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HontzoNTAL parallac. See PARALLAx.

HoR1zoNtAL plane, that which is parallel to the horizon of the place, or nothin inclined thereto. The business of levelling is to find whether two points be in the horizontal plane, or how much the deviation is.

Hontzost AL plane, in perspective, is a plane parallel to the horizon, passing through the eye, and cutting the perspective plane at right angles.

Homizostal range, of a piece of ordnance, is the distance at which a ball falls on, or strikes a horizontal plane, whatever be the angle of elevation or direction of the piece. When the piece is pointed parallel to the horizon, the range is then called the point-blank, or point-blank range. The greatest horizontal range, in the parabolic theory, or in a vacuum, is that made with the piece elevated to 45 degrees, and is equal to double the height from which a heavy body must freely fall, to acquire the velocity with which the shot is discharged. Thus, a shot being discharged with the velocity of v feet per second ; because gravity

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farthest in the resisting medium, is always below 45°, and gradually the more below it as the velocity is greater, so that the greater velocities with which balls are discharged from cannon with gunpowder require an elevation of the gun equal to but about 30°, or even less. And the less the size of the balls is too, the less must this angle of elevation be, to shoot the farthest with a given velocity. See GuxNERY and PRoj Ectiles. HORN, in physiology, a tough, flexible, semitransparent substance, intended for the defence or covering of animals. The hollow horns of the ox, goat, &c.; the hoof, the horny claw and nail, and the scale of certain insects, as the shell of the tortoise, resemble each other in chemical characters; but they differ very widely from stag’s horn, ivory, &c. Horn is distinguished from bone, in being softened very completely by heat, either naked, or through the medium of water, so as to be readily bent to any shape, and to adhere to other pieces of horn in the same state. Horn contains but a small portion of gelatine, and in this it differs from bone, which contains a great deal. Horn consists chiefly of condensed albumen, combined with a small and varying portion of gelatine, with a small |. of phosphate of lime. The fixed alalies readily and totally dissolve horn into a yellow saponaceous liquor. Horn and tortoise-shell are applied to mechanical purposes, which require them to be bent and united in various ways; this is performed by the aid of heat, ap[. either dry, with warmed irons or urning charcoal; or by softening the horn in boiling water, or in a weak solution of alkali: when thus softened, they will easily adhere. Mr. Aiken gives the following process for making the horn-ring that surrounds a common opera-glass: “A flat piece of horn is cut out, of the requisite shape, the ends to be joined are thinned down by a file, the piece is then put into boiling water till sufficiently supple, and is then rolled round a warm iron cylinder, and held in that position by a vice, so that the ends over-lap each other: another piece of iron, heated and grooved, is then laid upon the seam of the joined ends, and pressed upon the cylinder, and there confined by an iron wire; and the heat of the two partially melts that portion of the horn, and cements the ends so completely, that no seam or joining can be observed when cold.” For the manner of making horn to imitate tortoise-shell, see Coxsh. Hon N is also a musical instrument of the wind kind, chiefly used in hunting, to animate the hunters and the dogs, and to call the latter together. The French horn is bent into a circle, and goes two or three times round, growing gradually bigger and wider towards the end, which in some horns is nine or ten inches over. HoRNs of insects, the slender oblong bodies projected from the heads of those animals, and otherwise called antennae, or feelers. The horns of insects are extremely various; some being forked, others plumose or feathered, cylindrical, tapering, articulated, &c. As to the use of these parts, some have imagined that they served as feelers, lest the creature should run against any thing that might hurt it; and others there are, who think them the organs of hearing. See ENToarology. HoRN ore, in mineralogy, is one of the species of silver ore; its most frequent colour is pearl-gray, of all degrees of intensity, which borders sometimes on milkwhite, and sometimes approaches to lavender and violet-blue. It passes also, though but rarely, into green. It is found massive, disseminated in thick membranes, in roundish hollow balls; also crystallized: specific gravity 4.8. When heated on charcoal before the blow-pipe, it melts quickly, and leaves a globule of silver; it is even fusible by the flame of a candle; it takes a polish by friction; and its constituent parts, according to Klaproth, are

Silver - - - 67.75 Muriatic acid - - 21. Sulphuric acid - - 6.25 Oxide of iron - - 6.0 Alumina - - - 1.75 Lime - - - 0.25 97.00 Loss - - - - 3.00 100,00

