« PreviousContinue »
*inte-in a short flight on the Sl jo wa–ter: in this respect relies †. exo-coeti, and several other o,” two other species are also P =an Seas. Hill, of M, in botany, crane's dria §.". of the Monadelphia HeptanStuinales * order. Natural order of thatacter. erania, Jussieu. Essential *gment Şılyx five-parted, the upper Tous .."ling in a capillary, nectarifeCorolla . running along the peduncle; ten ... etalled, irregular; filaments : o three of ". j *:::: o ve-grained, beaked; bea 3. bearded within. There are eighty.*Poies: almost all of them are naa, particularly those which
i.o. the pelican, in natural
seres.” A genus of birds of the order An00ked eneric character: bill straight, j at the point: nostrils in an almost eath "ated furrow ; face almost naked of o gullet naked, and capable of to distention; four toes, all webbed ...e. There are thirty species, of ich we shall notice the following.
SO onocrotalus, or the great pelican, is
*times of the weight of twenty-five Ponds, and of the width, between the ..one points of the wings, of fifteen *; the skin, between the sides of the "PPot mandible, is extremely dilatable, *hing more than half a foot down the **k, and capable of containing many *rts of water. This skin is often used \ \ailors for tobacco-pouches, and has . occasionally converted into elegant adies’ work-bags. About the Caspian *d Black Seas, these birds are very nuherous, and they are chiefly to be found "the warmer regions, inhabiting almost Wery country of Africa. They build in the small isles of lakes, far from the haitations of man. The nest is a foot and *halfin diameter, and the female if molested will remove her eggs into the watertill the cause of annoyance is removed, returning them then to her nest of reeds and grass. These birds, though living principally upon fishes, often build in the midst of desarts, where that element is rarely to be found. They are extremely dexterous in diving for their prey, and after having filled their pouch, will retire to some rock, and swallow what they have taken, at their leisure. They are said to unite with other birds in the pursuit of fishes. The pelicans dive, and
drive the fish into the shallows. The cormorants assist by flapping their wings on the surface, and forming a crescent, perpetually contracting, they at length accomplish their object, and compel vast numbers into creeks and shallows, where they gratify their voracity with perfect ease, and to the most astonishing excess. P. occidentalis, or the American pelican, is about the size of a goose : of this bird it is reported, that it will bring supplies of food to any disabled and diseased companion: and that the natives of the island of Assumption, by confining one near the shore, frequently induce others to make these generous presents, which are fraudulently converted to the purpose of food for the islanders. The red-backed pelican. One of these was in the possession of Mr. Latham, and was found, on an experiment purposely made, to store away ten fishes, weighing a pound each, in its pouch, arranging them with the head towards the throat. It then marched away to swallow them at its leisure; the pouch being extended nearly down to its feet. P. aquilus, or the man of war bird, is small in body, but between the extremities of the wings fourteen feet in width. It is seldom seen but within the tropics, and not unfrequently is observed two hundred leagues from land. It watches the movements of fishes from a very considerable height, and pounces upon them with unfailing success, returning from its immersion with equal rapidity. It also often obliges other birds to quit the prey which they have just made, and are flying off with, and seizes it as it drops from them with a dexterity truly admirable. During the movements of flying fishes over the surface of the sea, which are previously indicated to this bird by the bubbling of the water, it is one of their most vigilant and fatal enemies. P. carbo, or cormorant, is nearly as large as a goose, is found in many places both of the old and;the new world, and is to be met with very common on the sea coast and harbours of the United States, known in many parts by the name of “Negro-geese.” They swim with the body under water, the neck and head only erect and above the surface; at the flash of a gun or the approach of any danger, they dive instantaneously, remain a considerable time under water, and rise at a distance. They have a very offensive smell, and the flesh is not palatable. These birds are shy and crafty, but frequently eat to so great an excess as to induce a species of lethargy, in which, in England, they are caught by nets thrown over them, without their making an effort to escape. They are trained by the Chinese to fish for them. By a ring placed round their necks, they are prevented from swallowing what they take, and when their pouches are filled, they unload them, and, at the command of their owners, renew their divings: two will be seen combining their efforts to secure a fish, too large for the management of one only. When their work is finished to the employer's satisfaction, the birds have a full allotment of the spoil, for their reward and encouragement. In Macao, also, these birds are thus domesticated, taking extreme delight in the exercise, and constituting a source of very considerable profit to their owners. They were formerly trained, and used in the same manner in England; and Charles I. had an officer of his household, called master of the cormorants. See Aves, Plate XI. fig. 3. P. bassanus, or the island goose, cr gannet, weighs about seven pounds, and inhabits, in great numbers, the northern isles of Great Britain. It is migratory, and drawn to that country by the shoals
of herrings and pilchards, whose move-.
