

606. Besides the above, there are hygrometers, to measure the moisture of the air; rain gauges, to take the depth of rain; electrometers, to measure the electricity; and anemometers, to measure the velocity of the wind,

By these last, it appears, that wind is just perceptible when it moves two miles in an hour; that it is brisk, at 15 miles; high, at 35 miles; blows a storm, at 50 miles; and a hurricane, at

100 miles an hour: tearing up trees, and carrying away buildings.

In England, the wind blows twice, or nearly thrice as much from the west as from the east ; and the wind from the south is to that from th north, in the proportion of 3 to 2.

Of what important use to human kind,
To what great ends subservient is the wind?
Where'er th' aërial, active vapour flies,

It drives the clouds, and ventilates the skies:
Sweeps from the earth infection's noxious train;
And swells to wholesome rage the sluggish main.

605. The primary cause of all wind is the heat of the sun; which, during the diurnal rotation of the earth, passes over some parallel between the tropics, from east to west, every 24 hours.

Hence, as the air which is beneath the sun, is every way rarefied, there is a regular wind following the sun in the tropical parts of the world; which, in some parts, is so regular and so unruffled, particularly in the great Pacific Ocean, that the inhabitants have no idea of a change of wind, or of a storm.

Obs.-It is remarked, that the cause of all winds begins at the part towards which they flow or follow.

606. The variation of the parallel over which the sun passes vertical, from 23 north to 234 south, necessarily affects the winds at each pole; the heights of mountains, and local causes of heat in particular situations, also generate constant changes in the wind, in some northern and middle latitudes.

In other situations, where mountains aid the