It occurs in veins, and generally in their upper parts, and is usually accompanied with brown iron ocre, and with silver glance, but seldom with native silver and red silver ore. It occurs in considerable abundance in the mines of South America, in some parts of France,

and in Hungary. It derives its name from its property of cutting like horn; and is, of course, soft, flexible, and ductile, when obtained in thin plates. HoRN stone, or Honx steen, in mineralogy, a species of the flint genus, divided by Werner into three sub-species: the splintery, the conchoidal, and the woodstone. The most common colour of the splintery horn stone is gray; it is found in veins, in the shape of balls, in limestone, and forming the basis of porphyry, in several parts of Germany, and also in the Shetland islands. It appears to differ from quartz in containing a greater proportion of alumina; when it contains a very large quantity, it passes into jasper. It sometimes borders on chalcedony and flint. The best millstone, called French burr, is cellular-splinter hornstone. Conchoidal hornstone occurs in beds, accompanied with agate, and is distinguished from the splintery by the lightness of its colours, its fracture, and its inferior translucency and hardness. In the woodstone several colours occur together, and it commonly exhibits coloured delineations, as clouded and striped, and these arrange themselves in the direction of the original woody texture. Its shape is exactly conformable to its former woody shape, so that it occurs in the form of trunk, branches, and roots. It is found in sandy loam, in Germany, and in Ireland. It receives a good polish, and serves the purpose of agate. Mr. Jameson observes, on this mineral, that, “at first sight it may appear inconsistent, to consider a petrifaction as a particular fossil species; when we reflect, however, that woodstone differs in its external characters from all other fossils, the justness of the Wernerian method will become evident. Many other fossils occur in the shape of petrifactions, but they are almost always identical with some known species, and therefore are to be considered only as varieties of the exter. nal shape of the particular fossil to which they belong. HoRN work, in fortification, an out-work composed of two demi-bastions, joined by a Curtin. Honx geld, a tax paid for feeding of horned beasts in the forest. See FoREST. HORNBLENDE, in mineralogy, a species of the clay genus, of which there are four sub-species; viz. the common

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The common hornblende forms one of the essential ingredients of several mountain rocks; is sometimes accidentally disseminated in others, and occurs in beds. When in beds, it is frequently accompanied with ores, as magnetic iron-stone, iron pyrites, &c. It is found in all the northern parts of Europe; and when pure is a capital flux for iron ores, to which purpose it is applied in Sweden, where it is obtained in large quantities. The Labrador hornblende is found in the island of St Paul, on the coast of Labrador, is usually of a brownish black, and its specific gravity is 3.38. The hornblende slate is of a colour intermediate between greenish and raven black; it is massive, and is generally mixed with mica and felspar. It occurs in beds of primitive rocks, particularly in clay slate; also in gneiss and mica slate, and is found principally in the northern 'parts of Europe.—The basaltic hornblende is of a welvet black, occurs almost always in single imbedded crystals, which are small and middle sized. The surface is smooth and shining, except where it happens to be covered by a thin ochery crust. The specific gravity is rather less than that of the hornblende slate. It melts before the blow-pipe into a brownish black glass. Bergman has analyzed it, and found it contained.

Silica 58.0 Alumina . 27.0 Iron 9.0 Linne 4.0 Magnesia . 1.0 [99.0.

Loss . . . 1.0

100.0

*7-Y. "I,

It is found in Saxony, Bohemia, Scotland, Italy, &c. It resists decomposition longer than basalt, hence crystals of it are found in clay, formed by the decomposition of basaltic rock. Common hornblende is difficultly frangible, but the basaltic is very easily frangible. HORNET. See VESPA HOROLOGY. Horology is that branch of science which enables us to measure the portions of time. We judge of the lapse of time by the succession of sensible events; and the most convenient and accurate measures of its quantity are derived from motions, either uniform, or else repeated at equal ... Of the former kind, the rotation of the earth on its axis is the most exact, and the situation of its surface with regard to the fixed stars, or, less simple, with regard to the sun, constitutes the means for determining the parts of time as they follow each other. See AstroNoMy and DIALLING.-Of the latter kind, the rotation of machinery, consisting of wheel-work, moved by a weight or spring, and regulated by a pendulum or balance, affords instruments of which the utility is well known. The term horology is at present more particularly confined to the principles upon which the art of making clocks and watches is established. A considerable portion of this extended subject of research has been given under the articles Clock and ChronoMETER. In the present, we shall chiefly attend to the means by which the train of wheel-work is made to make a number of successive advances, all so very nearly equal in the measurement of time, that a surprising degree of precision is obtained in ascertaining the intended object. The machines which, for centuries, have been commonly used to measure time, consist of a movement, or train of wheels, drawn by a weight or spring, and a regulator, the object of which is to keep the motion of the train within the required degree of uniformity. A continual rotatory motion, which constantly tends to accelerate, is thus corrected by means of an alternate motion; while the power which carries round the movement restores also, to the regulator, the action lost by friction and other causes. The mechanism, by which the two principal parts act on one another, is called the escapement; and this most admirable contrivance may be reckoned the distinguishing characteristic of the modern art of time-piece making. One of the most ancient escapements is that which is now applied in almost all Y o

