For example,–if a free or dry thermometer indicates 63°, and the one with the wet bulb has by evaporation cooled down to 54°, the difference would be 9°. The dew-point would not be 54°, but that degree to which the mercury would fall in the free thermometer, for the atmosphere to become saturated with the quantity of moisture then actually existing in it. It would be 46.8°.

This dew-point, which figures so largely in all well-kept meteorological reports, is the key to many important conditions of the atmosphere, affecting health, vegetation, and climate.

It is found that the air at different degrees of heat has different degrees of elasticity, different degrees of tension, and different degrees of capacity to hold vapor. Dalton, by a series of experiments with barometer-tubes, into which he introduced air and vapor at certain temperatures, found what its force was upon the mercurial column from degree to degree. He also experimentally determined the ratio of the weight of moisture and of air, the former being five-eights of the latter,–in other words, how many grains of moisture additional could be held by the air, advancing from degree to degree of temperature. This being ascertained, a table of factors was constructed, in other words, a set of figures contrived, which should, by a multiplication of the subtracted difference between the range of the dry bulb and the wet bulb of the thermometers, furnish the amount of deduction from the former which would indicate the dew-point, or the point to which the mercury in the dry thermometer must fall to show how much more moisture the air could hold without its condensation. These tables of factors have been constructed at the Greenwich Observatory, and are generally used.

The Hygrometer, invented by Mr. Daniell, gives the dew-point by inspection.

It is an error to suppose that dew falls like rain from the air; it forms on the body which is cooled down below the temperature of the air. It differs in quantity with the radiating or cooling surface; that which has absorbed and retained the most heat during the day radiates the most at night and furnishes the most cold in return.

Hoar-frost, such as we find on our window-panes, or on the grass, is the moisture of the warm air cooled down and frozen, and is produced when the cold at the surface is below the freezing-point. What we in common parlance call the action of frost, and which in this climate is well known to be very powerful, is not particularly injurious to organized bodies.

Mists are the vapor near the ground rendered visible by the temperature of the air falling below that of the vapor. When we see our breath in a cold morning, we see a mist. Where the surface is comparatively warm and damp, and the air is cooler, we have mists, which, if dense, are called fogs. These are found plentifully on the banks of Newfoundland; and with icebergs on the one hand and the Gulf Stream on the other, we must always expect to have them.

The distribution of rain, which is one of the offices of the clouds, is another of the more important features of Meteorology. The amount of water taken up by evaporation into the atmosphere is almost incredible. It is calculated by Lieutenant Maury that there is annually taken up in the torrid zone a belt of water three thousand miles in breadth and sixteen feet deep. Rain occurs regularly and irregularly in different parts of the earth. In some places it may be calculated upon to a day; in others it is quite unknown. Latitude and longitude may indicate the points of distribution, but the causes are dependent on temperature, winds, locality, and, what may seem a strange assertion, upon the conduct of man himself. The greatest quantity falls near the equator, diminishing towards the poles. Much more falls on islands and coasts than in the interior of continents,–more in the region of the variables and less in that of the trades. There are, however, tropical countries of great extent where rain is scarcely ever seen.

The influence of man upon rain is seen in the progress of civilization, the destruction of forests, and the drying-up of meres, swamps, and water- courses.

Forests undoubtedly affect the distribution of rain, and the supplies of streams and springs. Their cooling influence precipitates the vapor passing over them, and the ground beneath them not getting heated does not readily evaporate moisture. Lands, on the contrary, which are cleared of forests become sooner heated, give off larger quantities of rarefied air, and the passing clouds are borne away to localities of greater atmospheric density.

The Canary Islands, when first discovered, were thickly clothed with forests. Since these have been destroyed, the climate has been dry. In Fuerteventura the inhabitants are sometimes obliged to flee to other islands to avoid perishing from thirst. Similar instances occur in the Cape Verdes. Parts of Egypt, Syria, and Persia, that once were wooded, are now arid and sterile deserts.

In the temperate zones these results are not so immediately apparent. It is now much in doubt whether the climate of our country has changed its character within the last two hundred years. Jefferson and Dr. Rush both contended that it had. Our oldest inhabitants assert that in their day our winters began nearly two months earlier than they do now.

The general laws laid down in relation to rain are these:–

1. It decreases in quantity as we approach the poles.

2. It decreases as we pass from maritime to inland countries.

3. It decreases in the temperate zones on eastern coasts as compared with western coasts, but within the tropics it is the reverse.

4. More rain falls in mountainous than in level countries.

5. Most rain falls within the tropics.

The rainless regions, not deserts, are parts of Guatemala, the table-land of Mexico, the Peruvian coast, parts of Morocco, Egypt, Arabia, Persia, etc.

