contrivance, by or through which the successive impulses,or actions, are made, is called a 'scapement or ESCAPEMENT, several of which are described at the articles referred to.

According to the nature of the escapement, and the part of the vibrating arc at which the impulses are applied, the vibrations of the balance may be made to employ a longer or shorter time than they would have employed if the balance had been separate from the works. Thus, in the common watch, these impulses quicken the vibrations; and consequently an increase in the maintaining force will make the watch go faster; as may be easily tried by gently forcing the key in the opposite direction to winding up.

If the balance and its spring were to continue unchanged in all temperatures, and under all circumstances, and if its long and short vibrations measured equal times when separate from the machine, it would only be required that the escape ment should be so constructed as neither to accelerate nor retard them. But none of these conditions can be had in the ordinary structure of watches, and in the superior time-pieces considerable difficulties are found in the attempt to obtain them.

By the natural contraction, to which all bodies are subject when cooled, the diameter of the balance will be less the lower the temperature: it will therefore be more easily carried by the vibrating forces, and will then vibrate more quickly.

The spring attached to the balance, which is called the pendulum spring, will likewise act with greater force when cold, and on this account also the vibrations will be quicker.

The remedies for these causes of imperfection are the following:

1. The Remontoire. As the irregularities in the transmission of force from the main spring are certainly increased by the number of wheels in the train, it was proposed, in the infancy of the art, to detach the last wheel, or that nearest the balance, or time measurer, from the rest, and to move this by a separate spring or weight: so that in this contrivance the time measurer is acted upon by one single wheel, and the rest of the train is employed in winding up the secondary first mover at short intervals, such, for example, as every half minute. We shall also have to mention some escapements, in which the winding up is performed in

every single vibration. With regard to remontoires, it may be remarked, that they either greatly shorten, or else destroy the periodical irregularities of the train, and those of the main-spring; but that with regard to the influence of oil, and other causes of more permanent difference, their advantage is not very considerable, because the remedy is not applied where the motion is quickest.

Whether the irregular action of the maintaining power be diminished by the remontoire or not, it is desirable that the impulse on the balance, through the escapement, should affect the natural measure of its vibrations as little as possible; or rather that it should tend to equalise them when the arcs of vibration vary. Some attention, but not much, has been paid to the equalizing quality of an escapement, principally by making the faces of the pallets of a figure suitable to that effect; but these are now for the most part abandoned, and the method of applying the force constitutes the distinguishing feature in this part of our modern chronometers. If a balance be set to vibrate by the mere action of its pendulum spring, its motion will soon decay; but if we suppose a lever or pallet to proceed from its axis, and a maintaining power to be applied to this, it is obvious, that if the power meet the pallet in its progress from the point of quiescence, it will shorten the time, and also the arc of that semi-vibration; and, agam, that if the power follow the pallet in its progress towards the point of quiescence, it will drive it home sooner, and consequently will shorten the time of that semi-vibration; and that actions contrary to these would lengthen the times. if,therefore, the action itself, which may be considered as an accelerating force, be not applied on both sides of the point of quiescence through a certain arc, determinable from the circumstances, the maintaining power, when it comes to be ap. plied, will alter the time; and if this vary, the time must also vary. Now the remedy at present adopted is, to make the balance vibrate through a very large arc, such as a semicircle or more, and to follow the pallet in each returning vibration by a strong power exerted through a very small arc, as, for example, 15° or 16°. By this contrivance the balance will vibrate at perfect liberty, out and home, through two semicircles, or 360°, excepting the small part during which the impulse is given; and if the impulse vary, the arc of vibration will vary, and with it

the time, unless the spring be made of a certain definite length, or tapered in its thickness according to the experience which many artists in this country possess.

The escapement generally used in our best chronometers, as we shall hereafter see, consists of a toothed wheel at the end of the train, which is prevented from running down by a detent or hook, and of two pallets, a longer and a shorter, fixed upon the verge or axis of the balance. These pallets are so placed, that when the face of the longer pallet has just arrived before one of the teeth of the wheel, the shorter pallet strikes out the hook, and allows the wheel to push forward the longer pallet with its tooth, during which action, the hook falls again into its place, to catch the succeeding tooth. The balance therefore proceeds in its vibration, and returns again without disturbing the train; because the short pallet does not strike out the hook in its backward course, but only acts on a slender spring, resembling those formerly used in the jacks of harpsichords. In this manner the vibrations are kept up; and so little do the variations in the maintaining power affect the rate, when all the adjustments are made, that if the main spring be let down to only a small part of its ordinary tension, these time-pieces will keep the same rate for many hours together.

