THE purpose of this article is to present, in a brief and simple manner, the leading principles on which the science of Meteorology is founded,—rather, however, in the spirit of an inquirer than of a teacher. For, notwithstanding the rapid progress it has made within the last thirty years, it is far from having the authority of an exact science; many of its phenomena are as yet inexplicable, and many differences of opinion among the learned remain unreconciled on points at first sight apparently easy to be settled.

Meteorology has advanced very far beyond its original limits. Spherical vapor and atmospheric space give but a faint idea of its range. We find it a leading science in Physics, and having intimate relations with heat, light, electricity, magnetism, winds, water, vegetation, geological changes, optical effects, pneumatics, geography, and with climate, controlling the pursuits and affecting the character of the human race. It is so intimately blended, indeed, with the other matters here named, as scarcely to have any positive boundary of its own; and its vista seems ever lengthening, as we proceed.

VOL. VI.

Without dwelling upon the numerous consequences which flow from meteorological influences, let us see what is properly included under the subject of Meteorology. And first, of the Atmosphere.

This is a gaseous, vapor-bearing, elastic fluid, surrounding the earth. Its volume is estimated at th, and its weight at about

43

oths, that of the globe. It is composed of 21 parts in weight of Oxygen and 77 of Nitrogen, with a little Carbonic Acid, Aqueous Vapor, and a trace of Carburetted Hydrogen. There are numerous wellknown calculations of the proportions of the various constituents of the atmosphere, which we owe to Priestley, Dalton, Black, Cavendish, Liebig, and others; but that given by Professor Ansted is sufficiently simple and intelligible. In 10 volumes or parts of it, he gives to

Oxygen, the great supporter of life, Nitrogen, (not condensible under 50 atmospheres, and not respirable or combustible,)

Aqueous Vapor

2.100

7.750

.142

of air as being 1 Oxygen to 4 Nitrogen.

It is a curious fact, that, while there are six varieties of compounds of nitrogen and oxygen, but one of these is fitted to sustain life, and that is our atmosphere.

for

It is well enough to note, that, when we use the word volume or measure, in speaking of the atmosphere or any gaseous body, we adopt the theory of GayLussac, who discovered that gases unite with each other in definite proportions whenever they enter into combination. This theory led to important results; by knowing the elements of a compound gas, we easily determine its specific gravity. It has been attempted to apply the principle to organic bodies; but it has not yet been carried to a full and satisfactory conclusion. It may be noticed, too, that Dalton affirmed that simple substances unite with each other in definite weights to form compound substances, thus supporting the idea of Lussac. These discoveries were made about the same time, Dalton having the credit of originating them. Various modifications of the principle have been from time to time presented to public attention.

Whether the constituents of the atmosphere are chemically or mechanically combined, one of the things about which the learned are not fully agreed,it is found to be chemically the same in its constituents, all over the world, whether collected on mountains or on plains, on the sea or on the land, whether obtain ed by aëronauts miles above the earth or by miners in their deepest excavations. On the theory of its mechanical combination, however, as by volume, and that each constituent acts freely for itself and according to its own laws, important speculations (conclusions, indeed) have arisen, both as regards temperature and climatic differences. It should be observed, that volume, as we have used the word, is the apparent space occupied, and differs from mass, which is the effective space occupied, or the real bulk of matter, while density is the relation of mass to volume, or

the quotient resulting from the division of the one by the other. Those empty spaces which render the volume larger than the mass are technically called its pores.

Has the composition of the atmosphere changed in the lapse of years? On this point both French and German philosophers have largely speculated. It is computed that it contains about two millions of cubic geographical miles of oxygen, and that 12,500 cubic geographical miles of carbonic acid have been breathed out into the air or otherwise given out in the course of five thousand years. The inference, then, should be, that the latter exists in the air in the proportion of 1 to 160, whereas we find but 4 parts in 10,000. Dumas and Bossingault decided that no change had taken place, verifying their conclusion by experiments founded on observations for more than thirty-five years. No chemical combination of oxygen and nitrogen has ever been detected in the atmosphere, and it is presumed none will be.

The atmosphere possesses, as may be readily imagined, many important characteristics. One of these is Weight.

This is demonstrated by simple, yet decisive experiments. The discovery of the fact is attributed to the illustrious Galileo, but to modern science we owe all the certainty, variety, and elegance of the demonstration. A vessel containing a quantity of air is weighed; the air is exhausted from it and it is weighed again. An accurate scale will then detect the difference of weight. A cubic foot of air weighs 1.2 oz. Hence a column of air of one inch in diameter and a mile in height weighs 44 oz.

The atmosphere is supposed to have an elevation of from 45 to 50 miles, but its weight diminishes in proportion to its height. The whole pressure at the surface of the earth is estimated to be 15 lbs. to the square inch; a person of ordinary size is consequently pressed upon by a weight of from 13 to 14 tons. Happily for us, the pressure from with

The weight of the air is of great importance in the economy of Nature, since it prevents the excessive evaporation of the waters upon the earth's surface, and limits its extent by unalterable laws. Water boils at a certain temperature when at the earth's surface, where the weight of the atmosphere is greatest, but at different temperatures at different elevations from the surface. At the level of the sea it boils at 212°. On the high plains of Quito, 8,724 feet above the sea, it boils at 194°, and an egg cannot be cooked there in an open vessel. At Potosí the boiling-point is still lower, being 188°, and the barometrical column stands at 18°. Indeed, the experiment is often exhibited at our chemical lectures, of a flask containing a small quantity of water, which, exhausted of air, is made to boil by the ordinary heat of the hand.