ments it watches with the most anxious vigilance. The young birds are sold in geeat plenty in Edinburgh, where they are frequently introduced before dinner as a stimulus to appetite. In St. Kilda, it is supposed that upwards of twenty thousand of these birds are taken annually. They constitute an important article of food to the inhabitants, who, to procure both the eggs and the young ones, expose themselves to the most imminent dangers on elevated and precipitous cliffs, and, in several instances, have fallen victims to the hardihood with which they have pursued their researches. See Aves, Plate XI. fig. 4. PELECOIDES, in geometry, a figure in form of an hatchet: such is the figure B C D A, Plate XII. Miscel. fig. 7, contained under the two inverted quadrantal arcs, A B and AD, and the semicircle B C D. The area of the pelecoides is demonstrated to be equal to the square, A C, and that again to the parallelogram, E. B. It is equal to the square, A C, because it wants of the square on the left hand the two segments A B, and A C, which are equal to the two segments B C, and C D. by which it exceeds on the right hand.
PELICAN. See PELECANUs. PELLETS, in heraldry, are those roundles that are black, called also ogresses and gun-stones, and by the French torteaux de sable. PELTA, in botany, a term by which the flower or flat fructification of the genus lichen or lever wort is characterized, which, in most of its species, is glued to the edges of the leaves. PELTARIA, in botany a genus of the Tetrady Hamia Siliculosa class and order. Natural order of Siliquosae. Cruciformes or Cruciferae. Essential character: silicle entire, suborbiculate, compressed, flat, not opening. There are two species, viz. P. aliacea, garlic scented peltaria, and P. capensis, cape peltaria. PELV1S, in anatomy, the lower part of the cavity of the abdomen, thus called from its resemblance to a basin, or ewer, in Latin called pelvis. It is formed by the ossa ilae and ischia, the os sacrum, the os coccygis, and the ossa pubis. See ANATOMY. PEN, fountain, is a pen made of silver, brass, &c. contrived to contain a considerable quantity of ink, and let it flow out by gentle degrees, so as to supply the writer a long time without being under the necessity of taking fresh ink. PENAEA, in botany, so named from Peter Pena, a genus of the Tetrandria Monogynia class and order. Essential character: calyx two-leaved, corolla bellshaped; style quadrangular; capsule four-cornered, four-celled, eight-seeded. There are nine species; these are shrubs which are rugged below, with the vestiges of fallen leaves, leafy above ; leaves opposite, crosswise, sessile, approximating, imbricately in a fourfold row, the upper ones near the flowers, like scales, and coloured; flowers terminating, sessile, solitary, or several together: fruit as in the order of Acanthi, but four-celled; this genus may perhaps be allied to them, but having been hitherto little examined, except in dried specimens, the natural order of the genus Penaea must yet remain uncertain. Jussieu.
PENAL Laws and Statutes, having been made on many occasions to punish and deter offenders, they ought to be construed strictly, and not be extended by equity, but the words of them may be interpreted beneficially, according to the intent of the legislator.
