RHYTHM, in general, means a measured division of time. The rhythm, in dancing, is made manifest to the eyes by steps, and, in music and language, to the ears by tones. (See the beginning of the article Dancing.) We must refer the fondness for rhythm, in the human mind, to its love of order, harmony, symmetry, which lies at the basis of all the arts. As song, music and dancing sprung from the same sources, and, in the earliest periods of nations, are actually united, the rhythm of all three has much in common. The rhythm of poetry is susceptible of the same exact divisions of time as the rhythm of dancing and music; but rhetorical rhythm is satisfied with a pleasing cadence of syllables-an approximation to the rhythm of verse, particularly at the beginning and end of periods. The orator or eloquent prose writer arranges his words in an expressive and pleasing succession, but he does not follow precise rules, like the versifier. The poetical rhythm, like every species of rhythm, requires a succession of motions of regular duration, which, variously interrupted, must yet be obvious, and combined according to the rules of beauty and grace, so as to form a harmonious whole. In order to make rhythm please, its constituent parts must excite the feeling of variety in harmony or unity. The various parts must form a whole, and exhibit a beginning, middle and end, by a measured rise and fall. Those parts which receive the ictus, the stress, of the rhythm are called arsis (elevation), the other parts thesis (positio, depression). To denote the arsis, the common acute accent is used ('), e. g.

Singula quaeque locum teneant sortita decenter. The arsis must by no means be confounded with the long syllable, nor the thesis

with the short syllable. As the short syllable is the smallest constituent part of a verse, it is considered as the original unit for the measure of time in the rhythm, and is called a time, or mora. The absolute duration of this unit depends upon the quickness or slowness with which the rhythmical composition is uttered. The smallest rhythmical magnitude is the foot, by which every union of arsis and thesis is understood. A single word may constitute a foot; or the beginning and end of the foot may be in the middle of words, as in the following verse:

Contém-nit, ár-tibús-que ví-vit dé-ditúm turpís-si

mis.

Rhythm can be imagined without words, and may be indicated by notes, or other signs of long and short syllables. Hence the rhythm may also be divided differently from the words, as we have just seen; and the division of the words should not agree with the rhythmical feet, except where a rhythmical series is concluded, or the pausing of a part of the same requires a break in the text. In all other cases, the divisions of the rhythm ought to separate the parts of words as much as possible, which is called cæsura. (q. v.) The Greeks distinguished the feet according to the number of units of time contained in them. The Romans divided them, according to the number of syllables, into four of two syllables, eight of three syllables, and sixteen of four syllables, and called them, with the Greeks, thus:

1. Feet of two Syllables. Pyrrhichius.

Spondeus.

Choréus, or Trochæus. Iambus.

SLATE. Clay slate (Thon Schiefer of the Germans) and argillite. By the early English geologists, this rock was called argillaceous schistus, and it is denominated phyllade by the modern French writers. The structure of slate is eminently fobated or schistose, separating, in some of its varieties (as in the roofing slate, for example), into laminæ as thin as pasteboard. Prevailing color gray of various shades: it is also bluish, reddish and greenish; opaque and dull; yields to the knife, but varies considerably, as respects hardness, in its different varieties; fissile; specific gravity 2.7. When moistened, it emits an argillaceous odor. The common roofing slate appears to consist very nearly of the following ingredients:

Ionicus a minore.

Ionicus a majore.

Pæon primus.

Pæon secundus.

Pæon tertius.

Pæon quartus. Epitritus primus. Epitritus secundus. Epitritus tertius.

Epitritus quartus.

These feet are simple or compound, redundant or retrenched. The first consist of but one arsis and thesis, as,

、 L~,~~, &c. The second consist of two of each sort, of which one arsis and thesis, taken together, is considered as a single arsis or thesis, as budy, tutu, ~—~—, ————or

- The third are such as contain, besides a simple foot, a short prefix or affix, or in which feet of unlike quantity are connected with each other, as, ༦╚, L~; ~---, ~`~, The fourth sort are feet which, with two arsises, have but one thesis, or have two arsises immediately succeeding each other without a thesis between, e. g. Lut, tuu, or,;,

utt, buty,, ~~; ~~ ↓ ↓ ↓ ↓——,-. Of the simple feet, those only which have a long syllable in the arsis, and a short syllable in the thesis, afford natural variety, as Lu, tum, ut,. The trochee and dactyle, therefore, the iambus and anapest, are considered as the fundamental feet of all rhythm, with which the other feet can be brought into connexion by resolving a long syllable into two short ones, or by contracting two short into one long. (For more information, see Verse.)