How ever perfect, practically speaking, the application of the maintaining power may be, yet if the balance and its spring be subject to vicissitudes from heat and cold, it will be in vain to expect accuracy. There are two ways of correcting this compound time-measurer. The first, which was invented by Peter Leroy,consists in causing the balance to enlarge itself, instead of contracting by heat; by which means the spring, when in the state of greater rigidity, has more work to do; and the other acts by lengthening or shortening the spring, when cold or heat may have given it more or less of force. This was invented by Harrison, and depends on the well-known fact, that a short spring is stiffer than a longer; so that by shortening his spring at the time when it was weakened by heat, and the balance enlarged by the same cause, he gave it the stiffness requisite to compensate for these alterations; and the same contrivance produced the contrary effect in cold temperatures. As we shall more fully exhibit these inventions under the article HOROLOGY, it is only necessary to observe, that Peter Leroy constructed

his first time-piece with fluid thermometers on the balance, and that he also invented our present expansion balance of brass and steel, soldered or fused together in the rim, which was afterwards introduced and brought to great perfection by Arnold.

Machines, made upon the principles here cursorily pointed out, have measur ed time to a wonderful degree of perfection; and from the immense maritime trade of the British empire, and the scientific disposition of many wealthy individuals, the demand has been so great, as to have produced a very great number of able workmen, fully equal to their construction, at the same time that the prices have been considerably reduced. Most sea commanders of any respectability are provided with two or more of them.

The

Among the other causes of irregularity in time measurers, the resistance of the air has been occasionally considered by authors. But artists seem to suppose, ei ther that it is a constant quantity, or that its variations are not considerable enough to be brought into the account. very accurate performance of some chronometers, and the steady going of astronomical clocks, seem to give weight to this supposition: but on the other hand it may be remarked,that though the slow motion of heavy pendulums vibrating through small arcs in astronomical clocks must be subject to very little resistance indeed from the air, yet it does not follow that the rapid vibrations of a balance may not be affected by this cause; and the extreme precision of some chronometers will not, perhaps, be admitted as a very strong argument, when we consider that the changes from barometrical causes may have compensated each other, and that the most perfect machines will vary as much as one second per day, from causes which have not been yet clearly detected, though these are probably resolved into that before us. We are more particularly led to these reflections by a communication from Mr. Manton, of Davies-street, who found by experiment that a chronometer, which was going upon a gaining rate of five seconds per day, did increase its arc of vibration by an additional 50 degrees immediately upon the air being exhausted; and that being kept in vacuo, its rate became 37 seconds per day, the gain being 34 seconds upon the former rate. Hence it follows, that as the difference between the highest and the lowest stations of the barometer indicate a change of about one-fourteenth part in

the density of the air, the correspondent change per day, in the rate, may be two seconds and a half, or about one second per inch. Hence it may happen that a capital time-keeper shall indicate a more steady rate from week to week than from day to day.

The causes of imperfection in chronometers, which still call for farther exertions of sagacity in our artists, are, 1. The spring gradually tires or falls off from its strength, and neither the law of this vari ation nor its remedy are known. The ef fects of this change are, that all the adjustments are disturbed by it. 2. There is great reason to apprehend that the expansion-bars of brass and steel do change in their relative powers of flexure by their continued action on each other, though it is probable they settle at last. 3. The wear of the acting parts is uncertain, and will affect the time of striking out the detent and the arc of impulse. 4. No certain rules have been given, or are perhaps known, for making all the vibrations equal in time. If we suppose the long and short vibrations to be at first adjustable, with certainty, to equal times, not only for the extremes, but for all the means or intermediate arcs, it will not follow that the falling off from wear or from tiring, or from change in the balance, will continue to be accompanied by the same isochronism. 5. The best artists find very great difficulty in adjusting a pocket chronometer for all positions, preserving at the same time the other needful adjustments. See ESCAPEMENT, HOROLOGY, PENDULUM, TRAIN, and the articles thence referred.

CHRYSALIS, in natural history, a state of rest and seeming insensibility, which butterflies, moths, and several other kinds of insects, must pass through, before they arrive at their winged or most perfect state. The first state of these animals is in the caterpillar or reptile form; then they pass into the chrysalis-state, wherein they remain, immoveably fixed to one spot, and surrounded with a case or covering, which is generally of a conical figure; and, lastly, after spending the usual time in this middle state, they throw off the external case wherein they lay imprisoned, and appear in their most perfect and winged form of butterflies, or flies. See CATERPILLAR.

CHRYSANTHEMUM, in botany, a genus of the Syngenesia Polygamia Superflua class and order. Natural order of Composite Dioscoidea. Corymbiferæ, Jussieu. Calyx, hemispherical, imbricat ed; the marginal scales membranaceous;

pappus margined; receptacle naked. There are twenty-seven species. One of these, the Chrysanthemum leucanthemum, or ox-eyed daisy, has been introduced from Europe, and become naturalized in the United States; it has in fact become one of the most troublesome and pernicious weeds which infest our country. It covers hundreds of acres of ground with its white flowers, in the month of June.