Fahrenheit proposed to ascertain the height of mountains by this principle, and a simple apparatus was contrived for the purpose, which is now in successful use. The late Professor Forbes of Edinburgh, whose untimely death the friends of science have had so much reason to deplore, ascertained that the temperature of boiling water varied arithmetically with the height, and at the rate of one degree of the thermometric scale for every 549.95 feet. Multiplying the difference of the boiling-point by this number of feet, we have the elevation. The weight of the atmosphere, as indicated by the barometer, is also a means for ascertaining the height of mountains or of plains; but correction must be made for the effects of expansion or contraction, and for capillarity, or the attraction between the mercury and the glass tube, at least whenever great exactness is required. Tables for the convenience of calculation are given in several scientific works, and particularly in a paper of Professor Forbes, Ed. Trans. Vol. 15. Briefly, however, we may state, that between 0° and 32°, 34 thousandths of an inch must be allowed for depression or

A common experiment to prove the weight of air is that of the Magdeburg Hemispheres, a simple contrivance of Otto Guericke, a merchant of that city. It is a part of every complete philosophical apparatus. It consists of brass caps, which, when joined together, fit tightly and become a globe. The air within being exhausted, it will be found difficult to separate them. If the superficies be 100 square inches and the height of the mercury be 30 inches, the atmosphere will press on these hemispheres with a weight of 1,475 lbs, requiring the efforts of seven or eight powerful men to tear them asunder. One of these instruments, of the diameter of a German ell, required the strength of 24 horses to separate it. The experiment was publicly made in 1650 at the Imperial Diet at Rendsborg, in the presence of the Emperor Ferdinand III. and a large number of princes and nobles, much to their astonishment.

As compared with water, the air (the barometer indicating 30°, and the thermometer 55°) is 833 times lighter.

It is this weight of the atmosphere which counterbalances that of a column of mercury 29 inches in height, and a column of water 32 to 34 feet in height. The old quaint notion of Nature's abhorring a vacuum was found to be practically only an assertion that the air had weight. The ordinary pump, commonly called the suction-pump, is constructed on this principle. The weight of the atmosphere at the level of the sea is found to be the same all over the world.

We find the atmosphere with another characteristic,- Elasticity.

However it may be compressed, air returns, on liberation, to its original volume, and while thus perfectly elastic it

is also the most compressible of bodies. This elasticity arises from the repulsive force of its particles, and is always equal to the compressive force which it balances. A glass vessel full of air, pla

ced under a receiver and then exhausted by the air-pump, will burst into atoms. Water, on the other hand, is almost the reverse. Twenty cubic inches, introduced into a cannon whose sides are three inches thick, cannot be compressed into nineteen inches without bursting it. This non-elastic property of water, with another, that of communicating, when under the action of any force, an equal pressure in all directions, led to the invention of the hydraulic press. The elasticity of the air enables fishes to rise and sink in water, through the action of the air-bladder..

The sudden compression of air liberates its latent heat, and produces fire. On this principle the pneumatic tinderbox is constructed.

As we proceed in the consideration of our general subject, we shall find, under the appropriate heads, that density is not without material influence on reflection

and refraction, on transparency and the transmission of light, the presence or absence of moisture, and the amount of heat at the earth's surface, - and we might add, on health, and the increase or diminution of the vital energies.

Temperature is another branch of our subject, and one involving a series of subordinate topics on which volumes have been written, and to which are still devoted the labors of the most learned men of our day. In this place, merely an outline can be attempted.

Temperature is the degree of heat or cold in the particles of all bodies, which is perceptible by sensation, and is measurable by their expansion or contraction. It is the key to the theory of the winds, of rain, of aerial and oceanic currents, of vegetation and climate with all their mul

For every degree of heat between the freezing-point and the boiling-point, 32° and 2120, the expansion of air is about 4th part, so that any invention which seeks to use rarefied air as a motive power must employ a very intense degree of heat, enough to fuse many kinds of metals.

To the celebrated Mr. Boyle and to Henry Cavendish, both of Great Britain, we are indebted for most of what we know of this particular property of the air.

Density, or closeness, is another quality of the atmosphere. It has been found to be 770 times less than that of water, and 770 cubic inches of air weigh as much as a cubic inch of water. It is in direct ratio with its elasticity, and there are tables by which it may be determined at different altitudes. At the surface of the earth, this density is indicated as 1; at 2 miles, as; at 5 miles, as ; and so on, the difference being in a geometrical progres

sion.

the inclined position of the earth on its axis and its movement in its elliptical orbit influence the general amount of heat, it is rather to the consequences of these in detail that we are called when we speak of temperature. If the sun shone on a uniformly level surface, everywhere of the same conducting and radiating power, there would be but little difficulty in tracing the monotonous effects of temperature.

The reformer Luther, as eccentric as he was learned and sincere, is reported to have said, that, if he had been consulted at the Creation, he would have placed the sun directly over the centre of the world and kept it there, to give unchanging and uniform light and heat! It is certainly much better that he was not consulted. In that case, every parallel of latitude would have been isothermal, or of equal mean annual temperature. The seasons would have been invariable in character. Some portions of the earth would have been scorched to crispness, others locked up in never-changing ice.