PENALTY, is a forfeiture inflicted for not complying with the regulations of certain acts of parliament; a penalty is
also annexed to secure the performance of certain covenants in a deed, articles of agreement, copartnership, &c. In a bond also for payment of money, it is usual to annex a penalty in double the amount of the obligation. See Bonn. PENCIL, an instrument used by painters for laying on their colours. Pencils are of various kinds, and made of various materials; the larger sorts are made of boar’s bristles, the thick ends of which are bound to a stick, bigger or less, according to the uses they are designed for; these, when large, are called brushes. The finer-sorts of pencils are made of camel’s, badger’s and squirrel’s-hair, and of the down of swans; these are tied at the upper end with a piece of strong thread, and inclosed in the barrel of a Quill. All good pencils on being drawn between the lips come to a fine point. PEN.cIL is also an instrument used in drawing, writing, &c. made of long pieces of black lead, or red chalk, placed in a groove cut in a slip of cedar, on which other pieces of cedar being glued, the whole is planed round, and one of the ends being cut to a point, it is fit for use. PEN DANT, an ornament hanging at the ear, frequently consisting of diamonds, Pearls, and other precious stones. PEN DANTs, in heraldry, o: hanging down from the label, to the number of three, four, five, or six at most, resembling the drops in the Doric frieze. When they are more than three they must be specified in blazoning. PENDANts, of a ship, are those streamers or long colours which are split and divided in two parts ending in points, and hung at the head of masts, or at the yard-arm ends. PENDULOUS, a term applied to any thing that bends or hangs downwards; thus, the flowers, whose slender stalks are not able to sustain their heads upright, are called pendulous flowers. See BotANY and Flow ER. PENDULUM, in mechanics, denotes any heavy body, so suspended as that it may vibrate or swing backwards and forwards, about some fixed point, by the force of gravity. The vibrations of a pendulum are called its oscillations. See OscILLATION. A pendulum, therefore, is any body, B, (Plate XII. Miscell. fig. 8.) suspended upon, and moving about, a fixed point, A, as a centre. The nature of a pendulum consists in the following particulars: 1. The times of the vibrations of a pendulum, in very small
arches, are all equal. 2. The velocity of the bob, in the lowest point, will be nearly as the length of the chord of the arch which it describes in the descent. 3. The times of vibration in different pendulums, A B, A C, are as the square roots of the times of their vibrations. 4. The time of one vibration is to the time of the descent, through half the length of the pendulum, as the circumference of a circle to its diameter. 5. Whence the length of a pendulum, vibrating seconds, will be found 39.2 inches nearly; and that of an half second pendulum 9.8 inches. 6. An uniform homogeneous body BC (fig. 9.) has a rod, staff, &c. which is one-third part longer than a pendulum A D, and will vibrate in the same time with it. From these properties of the pendulum we may discern its use as an universal chronometer, or regulator of time, as it is used in clocks, and such like machines. See CHRonoMETER, HoRology, &c. By this instrument also we can measure the distance of a ship, by measuring the interval of time between the fire and the sound of the gun; also the distance of a cloud, by numbering the seconds, or half seconds, between the lightning and thunder. Thus, suppose, between the lightning and thunder, we number 10 seconds ; then, because sound passes through 1142 feet in one second, we have the distance of the cloud equal to 11,420 feet. Again, the height of any room, or other object, may be measured by a pendulum vibrating from the top thereof.
Thus, suppose a pendulum from the
height of a room vibrates once in three seconds; then say, as 1 is to the square of 3, viz. 9, so is 39.2 to 352.8 feet, the height required. Lastly, by the pendulum, we discover the different force of gravity on different parts of the earth’s surface, and thence the true figure of the earth. When pendulums were first applied to clocks, they were made very short; and, the arches of the circle being large, the time of vibration through different arches could not in that case be equal; to effect which, the pendulum was contrived to vibrate in the arch of a cycloid, by making it play between two semi-cycloids C B, C D (fig. 10.) whereby it describes the cycloid B E A D ; the property of which curve is, that a body vibrating in it will describe all its arches, great or small, in equal times. These are, however, which concur in rendering the ap
plication of this curve to the vibration of pendulums designed for the measures of time, the source of errors even greater than those which by its peculiar property it is intended to obviate, and it is now not used. Although the times of vibration of a pendulum in different arches be nearly equal, yet, if the arches differ very considerably, the vibrations will be performed in different times, and the difference, though very small, will become sensible in the course of one day or more. In clocks for astronomical purposes, the arc of vibration must be accurately ascertained, and if it be different from that described by the pendulum, when the clock keeps time, a correction must be applied to the time shown by the clock. This correction, expressed in seconds of time, will be equal to the half of three times the difference of the square of the given arc, and of that of the arc described by the pendulum when the clock keeps time, these arcs being expressed in degrees; and so much will the clock gain or lose, according as the first of these arches is less or greater than the second. Thus, if a clock keeps true time when the pendulum vibrates in an arch of 3°, it will lose 10% seconds daily in an arch of 4°, and 24 seconds in an arch
and generally Bo – A* x * gives the time lost or gained. See Simpson’s Fluxions, vol. ii. prob. xxviii.
In all that has been hitherto said, the power of gravity has been supposed constantly the same. But, if the said power varies, the lengths of pendulums must vary in the same proportion, in order that they may vibrate in equal times; for we have shewn, that the ratio of the times of vibration, and descent through half the lengths is given, and consequently the times of vibration and descent through the whole length is given; but the times of vibration are supposed equal, therefore the times of descent through the lengths of the pendulum are equal. But bodies descending through unequal spaces, in equal times, are impelled by powers that are as the spaces described, that is, the powers of gravity are as the lengths of the pendulums.