CHRYSIS, golden fly, in natural history, a genus of insects of the order Hymenoptera. Mouth horny, projecting; lip much longer than the jaw, which is linear, membranaceous, and emarginate at the tip; no tongue; feelers four, unequal, filiform; antennæ short, filiform, of twelve articulations, the first longer; body gilt polished; abdomen arched beneath, with a scale on each side; tail generally toothed; sting pungent, nearly concealed; wings flat. These are generally found in the holes of old walls. There are more than thirty species.

CHRYSITRIX, in botany, a genus of the Polygamia Dioecia class and order. Natural order of Calamariæ. Cyperoideæ, Jussieu. Essential character: hermaphrodite; glume bivalve; corolla of numerous setaceous chaffs; stamina many, solitary, between the chaffs; pistil one :male as in the hermaphrodite; pistil one. There is but one species; viz. C. capensis, a perennial plant; native of the Cape of Good Hope.

CHRYSOBALANUS, in botany, a ge nus of the Icosandria Monogynia class and order. Natural order of Pomaceæ. Rosacea, Jussieu. Essential character: calyx five-cleft; petals five; style lateral; drupe with a five-furrowed, fivevalved nut. There is but one species: viz. C. icaco, cocoa plumb, a shrub about eight feet high. Native of the Caribbee islands, and the neighbouring continent

near the sea.

CHRYSOBERYLL, in mineralogy, a species of the flint genus. Its chief colour is asparagus green, passing on the one side into an apple-green, mountain green, and greenish white: on the other side it passes through light olive and oil green into yellowish grey, which inclines to brown. It occurs but seldom crystalized, and then the crystals are small, externally shining, internally splendent, and intermediate between the resinous and vitreous. It is brittle, not easily frangible; specific gravity from 3.6 to 3.8. Before the blow-pipe it is infusible without addition: it is found in Brazil, in the sands of Ceylon, along with rubies and

CHRYSOCOMA, in botany, English goldy locks, a genus of the Syngenesia Polygamia Equalis class and order. Natural order of Composite Discoideæ. Corymbiferæ, Jussieu. Essential character: calyx hemispherical, imbricate ; style scarcely longer than the florets; pappus simple; receptacle naked. There are thirteen species: almost all of them na tives of the Cape of Good Hope.

CHRYSOGONUM, in botany, a genus of the Syngenesia Polygamia Necessaria class and order. Natural order of Compositæ Oppositifolia. Corymbiferæ, Jussieu. Essential character: calyx fiveleaved; seeds involved in a four-leaved calycle; pappus one-leafed, three-toothed; receptacle chaffy. There is but one species; viz. C. virginianum, a native of Virginia.

CHRYSOLITE, in mineralogy, a species of the flint genus; the chief colour of which is pistachio green, of all degrees of intensity: it occurs sometimes in original, angular, pretty sharp-edged pieces, which are frequently notched, and exhibit a peculiar, rough, scaly, splintery surface; also in rolled pieces, and crystallized; brittle; easy frangible; specific gravity about 3.4; infusible before the blow-pipe without addition; constituent parts, according to Klaproth, Silica Magnesia Oxide of iron Loss

38.0

39.5

19.0

3.5

100.0

It is found principally in Upper Egypt; but has been met with in Bohemia, and in the isle of Bourbon. It is employed as a precious stone in different kinds of jewelry, but of no very great value. Werner thinks that the stone described by the ancients under the name of yellow chrysolite answers to our topaz.

CHRYSOMELA, in natural history, a genus of insects of the order of Coleoptera. Antennæ moniliform; six feelers, growing larger towards the end; thorax marginate; shells immarginate; body mostly oval. Of this genus there are several hundred species. They are separated into three distinct divisions. A. lip entire; hind legs equal. B. oblong; lip bifid; hind thighs equal. C oblong; lip bifid; hind thighs thickened. This numerous and very beautiful tribe is found every where in woods and gardens. Their motion is slow, and some of them when caught emit an oily liquor of a disagreeable smell. The larva of this genus, and also of the Cryptocephalus, feed on the leaves of trees and plants.