The electric character of the air is another subject of interest, and a leading one in Meteorology. What can be more magnificent, what more awful, than those storms of lightning and thunder which are witnessed sometimes even in our own latitudes?

Faraday, who as a chemist and philosophical writer is of the highest authority, professes to have demonstrated that one single gram of water contains as much electricity as can be accumulated in eight hundred thousand Leyden jars, each requiring to charge it thirty turns of the large machine at the Royal Institution.

It is not intended that this astounding statement should be received without some grains of allowance; but a very elegant and scientific writer, who adopts it without hesitation, adds, "We can from this crystal sphere [of water] evoke heat, light, electricity in enormous quantities, and beyond these we can see powers or forces for which, in the poverty of our ideas and our words, we have not names."

Flashes of electricity have been detected, during warm, close weather, issuing from some species of plants. The Tuberose and African Marigold have been seen to emit these mimic lightnings. (Goethe is the authority for this.) To atmospheric electricity we doubtless owe the coruscations of the Aurora, one of the most beautiful of our meteors.

The usual forms of lightning are the zigzag or forked sharply defined,–the sheet-lightning, illuminating a whole cloud, which it seems to open,–heat- lightning, not emanating from any cloud, but apparently diffused through the air and without report. There are also fireballs which shoot across the sky, leaving a train often visible for seconds and minutes. These last, when they project any masses to the earth, are termed aërolites.

Atmospheric electricity has much to do with the distribution of rain, the precipitation of vapor, the condition of our nervous system, and, according to Humboldt, with the circulation of the organic juices. Atmospheric electricity has heretofore been a great obstacle to the success of the Magnetic Telegraph, and curiously disturbs its operation; but there has recently been invented an instrument called a Mutator, which is connected with the wires, and carries off all the disturbing influences of the atmosphere without interfering with the working current. On the other hand, artificially created electricity has led to important advances in many of the arts and sciences.

Ice is water frozen under a very curious and peculiar law. Hail is the congelation of drops of rain in irregular forms, always sudden,–by some attributed to electricity and currents of air violently rarefied by it, and by others to rain-drops falling through a cold stratum of air and suddenly congealed. Snow, the ermine of the earth, is the crystallized moisture of the air, and is in subjection to unchanging laws.

Water contracts as it grows colder, until it falls in temperature to 42°. It then expands till it reaches 32°, when it becomes solid, though its density is actually diminished, and its specific gravity is reduced to .929, while that of unfrozen water is 1.000. Of course it is much lighter, and it floats. This admirable arrangement prevents our rivers being frozen up and our lakes becoming solid. Ice thickens because it is porous, and allows the heat of the water to pass up and the cold to descend; but this is happily a slow process, as ice is a bad conductor. Salt water freezes at the temperature of 7°, 25° below freezing- point. There are many things to be said about ice, whether as glaciers, or Arctic bergs, or, as it is found sometimes, contrary to its general law, at the bottom of rivers and ponds, its geological movements in the transportation of boulders, and as an article of luxury;–but we are compelled to leave them for the present.

Snow, which, in its crystallization, surpasses the most perfect gems, is invariably found arranged in determinate angles, to wit, 60°, and its double, 120°, and formed of six-sided prisms. More than one hundred kinds have been described by Dr. Scoresby and others, and all these are combinations of the six- sided prism. The uses of snow, from its non-conducting qualities, whether as appreciated by the Esquimaux as a material for huts, or by the agriculturists of our own climate as sheltering the seed, are too well known to require any particular remarks. Strange as it may appear, the proximate cause of the formation of snow is not yet fully agreed upon by the learned.

The connection between Sound and the atmosphere is an important one. The air is a conductor of sound, and in some conditions one of the best. A bell rung in an exhausted receiver gives no sound. In the Arctic regions ordinary conversations have been distinctly heard for the distance of a mile and a half.

All that we have thus far said in this article bears directly, in some form or other, on another of the great features of Meteorology, one of its great objects, and an unceasing topic,–namely, Climate.

The term Climate, in its general sense, indicates the changes and condition of the atmosphere, such as we have been considering. It has something to do with all of them; it is not entirely controlled by any. Thus, places having the same mean annual temperature often differ materially in climate. In some (we quote Mrs. Somerville) the winters are mild and the summers cool, whereas in others the extremes of heat and cold prevail.

Climates are not found coincident with lines of latitude; they are quite as often found parallel to lines of longitude. If you connect the extreme points of the mean annual temperatures by a line passing round the earth, you have a zone, but never a true circle. The curves are longitudinal.