But slate varies exceedingly in its chemical constitution, as might very naturally be expected, since it is a mixed rock, consisting of very minute individuals of quartz, feldspar and mica, to which are occasionally added scales of talc, and particles of carbonaceous matter. Those slates which contain a large proportion of quartz, are called whet-slate. In these, the mechanical composition is impalpable, and the fracture splintery in the small, though slaty in the large. They are translucent, and of a greenish-white color. When magnesia enters largely into the composition of slate rocks, they are distinguished by their green color, and by their unctuous feel. These are the slates which, for the most part, have tale as an ingredient, and are often called tale, or chlorite slates. When carbonaceous matter prevails to the proportion of eight or ten per cent., the slate soils more or less, and even writes. It is then called drawing slate, or black chalk. This variety is softer than the preceding kinds, and sometimes possesses the property of adhering to the tongue. Its specific gravity is only 2.18. A variety of slate called adhesive slate, from its property of adhering to the tongue, deserves to be mentioned, although it is very remote in its properties from the roofing slate, which may be considered as the type of the present rock. Fracture in the large slaty; in the fine earthy; color light gray; specific

Still another argillaceous aggregate, which has been treated of along with the slates, is the polishing slate. It differs from adhesive slate in not adhering forcibly to the tongue, in being very soft, and in having a low specific gravity, namely, 0.50 to 0.60.-Slate, in varieties approaching roofing slate, occurs in vast strata in primitive countries, and is often observed graduating into mica slate. Wherever its strata are contiguous to granite, gneiss or mica slate, it is noticeable that it has a more shining lustre: as it recedes, however, from the primary rocks, its texture is more earthy. It is commonly divided into beds of various degrees of thickness, which are generally much elevated; and, from the natural divisions of the rock, they often form peaked and serrated mountains. The cleavage of these beds is in a transverse direction, making with the slope of the bed an angle of about 60°. The finest variety which is used for roof slate seldom forms entire mountains, but is generally imbedded in slate rocks of a coarser kind. Those kinds are selected for the covering of buildings which have the smoothest surface, and split into the thinnest plates. Quarries of slate of this description are worked extensively in Westmoreland, Yorkshire, Leicestershire, North Wales, Cornwall and Devonshire. Excellent deposits of roofing slate occur at several places in Vermont, one of which is situated near Brattleborough, upon the Connecticut river; also in Massachusetts, in Worcester county, where it is associated with the peculiar mica slate that contains anthracite coal. Excavations of considerable extent have been made in Harvard and Pepperell; and the slate obtained has been employed both as a roofing slate and for grave-stones. Whet slate is found in beds between strata of common slate in transition formations. The use of this variety for hones and whet-stones is well known. The most valuable kinds come from Sonnenberg, in Meiningen, and from Saalfeld. They are likewise brought from the Levant. It has been discovered, within a few years, in great perfection, over a very large extent of country in North Carolina. An inferi

50 *

or variety occurs extensively in the vicinity of Boston, at Charlestown, Quincy and Malden. Talcose and chlorite slates are found abundantly in various parts of New England, and afford the gangue for the most part of the native gold of the Southern States. The drawing slate, which is used as a drawing material, comes from Italy, Spain, and Bayreuth in Thuringia. It has been observed also at several places in Rhode Island, and in the neighborhood of coal measures generally in the U. States. Adhesive slate occurs only at Menil, Montant and Montmartre, near Paris. Polishing slate, which is believed to have been formed from the ashes of burnt coal, occurs at Planitz, near Zwickau, and near Bilin, in Bohemia. It is used as a polisher of metals.