CHRYSOPHRAS, in mineralogy, a species of the Flint genus, of an apple-green, of all degrees of intensity, passing through the various shades of greenish grey. It is found massive in angular pieces, and thick plates. Internally it is dull; some rare varieties are glimmering. Specific gravity 3.25. Before the blow-pipe it loses its colour and transparency, and is infusible without some addition. By ana lysis it is found to contain

A trace of alumina and oxide of iron.It is found with quartz, opal, chalcedony, &c. in serpentine, in Lower Silesia. It is chiefly used for ring-stones; but is diffi cult to cut and polish. The apple-green variety is the most highly valued, and ring-stones of that colour will fetch 10, or 121. It passes into horn-stone and chalcedony, and into a fossil which is intermediate between chrysophras and opal. It loses much of its colour when kept in a warm and dry place, or when much exposed to the air. Very elegant specimens of this beautiful fossil are to be seen in the great cathedral at Prague, where a closet is inlaid with it.

CHRYSOPHYLLUM, in botany, a ge nus of the Pentandria Monogynia class and order. Natural order of Dumosæ. Sapota, Jussieu. Essential character: corolla bell-shaped, ten-cleft; segments alternate, spreading; berry ten-seeded. There are six species; natives of the West Indies.

CHRYSOSPLENIUM, in botany, a genus of the Decandria Digynia class and

order. Natural order of Succulenta. Essential character: calyx four or five-cleft, coloured; corolla none; capsule twobeaked, one-celled, many-seeded. There are two species; viz. C. alternifolium, alternate leaved golden saxifrage: and C. oppositifolium, opposite-leaved golden saxifrage. These plants are found in moist shady places, by the sides of rivulets, in Lapland, Sweden, Denmark, Germany, Switzerland, and with us, about Norwich, and Worcestershire. The latter species is native in the United States. CHURCH, has different significations, according to the different subjects to which it is applied. 1. It is understood of the collective body of christians, or all those over the face of the whole earth, who profess to believe in Christ, and acknowledge him to be the Saviour of mankind. This is what the ancient writers call the catholic or universal church. 2. Church is applied to any particular congregations of Christians, who at one time, and in one place, associate together, and concur in the participation of all the institutions of Jesus Christ, with their proper pastors and ministers. Thus we read of the church of Antioch, the church of Alexandria, the church of Thessalonica, and the like. 3. Church denotes a par ticular sect of Christians, distinguished by particular doctrines and ceremonies. In this sense we speak of the Romish church, the Greek church, the Reformed church, the church of England, &c.

The Latin or Western church comprehends all the churches of Italy, France, Spain, Africa, the North, and all other countries whither the Romans carried their language. Great Britain, part of the Netherlands, of Germany, and of the North, have been separated from hence ever since the time of Henry VIII. and constitute what we call the Reformed church, and what the Romanists call the Western Schism. The Greek or Eastern church comprehends the churches of all the countries anciently subject to the Greek or Eastern empire, and through which their language was carried; that is, all the space extended from Greece to Mesopotamia, and Persia, and thence into Egypt.

CHURCH, the place which Christians consecrate to the worship of God. By the common law and general custom of the British realm, it was lawful for earls, barons, and others of the laity, to build churches; but they could not erect a spiritual body politic, to continue in success.on, and capable of endowment, with out the king's license; and before the

law shall take knowledge of them as such, they must also have the bishop's leave and consent, to be consecrated or dedicated by him.

en

CHURCHWARDENS, the guardians or keepers of the church, are persons annually chosen in Easter week, by the joint consent of the minister and parishioners, or according to the custom of the respective places, to look after the church and church-yard, and things thereunto belonging. They are trusted with the care and management of the goods and personal property of the church, which they are to order for the best advantage of the parishioners; but they have no interest in, or power over, the freehold of the church itself, or of any land or other real property belonging to it; these are the property of the parson or vicar, who alone is interested in their loss or preservation. The churchwardens therefore may purchase goods and other articles for the use of the parish; they may likewise, with the assent of the parishoners, sell or otherwise dispose of the goods of the church; but without such consent they are not authorised to alienate any of the property under their care.

All peers of the realm, clergymen, counsellors, attorneys, clerks in court, physicians, surgeons, and apothecaries, are exempt from serving the office of churchwarden, as is every licensed dissenting teacher, pretending to holy orders.

CHURN, an implement for agitating cream, or milk, so as to separate the butyrous particles from the serous, and to effect the production of butter. Some churns are made upright, of a tapering form, and are worked by means of a pole and cross, the former passing through a hole in the lid. These are pail or bell churns. Many churns are in the form of a barrel; in some of these beaters, or projecting battens, are affixed within four or five of the staves, which strike the cream as the barrel is moved round by means of a winch: in others, the barrels are at rest, while a cross fly, of four or more leaves, is turned within it: in either case the barrel is supported on a frame. The Indian churn has an alternate motion, being worked by a vertical pole, which is turned much the same as a hand-lathe; having its lower part split, the pole occasions great agitation in the cream. A great variety of churns are in use but, in general, their formation evinces more ingenuity than practical knowledge. Those moved by pedals,