common pocket watches. It is represented in fig. 1. Plate Honology, and is best suitcd to the long vibrations of the balance, which was invented earlier than the pendulum. A B denotes the rim of a contrate wheel, called a crown wheel, having its teeth pointed and sloped on one side only, so that the points advance before any other part of the teeth during the motion. C and i) are two pallets or flaps proceeding downwards from the verge E. F. The pallets are nearly at right angles to each other; and when the balance F G, fixed to the verge, is at rest, the pallets remain inclined to the plane of the wheel, in an angle of about forty-five degrees; but when it is made to vibrate, one of the pallets is brought nearer to the perpendicular position, while the other becomes more nearly parallel. The wheel must be supposed to have one of its teeth resting against a pallet, by virtue of the maintaining power. This tooth will slip off or escape, as the pallet rises towards the horizontal position, at which instanta tooth on the opposite side of the wheel will strike against the other pallet which is down.— The returning vibration, by raising this last pallet, will suffer that tooth to escape, and another tooth will apply itself to the first-mentioned pallet. By this alternation, the crown-wheel will advance the quantity of half a tooth each vibration, and the balance or pendulum will be prevented from coming to rest, because the impulse of the teeth against the pallets will be equal to the resistances from friction and |. re-action of the air. The common escapement here described was well known to Leonardo de Vinci, who describes an instrument acting by an escapement of this kind, similar, as he says, to the verge of the balance in watches, which he does not seem to mention as a new thing : he died about 1313. "The isochronism of the pendulum was known to Galileo, in 1600, who, before his death, namely, about 1633, proposed to apply it to clocks. The actual application by Huygens was made before 1658, when he published his “Horologium Osciliatorium.” He applied it by means of the common escapement already in use with the balance, and still retained in our table-clocks. Sanctorius had made the same application about forty years before that time, as appears by his “Commentarii in Avicennam,” (quest. 56,) printed in 1625, in which several instruments are ‘lescribed as having been publicly exhibited and explained to his auditors, at his lectures in Padua, for thirteen years previous to that time.

This escapement not being adapted to such vibrations as are performed through arcs of a few degrees only, another construction has been made, which has been in constant use in clocks for this century past, with a long pendulum beating seconds. (Fig. 2.), A B represents a vertical wheel, called the swing wheel, having thirty teeth. C D represents a pair of pallets connected together, and moveable in conjunction with the pendulum, on the centre of axis F. One tooth of the wheel,in the present position,rests on the inclined surface of the inner part of the pallet C, upon which its disposition to slide tends to throw the point of the pallet further from the centre of the wheel, and consequently assists the vibration in that direction. While the pallet C moves outwards, and the wheel advances, the point of the pallet D, of course, approaches towards the centre, in the opening between the two nearest teeth; and when the acting tooth of the wheel slips off, or escapes from the pallet C, another tooth on the opposite side immediately falls on the exterior inclined face of D, and, by a similar operation, tends to push that pallet from the centre. The returning vibration is thus assisted by the wheel, while the Sallet C moves towards the centre, and receives the succeeding tooth of the wheel after the escape from the point of D. In this manner the alternation may be conceived to go on, without limit.

