bars EG and I K. The two rods adjacent to the central rod, being of brass, are fastened to the cross pieces I K and LM; and the central rod, to which the ball of the pendulum is attached, is suspended from the cross piece L M, and passes freely through a perforation in each of the cross bars IK, BD. From this disposition of the rods, it is evident that, by the expansion of the extreme rods, the cross piece BD, and the two rods attached to it, will descend: but, since these rods are expanded by the same heat, the cross piece E G will consequently be raised, and theretore also the two next rods; but, because these rods are also expanded, the cross bar I K will descend; and, by the expansion of the two next rods, the piece LM will be raised a quantity sufficient to counteract the expansion of the central rod. Whence it is obvious that the effect of the steel rods is to increase the length of the pendulum in hot weather, and to diminish it in cold weather, and that the brass rods have a contrary effect upon the pendulum. The effect of the brass rods must, however, be equivalent, not only to that of the steel rods, but also to the part above the frame and spring, which connects it with the clock, and to that part between the lower part of the frame and the centre of the ball.

PENDULUM, MERCURIAL, was invented by the celebrated Mr. George Graham, and is considered as the compensating pendulum. In this the rod of the pendulum is a hollow tube, in which a sufficient quantity of mercury is put. Mr. Graham first used a glass tube, and the clock to which it was applied was placed in the most exposed part of the house. It was kept constantly going, without having the hands or pendulum altered, from the 9th of June 1722 to the 14th of October 1725, and its rate was determined by transits of fixed stars. Another clock made with extraordinary care, having a pendulum abont sixty pounds weight, and not vibrating above 1° 30 from the perpendicular, was placed beside the former, the more readily to compare them with each other, and that they might both be equally exposed. The result of all the observations was this, that the irregularity of the clock with the quicksilver pendulum exceeded not, when greatest, a sixth part of that of the other clock with the common pendulum, but for the greatest part of the year not above an eighth or ninth part; and even this quantity would have been lessened, had the column of mercury been a little shorter: for it differed a little the contrary way from the other clock, going faster with heat and slower with cold. To confirm this experiment more, about the beginning of July 1723 Mr. Graham took off the heavy pendulum from the other clock, and made another with mercury, but with this difference, that instead of a glass tube he used a brass one, and varnished the inside to secure it from being injured by the mercury. This pendulum he used afterwards, and found it about the same degree of exactness as the other.

M. Thiout's Pendulum.-Another excellent contrivance for the same purpose is described by M. Thiout, a French author on clock-making. Of this pendulum, somewhat improved by Mr. Crosthwaite, watch and clock maker, Dublin, we

have the following description in the Transactions of the Royal Irish Academy, 1788:- A and B, fig. 8, are two rods of steel forged out of the same bar, at the same time, of the saine temper, and in every respect similar. On the top of B is formed a gibbet C; this rod is firmly supported by a steel bracket D, fixed on a large piece of marble E, firmly set into the wall F, and having liberty to move freely upwards between cross staples of brass, 1, 2, 3, 4, which touch only in a point in front and rear (the sta ples having been carefully formed for that purpose); to the other rod is firmly fixed by its centre the lens G, of twenty-four pounds weight, although it should in strictness be a little below it. This pendulum is suspended by a short steel spring on the gibbet at C; all which is entirely independent of the clock. To the back of the clock-plate I are firmly screwed two cheeks nearly cycloidal at K, exactly in a line with the centre of the verge L. The maintaining power is applied by a cylindrical steel stud, in the usual way of regulators, at M. Now it is very evident that any expansion or contraction that takes place in either of these exactly similar rods is instantly counteracted by the other; whereas in all compensation pendulums composed of dif ferent materials, however just calculation may seem to be, that can never be the case, as not only different metals, but also different bars of the same metal that are not manufactured at the same time, and exactly in the same manner, are found by a good pyrometer to differ materially in their degrees of expansion and contraction, a very small change affecting one and not the other.' Theory has pointed out several other pendulums, known by the names of elliptic, horizontal, rotulary, &c., pendulums. We can only select two or three of the more modern inventions of this kind.

Elliott's compensating pendulum.-The adjustment of the rods for the temperature in the Gridiron pendulum of Harrison being found in convenient, and accompanied sometimes by a considerable change in the rate in the pendu lum of Elliott two levers are adopted instead o. one, and they are applied at the bob instead of at the superior end of the verge.