The

SOUTH AMERICAN GEOLOGY. equatorial regions of America exhibit the same composition of rock that we meet with in other parts of the globe. The only formations which Humboldt could not discover in his travels were those of chalk, roe-stone, gray-wacke, the topazrock of Werner, and the compound of serpentine with granular limestone, which occurs in Asia Minor. Granite, in South America, constitutes the great basis which supports the other formations: above it lies gneiss: next comes micaceous schist, and then primitive schist. Granular limestone, chlorite schist, and primitive trap, often form subordinate beds in the gneiss and micaceous schist, which is very abundant, and sometimes alternates with serpentine and sienite. The high ridge of the Andes is every where covered with formations of porphyry, basalt, phonolite, and green-stone; and these, being often divided into columns, that appear from a distance like ruined castles, produce a very striking and picturesque effect. At the bottom of these huge mountains occur two different kinds of limestone; the one with a silicious base, enclosing primitive masses, and sometimes cinnabar and coal; the other with a calcareous base, and cementing secondary rocks together.-Plains of more than 600,000 square miles are covered with an ancient deposit of limestone, containing fossil wood and brown iron ore: on this rests the limestone of the higher Alps, presenting marine petrifactions at a vast elevation. Next appears a lamellar gypsum, impregnated with sulphur and salt; and, still higher, another calcareous formation, whitish and homogeneous, but sometimes cavernous. Again occurs calcareous sand-stone, then lamellar gypsum mixed with clay; and the

series terminates with calcareous masses, involving flints and hornstone. But what may perplex some geologists, is the singular fact noticed by Humboldt, that the secondary formations in the new world have a most enormous thickness and elevation. Beds of coal are found in the neighborhood of Santa Fé, 8650 feet above the level of the sea, and even at the height of 14,700, near Guanuco, in Peru. The plains of Bogota are covered with sandstone, gypsum, shell-limestone, and, in some parts, with rock-salt. Fossil shells, which, in the old continent, have not been discovered higher than the summits of the Pyrenees, or 11,700 feet above the sea, were observed in Peru, near Micuipampa, at the height of 12,800; and again at that of 14,120; besides at Guancavelica, where sandstone also appears. The basalt of Pichincha, near the city of Quito, has an elevation of 15,500 feet; while the top of the Schneekoppe, in Silesia, is only 4950 feet above the sea, the highest point in Germany where that species of rock occurs. On the other hand, granite, which in Europe crowns the loftiest mountains, is not found in the American continent above the height of 11,500 feet. It is scarcely known at all in the provinces of Quito and Peru. The frozen summits of Chimborazo, Cayambe and Antisana, consist entirely of porphyry, which, on the flanks of the Andes, forms a mass of ten or twelve thousand feet in depth. The sandstone near Cuença has a thickness of 5000 feet; and the stupendous mass of pure quartz, on the west of Caxamarca, measures, perpendicularly, 9600 feet. It is likewise a remarkable fact, that the porphyry of those mountains very frequently contains hornblende, but never quartz, and seldom mica. The Andes of Chile have a distinct nature from those three chains called the Maritime mountains, which have been successively formed by the waters of the ocean. This great interior structure appears to be coeval with the creation of the world. It rises abruptly, and forms but a small angle with its base; its general shape being that of a pyramid, crowned at intervals with conical, and, as it were, crystallized elevations. It is composed of primitive rocks of quartz, of an enormous size and almost uniform configuration, containing no marine substances, which abound in the secondary mountains. From the Cordillera of this part of the Andes are obtained blocks of crystal of a

size sufficient for columns of six or seven feet in length. The central Andes are

rich beyond conception in all the metals, lead only excepted. One of the most curious ores found in the bowels of those mountains is the pacos, a compound of clay, oxide of iron, and muriate of silver, with native silver. The mines of Mexice and Peru, hitherto worked with remark able success, so far from being exhausted, promise, under a liberal and improved system, to become more productive than ever. Nature has, however, blended with those hidden treasures the active aliments of destruction. The whole chain of the Andes is subject to the most terrible earthquakes. From Cotopaxi to the South sea, no fewer than forty volcanoes are constantly burning, some of then, especially the lower ones, ejecting lava, and others the muriate of ammonia, scorified basalt, and porphyry, enormous quantities of water and moya, or clay, mixed with sulphur and carbonaceous matter. (Sec, further, the articles Mines, Diamond District, Peru, Potosi, Brazil, &c.)

SPARROW, OF FINCH (fringilla). The common European sparrow is almost domesticated in that portion of the globe, frequenting the habitations of man, even in the midst of populous cities, and nestling under the eaves of houses, in holes in the walls, in pots placed for their use, &c. It is of a robust form, and has a stouter bill than the majority of the finches. It is found almost throughout the Eastern continent, supporting equally well severe cold and extreme heats. It is inconveniently familiar, and its incessant and monotonous note is fatiguing to the ear. In many districts it is so numerous as to do great injury to the grain fields. Its voracity is extreme; neither can its flesh or plumage be applied to any useful purpose. Fortunately, we are free from this pest on this side of the Atlantic. We have, in its place, the chipping sparrow, a delicate bird, almost as familiar, but nowise obtrusive. We have, besides, numerous species of finch in the U. States. They are readily distinguished from other small birds, by the short, conical bill, with cutting edges, which seems peculiarly adapted to the purpose of freeing seeds of the hulls-an operation which these birds accomplish with great adroitness Though granivorous, they feed their young on worms and soft insects only.