Pendulums’ length in latitude of London, to swing
Seconds 39.2 * Seconds 9.8 # Seconds 2.45
Length of Pendulums to vibrate Seconds at every Fifth Degree of Latitude.
lum to make any number of vibrations, and vice versa. Call the pendulum making 60 vibrations the standard length ; then say, as the square of the given number of vibrations is to the square of 60, so is the length of the standard to the length sought. If the length of the pendulum be given, and the number of vibrations it makes in a minute be required; say, as the given length is to the standard length, so is the square of 60, its vibrations in a minute, to the square of the number required. The square root of which will be the number of vibrations made in a minute. The greatest inconvenience attending this most useful instrument is, that it is constantly liable to an alteration of its length, from the effects of heat and cold, which very sensibly expand and contract all metalline bodies. See HEAT, PyRoMETER, &c. To remedy this inconvenience, the common method is by applying the bob of the pendulum with a screw ; so that it may be at any time made longer or shorter, according as the bob is screwed downwards or upwards, and thereby the time of its vibrations kept always the same. Again, if a glass or metalline tube, uniform throughout, filled with quicksilver, and 58.8 inches long, were applied to a clock, it would vibrate seconds for 39.2 = # of 58.8, and such a pendulum admits of a twofold expansion and contraction, viz. one of the metal, and the other of the mercury, and these will be at the same time contrary, and therefore will correct each other. For by what we have shewn, the metal will extend in length with heat, and so the pendulum will vibrate slower on that account. The mercury also will expand with heat, and since by this ex
pansion it must extend the length of the column upward, and consequently raise the centre of oscillation; so that by this means its distance from the point of suspension will be shortened, and therefore the pendulum on this account will vibrate quicker; wherefore, if the circumstances of the tube and mercury are skilfully adjusted, the time of the clock might, by this means, for a long course of time, continue the same, without any sensible gain or loss. This was the invention of Mr. Graham, in the year 1721, who made a clock of this sort, and compared it with one of the best of the common sort for three years together, and found the errors of the former but about one-eighth part of the latter. Mr. Graham also made a pendulum consisting of three bars, one of steel between two of brass, and the steel bar acted upon a lever, so as to raise the pendulum, when lengthened by heat, and to let it down, when shortened by cold; but he found this clock liable to sudden starts and jerks in its motion. The ingenious Mr. Ellicott, in the Transactions of the Royal Society, describes a pendulum of his invention, composed of brass and iron, with the method of applying it, so as to avoid the many jerks to which the machine might be liable. But, besides the irregularities arising from heat and cold, pendulum clocks are liable to others from friction and foulness; to obviate which, Mr. Harrison has several excellent contrivances, whereby his clocks are almost entirely free from friction, and never need to be cleaned. See Long ITUDE. The gridiron pendulum is a contrivance for the same purpose. Instead of one rod, this pendulum is composed of any convenient odd number of rods, as five, seven, or nine; being so connected, that the ef. fect of one set of them counteracts that of the other set; and therefore, if they are properly adjusted to each other, the centres of suspension and oscillation will always be equidistant. Fig. 11. represents a gridiron pendulum composed of nine rods, steel and brass alternately. The two outer rods, A B, C D, which are of steel, are fastened to the cross pieces, A C, B D, by means of pins. The next two rods, E F, G H, are of brass, and are fastened to the lower bar B D, and to the second upper bar E. G. The two following rods are of steel, and are fastened to the cross bars EG and IK. The two rods VOL. IX.
adjacent to the central rod being of brass, are fastened to the cross pieces I K and L M ; and the central rod, to which the ball of the pendulum is attached, is suspended from the cross piece LM, and passes freely through a perforation in each of the cross bars I K, B. D. From this disposition of the rods, it is evident that, by the expansion of the extreme rods, the cross piece B D, 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 therefore also the two next rods; but because these rods are also expanded, the cross bar 1 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. Another excellent contrivance for the same purpose is described in a French author on clock-making. It was used in the north of England by an ingenious artist about fifty years ago. This invention is as follows: a bar of the same metal with the rod of the pendulum, and of the same dimensions, is placed against the back part of the clock-case: from the top of this a part projects, to which the upper part of the pendulum is connected by two fine pliable chains or silken strings, which just below pass between. two plates of brass, whose lower edges will always terminate the length of the . pendulum at the upper end. These plates are supported on a pedestal fixed to the back of the case. The bar rests upon an immoveable base at the lower part of the case; and is inserted into a groove, by which means it is always retained in the same position. From this construction, it is evident that the extension or contraction of this bar, and of the rod of the pendulum, will be equal, and in contrary directions. For suppose the rod of the pendulum to be expanded any given quantity by heat ; then, as the X