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