of

SPHAGNUM; a very natural genus mosses, easily recognised when once known, and remarkable for the whitish color of the leaves. These plants are soft, flaccid, and, when moistened, absorb

water like a sponge, but become friable in drying. They grow in moist places, and are usually saturated with water, often Occupying, exclusively, considerable tracts of marshy ground. The formation of peat, in such situations, is often owing, in a great measure, to the presence of these plants. They are found in all parts of the globe, from the equator to the polar regions, and to the summits of the highest mountains. They are excellent for enveloping the roots of plants intended for distant transportation.

SPIDER (aranea). These well-known animals are among the most interesting of the insect world, from their habits and mode of life. They differ essentially, in their internal structure, from insects proper, and their external form is so peculiar that they are easily recognised. The body is composed of two pieces only, the head being united with the thorax; and the feet are always eight in number; the jaws consist of two cylindrical, horny pieces, each having a palp, or feeler, at base, and terminated by a movable hook; the eyes are six or eight, variously disposed in the different genera, but always simple. They do not undergo metamorphoses: all envelope their eggs in a cocoon of silk, varying, however, in form and texture. The spiders are now arranged under numerous genera, differing, not only in anatomical characters, but very generally in their habits and the form of the web which they construct. The most familiar form of web is that of a wheel, perpendicular or slightly inclined, having its radii or spokes crossed by numerous concentric circles: in the centre of this the spider remains immovable, or more frequently in a little cell, at some distance from the web, but connected with it by threads. The struggles of an entangled insect communicate an undulatory motion to the whole web, which gives notice to the spider, who immediately sallies forth, and, if his victim be small, seizes it at once, and immediately sucks its blood: if, however, it be too large to be thus disposed of, the spider rolls it with his hinder feet, encircling it with a new thread at every turn, until, sometimes, the insect is completely coated, when it may be devoured at pleasure. Other spiders spin an irregular web, consisting of threads intersecting each other at every angle. Others, again, make a horizontal, closelymatted web, having a funnel-shaped retreat, into which they convey their prey. Others make only a retreat by binding a few leaves together, from which they sally

forth and seize insects which approach them. Some of these seem to be extremely venomous; for it is observed that no insect that has been once bitten by them, ever recovers, even though it be many times larger and more powerful than its adversary. Some are aquatic, and spin a cup-like web, which answers the purpose of a diving-bell, under which they disengage the air they bring down from the surface, and pass their lives feeding on aquatic insects. Some spiders spin no web, but take their prey by running; others, by approaching quietly till within a certain distance, when they suddenly leap upon their prey. Other spiders form cylindrical and perpendicular holes in the ground, into which they retreat on the approach of danger.

SPINAGE (spinacia oleracea). This plant is a native of Persia, and has been cultivated in Europe, as an esculent, for about two centuries. The root is annual; the stem herbaceous, smooth, upright, a foot or more high, and somewhat branching; the leaves alternate, petiolate and arrowshaped; the flowers small and greenish, disposed in several little bunches, in the axils of the superior leaves, and diœcious: in short, the whole plant much resembles some of the species of goose-foot (chenopodium), to which genus it is allied in its botanical characters. It is eaten sometimes in salads, but more frequently, cooked in various manners. It is a wholesome and agreeable aliment, but contains little nutriment, and is not suitable for delicate stomachs. The plant is of the easiest culture, and may be procured nearly all the year round, by sowing at intervals of time. It requires a rich soil, and frequent watering in dry weather.

SPINOZA, Baruch, or, as he translated his name, Benedict, was born, in 1632, at Amsterdam, of a Jewish-Portuguese family, and early gave proof of a reflecting mind and an independent spirit, which a scanty education only excited to new efforts. He was early dissatisfied with the instructions of the rabbins, and determined to examine for himself. The goodness of his disposition could not preserve him from persecution when his mode of thinking was discovered. He was calumniated and accused before the synagogue. He refuted the accusations with calmness, in spite of menaces on one hand, and zealous attempts to convert him on the other, but was at length excommunicated. He received the sentence with equanimity, and thenceforth joined no particular religious denomina