Fig. 1, plate II., PENDULUMS, represents this pendulum; ab is a bar of brass made quite fast at the upper end by pins, and held contiguous at several equal distances, by the screws, 1, 2, &c., to the rod of the pendulum, which is a bar of iron; and, so far as the brass bar reaches, is filed of the same size and shape, though it does not appear so in the figure, but, a little below the end of the brass, the iron is left broader, as at dd. for the conveniency of fixing the work to it, and is made of a sufficient length to pass quite through the ball of the pendulum to c. The holes 1, 2, &c., in the brass, through which the screws pass into the iron rod of the pendulum, are filed of a sufficient length to suffer the brass to contract and dilate freely by heat and cold under the heads of the screws: eece represent the ball of the pendulum; f, f, two strong pieces of steel, or levers, whose inner centres, or pivots, turn in two holes drilled in the broad part of the pendulum rod, and their outer ones in a strong

bridge, or cock, screwed upon the same part of the rod, but omitted in the figure, to show the mechanism: g, g, are two screws entering at the edges, and reaching into the cavity near the centre of the ball. The ends of the screws next the centre are turned into the form seen in the figure, which, pressing with the weight of the ball against the longer arms of the levers, cause the short ends to press against the brass bar at b. Let us now suppose that the rod of the pendulum, and the brass annexed to it, grow longer by heat, and that the brass lengthens more than the iron of the same length; then the brass, by the excess of its dilation, will press the short ends of the levers downwards at b, and at the same time necessarily lift up the ball, which rests on the long ends of the same lever, at ff, to any proportion necessary for due compensations. The calculated proportion of the short arms of the levers must be to the long ones as the excess in the expansion of brass is to that of the whole length of iron; and, if this calculation be found on trial not perfectly accurate, the side screws g, g, will produce the exact compensation at a few adjustments. At ik, the inferior end of the iron verge, a strong double spring is fixed to bear the major part of the ball's weight, by its pressure upwards against two points of the ball, equidistant from the vertical line. This is the description of the first pendulum made in this way; and the only alteration that the inventor made afterwards was placing the side screws in the body of the ball. Cumming proposes, in his Elements of Clock and Watch-work, that the brass bar should elongate between two iron bars, in order to keep it straight, and to prevent the jerks to which he conceives the foregoing construction liable: he proposes also to alter the structure of the short arms of the levers, by making them turn each on an axis of motion, and to change their places of pressure mutually, in order that their united action may be applied in the same point, at the centre of the brass bar, in direct opposition to the line of downward expansion. See his Elements, p. 106, and plate xi. Mr. Hardy of Clerkenwell has also improved considerably Elliott's pendulum.

Mr. Troughton's mercurial pendulum.-This is an improvement on Graham's mercurial penduJum. About the year 1790 Mr. Edward Troughton, with a view of doing away the most material prejudice against this useful instrument, contrived a modification of the mercurial pendulum, which must be considered as a distinct invention; the materials being differently arranged both as to quality, quantity, and situation. The prejudice alluded to was that the metallic rod of Graham, and the vessel of mercury, would not be affected by changes of temperature in a contemporaneous manner, but that the mercury being below the rod, and having on that account a quicker motion, would cool sooner than the metallic rod, and be liable to more frequent changes from its greater susceptibility. Mr. Troughton therefore substituted a strong glass tube with a bulb at the lower extremity, to be filled to a certain height with mercury, which should rise and fall precisely as in a thermome

which instrument, in fact, his tube is; and,

to make it useful as such, it carries a graduated scale. The tube in question is about the size of a barometer tube, and the bulb large enough to contain forty-five ounces of pure mercury by the aid of one-half the tube. Fig. 2 of plate II. exhibits Mr. Troughton's arrangement. A B is the mercurial rod filled with mercury to the middle point of the rod; CD the bulb guarded by the surrounding lenticular bob of brass and lead made in the usual way, except that it is hollow in the centre: the metallic parts of the pendulum weigh about nine pounds exclusively of the mercury; the rim that surrounds the bulb, and by which the weight of the lenticular bob is supported, projects into two notches cut in the interior edges of the metallic leus, at each side of the cavity, and the nice fitting of the bulb to its rim renders the glass capable of bearing the weight of metal supported by it. The compensation of this pendulum is effected entirely by the ascent and descent, alternately, of the mercury up and down the glass tube as the heat varies; and as the expansion of glass varies much less than that of any of the metals, the small column of mercury contained in the tube is found sufficient to answer its purpose completely after a trial of more than twenty years.