The celebrated George Graham improved this escapement very much, by taking off part of the slope furthest from the points of the pallets; instead of which É. he formed a circular or cylindrical

ace, having its axis in the centre of mo

tion. Pallets of this kind are seen on the opposite side of the wheel at E and G, having H for their centre or axis. A tooth of the wheel is seen resting upon the circular inner surface of the pallet G, which is not therefore affected by the wheel, excepting so far as its motion, arising from any other cause, may be af. fected by the friction of the tooth. If the vibration of the pendulum be supposed to carry G outwards, the slope surface will be brought to the point of the tooth, which will slide along it, and urge the pallet outwards during this sliding action. When the tooth has fallen from the point of this pallet, an opposite tooth will be received on the circular surface of E. and will not affect the vibration, exce pting when the slope surface of E is carried out so as to suffer the tooth to slide **ong it. In the two former escapements, theris always a certain portion of vibration takes place after the drop which drives the pallets back, and causes the index also to recede through a small arc: this has been distinguished by the name of a recoil. Other considerable objections, besides that of the continued action of the maintaining power, have been made against escapements with a recoil; but it would lead us too far into the minute departments of this subject to discuss them. The escapement of Graham, and all such as have no recoil, have been called dead beat escapements, because the index for seconds falls directly through its arc, and remains motionless on the line of division till the next vibration. It may be observed, that the maintaining power in Graham's escapement may be applied during a small portion only of the vibration; and that an increase of the maintaining power tends to enlarge the arc of vibration, but scarcely interferes with its veloCity. The effect of the escapement which has been termed horizontal, because the last Wheel in watches of this construction has its plane parallel to the rest of the system, is similar to that of the dead beat escapement of Graham. In fig. 3, the horizontal wheel is seen with twelve teeth, upon each of which is fixed a small wedge supported above the plane of the wheel, as may be seen at the letters A and B. On the verge of the balance there is fixed part of a hollow cylinder of steel, or other hardmaterial, the imaginary axis of which Passes through the pivots of the verge. C, represents this cylindrical piece, into which the wedge D. may be supposed to have fallen. While the vibration causes the cylindrical piece to revolve in the direction which carries its anterior edge towards the axis of the wheel, the point of the wedge will merely rub the internal surface, and no otherwise affect the vibration of the balance than by retarding its motion. But when the return of the vibration clears the cylinder of the point of the wedge D, the wheel will advance, and the slope surface of the wedge, act. ing against the edge of the cylinder, will assist the vibration of the balance. When the edge of the cylinder arrives at the outer point of the wedge D, its posterior edge must arrive at the position denoted by the dotted lines of continuation; immediately after which the wedge or tooth E will arrive at the position e, and rest on the outer surface of the cylinder, where it will produce no other effect than that of retardation from friction, as was remarked with regard to the wedge D, until the course of the vibration shall bring

the posterior edge of the cylinder clear of the point of the wedge. In this last situation the wedge will act on the edge of the cylinder, and assist the vibration, as in the former case, until that edge shall arrive at the outer or posterior point of the wedge; immediately after which the leading point will fall on the inner surface of the cylinder in the first position, as was shewn in the wedge D. Time-pieces, with a pendulum regulator, are certainly the most perfect, when they are kept in a fixed situation; and, for that reason, these are the only sort used in astronomical observatories. But external motion is so contrary to the regularity of their performance, that no sea chronometer has been since attempted to be constructed upon that principle. The balance regulator remained, as affording the only method by which the desired uniformity might be obtained in portable machines; and the great improvement made in that regulator, by the addition of a spiral spring, may be considered as one principal cause of the perfection which has been since attained in them. The first invention of attaching a spring, to give to the balance, by its elasticity, a power which renders the action of this sort of regulator similar to that of gravity in the pendúlum, is undoubtedly due to Dr. Hooke, though it is not so clear whether he ever applied it in the shape of a spiral, as has been so long practised since. F. Berthoud, in his “Histoire de la Mesure du Temps,” (vol. i. pp. 134 to 141,) gives a body of extracts from several works relative to this subject; and concludes, that Dr. Hooke only applied a straight spring to the balance, and that M. Huygens improved upon that idea, and contrived the spiral spring, which is more favourable to the vibrations of the balance. M. Huygens, indeed, applied in France a balance spring, the account of which has been published in the Philosophical Transactions for 1675, No. 112 ; but Dr. Hooke, in the Postscript to his Description of Helioscopes, asserts, that the hint was taken from the experiments he had made in 1664, in Gresham College, where he explained above twenty several ways, by which springs might be applied to do the same thing. In relating the progress of time-piece making, we must not omitmentioning the use of precious stones, particularly rubies, to form the holes in which the pivots of the wheels turn, and the ol. upon which the action of the teeth is exercised. These jewels, by the high polish given to them, reduce the quantity of

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