Climate is dependent on temperature, winds, the elevation of land, soil, ranges of mountains, and proximity of bodies of water; and it is also the expression, if we may so term it, of the changes in the atmosphere sensibly affecting our organs. Humboldt refers it to humidity, temperature, changes in barometric pressure, calmness or agitation of the air, amount of electric force, and transparency of the sky.

When mountains range themselves in lines of latitude across a continent, they are barriers to civilization, to the mingling of races, and the union of states. Thus, the Pyrenees have always kept France and Spain apart, the Alps and the Apennines have secluded Switzerland from its neighbors. In our own country, Providence has placed our great mountains on a northern and southern axis; the slopes, the direction, the prevailing winds, the facilities for transportation and travel favor no one of our northern, southern, and western States more than another.

Climate affects vegetation and the distribution of animal life, and thus greatly modifies commerce.

Whatever of importance is accomplished in those countries where climate has overpowered a race is best and principally done by the men of the temperate zones, who carry with them perseverance, courage, and ability, and maintain their ascendency, true to their type, while they have their life to live.

But with our own eyes we may perceive how much climate affects agriculture. The humidity or dryness of soils, their natural or acquired heat or cold, the prevailing winds, the quantity of rain, the snows, the dews, all affect the planter of the seed and the tiller of the ground; they increase or diminish the aggregate of the products of countries, the value of their imports and exports,– -in short, their material power, their resources, their influence, their very existence.

The climate of our own country is exceedingly variable. The transitions from heat to cold are very sudden, the range of the mercury is very great. In the North, we have almost the Arctic winters; in the South, almost the peculiarities of the tropics. Of the State of Pennsylvania it has been said, that in this respect it is a compound of all the countries in the world. Mr. Jefferson and Dr. Rush, as before observed, insisted that our climate has changed; and Williams, the historian of Vermont, contends that New England has deteriorated in its seasons, temperature, harvests, and health, since its early settlement. Our winds blow from every point of the compass, but a due north wind is very rare. Our great western lakes have a large influence on our climate. Some learned men have asserted, that, if they were land, their area being about ninety-four thousand square miles, the region would be so cold as to be scarcely inhabitable.

Such is an outline of our subject. The science itself is by no means systematized. Many things are taken for granted which may yet be disproved. If, says Humboldt, we perceive a want of connection in the phenomena of certain sciences, we may anticipate the revelation of new facts, whose importance will probably be commensurate with the attention directed to other branches of study. What we want is a larger class of observers, and not only those who are professional persons, but those who would commune with Nature, and seek to invigorate their minds by the acquisition of new ideas, and a recourse to rich and pure sources of enjoyment.

But more than this. It is a requirement of the present age, says the same authority, that there should be an equal appreciation of all branches of mathematical and physical science; for the material wealth and the growing prosperity of nations are principally based upon a more enlightened employment of the products and forces of Nature.

Much attention has of late years been paid to this subject. Many distinguished men in Europe have connected their great reputations indissolubly with it, and it is absolutely true that more persons are engaged in a common effort to promote this science than any other of our time. In Paris there is a large and flourishing society where the most brilliant of its savans combine their efforts. In London, that which was established in 1850 has met with remarkable success, and a most unexpected crowd of supporters. The finest instruments, the most accurate observations, and entire uniformity of purpose have been the result. In Germany, equal zeal prevails among its naturalists. There are more than eight hundred stations throughout the world where regular observations are made, and upwards of three hundred and sixty of them are in the United States. The Smithsonian Institution has been also a wise patron of this science, by its numerous publications, its lucid directions for observing meteorological changes, and the bestowal of standard instruments in large numbers to efficient and well-placed observers. By a recent arrangement, a portion of this work is to be performed by the Patent Office.

Observation, and accuracy in observation, are the foundation of this science. The results are compared to the leaves of a book, which will some day be arranged and bound together in one volume. The instruments in use are delicate, ingenious, and indispensable. Their history, uses, and importance would be topic enough for a separate article.

While at the first view Meteorology may appear to occupy but a limited sphere, upon a closer examination it will be found to embrace almost all the sciences, and to be commensurate with Nature itself. It is continually influencing us, by its agencies appealing to our senses, ministering to our wants, and governing our conduct.

Its influence upon its votaries is equally remarkable; for, as a rule, they are distinguished among the learned, their characters are in harmony with their pursuits, and they are recognized everywhere for disinterestedness, philanthropy, and public and private virtue. While Mental Philosophy, has made but little progress since the times of Plato, and the world is but little better for scholastic disputations, Natural Science has civilized man, elevated his condition, increased the circle of his exertions, and, by the development of some of its simplest principles, united the intelligent, the learned, the enterprising, and the virtuous of all nations into a recognized and a noble brotherhood.