Ward's compensation pendulum.-This is an invention of Mr. Henry Ward, of Blandford in Dorsetshire, who, in March 1806, communicated it to the Society of Arts, accompanied by a model, which gained him the silver medal of the society. It consists of three bars, two of iron and one of zinc, arranged in a manner very similar to Deparcieux's improvement of Regnauld's old French pendulum, but without the adjustment lever. The instrument appears in fig. 3, plate II., where hh and ii are two flat rods of iron or steel, about half an inch wide and an eighth of an inch thick each; and k k is a rod of zinc, of nearly a quarter of an inch thick, interposed; the two extreme or iron rods are cranked, one at the top to receive the suspension spring, and the other at the bottom to hold the ball in the same vertical line with the zinc rod. These three parallel rods are united by four screws, I, I, I, I, which, passing through oblong holes in the bars h h and kk, screw into the rod ii, while the rod hh is fastened to the zinc one kk, by the single screw m near its lower extremity, which screw is the screw of adjustment for temperature, and requires several holes to be made in the bar hh, and also in k k, that the length of the zinc bar may be limited to its due proportion for exact compensation. The iron bar ii rests by a chinpiece on the upper end of the zine bar, and therefore the ball supported by its lower tapped end stretches all the bars equally. If the zinc bar had rested on the crank at the bottom of the bar hh, instead of being held by the screw m, the arrangement would have been the same as in Deparcieux's, but with zinc substituted for brass, which consequently does not require the aid of a lever to increase its effect. The proportional expansions of zinc and iron were taken for this pendulum from Smeaton's table in the fortyeighth volume of the Phil. Trans. of London, and were made for hammered zinc as 373: 151; but subsequent observations on the rate of the clock,

to which the pendulum was applied, proved that the expansion of the hammered zinc was greater than Smeaton's table gives it, though the quan. tity was within the reach of the adjustment.

Reid's compensation pendulum.-Another compensation pendulum by Mr. Adam Reid of Woolwich was also presented to the Society of Arts in April 1809, and the inventor was rewarded with fifteen guineas for his contrivance. This is given in fig. 4, plate II., and has its compensation of a tube of zinc acting on the ball from the lower end of the steel verge thus: A B is the steel verge, made a little thicker where it enters the ball C, and also of a lozenge shape, to prevent the ball's turning, but above and below it is cylindrical; near the centre of the ball is a shoulder in the verge, against which the upper end of the zinc tube D presses the cross piece of the ball when the nut E is turned up close to its lower extremity, but when the nut is turned back, in adjusting for rate, the tube descends a little, and the ball with it, while it rests on the upper extremity of the tube. When the compensation is too much the tube may be shortened till it is found of the exact length, by a trial of the rate in extremes of temperature. In the figure the length of the zinc tube does not appear sufficiently long to compensate an entire verge of steel. The inventor observes that platina might be substituted for steel, and steel for zinc, for the formation of this kind of pendulum, but that the expense would be enhanced by the dearness of platina.

Ritchie's and Nicholson's compensation pendulum.-In March 1812 Mr. David Ritchie, of Clerkenwell, laid before the society a model of a compensation pendulum, for which he received twenty guineas. The compensation was effected by the flexure of two compound horizontal bars interposed between the verge and the ball, which act on the same principle as the compound bars in the ordinary compensation balances of a chronometer. Fig. 5 represents the pendulum, in which A is the spring of suspension, B the ball, and C the steel verge as usual, with the nut of adjustment for rate at E, below the ball. The compensating bars are F and G, the upper part of F and the lower part of G being steel, and the other parts brass; so that any additional heat may bring the bars nearer together by the convexity of the brass faces of the compound bars being contiguous.. These compound bars have each a sliding piece above and below, and f respectively, by the sliding of which the bars may have their effective lengths altered, in the adjustment for temperature; and are connected by the slender springs e and f, which bear a portion of the ball's weight; the remainder being borne by the springs and m attached by screws at h, one to the verge piece and the other to the ball piece b, while they themselves are united by screws at and m. The compensation frame, so constructed, is adjusted by sliding the compound bars, before they are fixed, till the centre of the ball falls in the vertical line of the verge, where the screws at h fix it for trial. It has been feared that when the crutch of the clock urges the verge the ball will not move with it till the springs e and fare bent a little; for when the

ball is heavy, as is now customary in seconds pendulums, its weight will place a great stress on the slender springs that connect it with the verge, and produce, probably, a vacillation, as in Doughty's, that must be very unfavorable to isochronism, allowing even that the compensation is perfect. To avoid this evil Mr. Nicholson placed a similar compensation bar on the cock of suspension above the verge of his pendulum, and also above the cock of limitation, as seen in fig. 6, but without any spring, and in this situation the varying curvature of the horizontal compound bar, E E, altering the length of the suspension spring, produced the alternate rising and falling of the ball, as the verge carried it in an opposite direction, so that the centre of oscillation was comparatively stationary. Sec Phil. Journal, vol. i. 4to. ed., plate V. fig. 3. The same author also contrived a compensation pendulum composed of four rods of steel and one of a compound, or alloy, of zinc and silver, which is described in vol. ii. of his 4to. Phil. Journal, p. 205, plate IX.

PENELOPE, in fabulous history, the daughter of Icarus, who married Ulysses, by whom she had Telemachus. During the absence of Ulysses, who was gone to the siege of Troy, and who staid twenty years from his dominions, several princes, charmed with Penelope's beauty, told her that Ulysses was dead, offered to marry her, and pressed her to declare in their favor. She promised compliance on condition they would give her time to finish a piece of tapestry she was weaving; but at the same time she undid in the night what she had done in the day, and thus eluded their importunity until Ulysses's return.

PENELOPE, in ornithology, a genus of birds of the order of gallinæ, the characters of which are these: The beak is bare at the base; the head is covered with feathers; the neck is quite bare; the tail consists of twelve principal feathers; and the feet are for the most part bare. Linné, in his Systema Naturæ, enumerates six species.

1. P. crax cumanensis, called by Latham, &c., yacou. It is bigger than a common fowl. The bill is black; the head feathers are long, pointed, and form a crest, which can be erected at pleasure. The irides are of a pale rufous color; the space round the eye is naked, similar to that of a turkey. It has also a naked membrane or kind of wattle, of a dull black color. The blue skin comes forward on the bill, but is not liable to change color like that of the turkey. The plumage has not much variation; it is chiefly brown, with some white markings on the neck, breast, wing coverts, and belly; the tail is composed of twelve feathers, pretty long, and even at the end; the legs are red. This species inhabits Cayenne, but is a very rare bird, being met with only in the inner parts, or about the Amazons country, though in much greater plenty up the river Oyapoc, especially towards Camoupi; and indeed those which are seen at Cayenne are mostly tame ones; for it is a familiar bird, and will breed in that state, and mix with other poultry. It makes the nest on the ground, and hatches the young there, but is at other times mostly seen on trees. It frequently

erects the

crest, when pleased or taken notice of, and likewise spreads the tail upright like a fan, in the manner of the turkey. It has two kinds of cry; one like that of a young turkey, the other lower and more plaintive; the first of these is thought by the Indians to express the word couyovoit, the other yacou.

2. P. maralia, the marail, about the size of a fowl, and shaped somewhat like it. The bill and irides are blackish; the space round the eye is bare, and of a pale red; the chin, throat, and fore part of the neck are scarcely covered with feathers; but the throat itself is bare, and the membrane elongated to half an inch or more; both this and the skin round the eyes change color, and become deeper and thicker when the bird is irritated. The head feathers are longish, so as to appear like a crest when raised up, which the bird often does when agitated; at which time it also erects those of the whole body; and so disfigures itself as to be scarcely known. The general color of the plumage is a greenish black; .the fore part of the neck is tipped with white; the wings are short; the tail is long, consisting of twelve feathers, which are even at the end, and commonly pendent, but can be lifted up, and spread out like that of the turkey; the legs and toes are of a bright red; the claws are crooked, and somewhat sharp. In a collection, says Latham, from Cayenne was a bird, I believe, of this very species. It was twenty-eight inches long; the bill is like that of a fowl, brown, and rather hooked; round the eyes bare; the head is crest ed; the feathers of the fore part of the neck are tipped with white; the breast and belly are rufous brown; the rest of the plumage is greenish brown; the tail is eleven inches long, and rounded at the end; the quills just reach beyond the rump; the legs are brown, and the claws hooked. This species is common in the woods of Guiana, at a distance from the sea, though it is less known than could be imagined; and generally found in small flocks, except in breeding time, when it is only seen by pairs, and then frequently on the ground, or on low shrubs; at other times on high trees, where it roosts at night. The female makes her nest on some low bushy trees as near the trunk as possible, and lays three or four eggs. When the young are hatched, they descend with their mother, after ten or twelve days. The mother acts as other fowls, scratching on the ground like a hen, and brooding the young, which quit their nurse the moment they can shift for themselves. They have two broods in a year; one in December or January, the other in May or June. The best time of finding these birds is morning or evening, being then met with on such high trees whose fruit they feed on, and are discovered by some of it falling to the ground. The young birds are easily tamed, and seldom forsake the places where they have been brought up: they need not be housed, as they prefer the roosting on tall trees to any other place. Their cry is inharmonious, except when irritated or wounded, when it is harsh and loud. Their flesh is much esteemed. Buffon supposes this bird to be the female of the yacou, or at least a variety; but that this cannot be, the anatomical inspection will at once determine. The windpipe of this

bird has a singular construction, passing along the neck to the entrance of the breast, where it arises on the outside of the flesh, and after going a little way downwards, returns, and, then passes into the cavity of the lungs. It is kept in its place on the outside by a muscular ligament, which is perceivable quite to the breast bone. This is found to be the case in both male and female, and plainly proves that it differs from the yacou, whose wind-pipe has no such circumvolution in either sex. If this be the bird mentioned by Fermin, in his History of Guiana, p. 176, he says that the crest is cuneiform, and of a black and white color; and observes that they are scarce at Surinam; but it does not seem quite certain whether he means this species or the yacou. Bancroft mentions a bird of Guiana by the name of marrodée, which he says is wholly of a brownish-black: the bill the same; and the legs gray. These he says are common, and make a noise not unlike the name given it, perching on trees. The Indians imitate their cry so exactly as to lead to the discovery of the place the birds are in, by their answering it. The flesh of them is like that of a fowl: it is therefore most likely the marail.

3. P. meleagris cristata, called by Ray penelope jacupeme, and by Edwards the guan, or quan, is about the size of a fowl, being about two feet six inches long. The bill is two inches long, and of a black color; the irides are of a dirty orange color; the sides of the head are covered with a naked purplish blue skin, in which the eyes are placed beneath the throat, for an inch and a half, the skin is loose, of a fine red color, and covered only with a few hairs. The top of the head is furnished with long feathers, which the bird can erect as a crest at pleasure; the general color of the plumage is brownish black, glossed over with copper in some lights; but the wing coverts have a greenish and violet gloss. The quills mostly incline to a purple color; the fore part of the neck, breast, and belly, are marked with white spots; the thighs, under tail coverts, and the tail itself, are brownish black; the legs are red; the claws black. Some of these birds have little or no crest, and are thence supposed to be females. They inhabit Brasil and 'Guiana, where they are often made tame. They frequently make a noise not unlike the word jacu. Their flesh is much esteemed.

4. P. meleagris satyra, the horned pheasant. Latham calls it the horned turkey. This species is larger than a fowl, and smaller than a turkey. The color of the bill is brown; the nostrils, forehead, and space round the eyes are covered with slender black hairy feathers; the top of the head is red. Behind each eye there is a fleshy callous blue substance like a horn, which tends backward. On the fore part of the neck and throat there is a loose flap of a fine blue color, marked with orange spots, the lower part of which is beset with a few hairs; down the middle it is somewhat looser than on the sides, being wrinkled. The breast and upper part of the back are of a full red color. The neck and breast are inclined to yellow. The other parts of the plumage and tail are of a rufous brown, marked all over with white spots, encompassed with black. The legs