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heliacally in the evening, after the new moon; and sets heliacally in the morning, when old and approaching to a conjunction with the sun.

The inferior planets, Venus and Mercury, which sometimes seem to go westward from the sun, and sometimes again have a quicker motion eastward, rise heliacally in the morning, when they are retrograde; but when direct in their motions they rise heliacally in the evening. The heliacal rising or setting of the moon, happens when she is seventeen degrees distant from the sun; but for the other planets, twenty degrees are required; and for the fixed stars, more or less according to their magnitude. HELIANTHUS, in botany, sun-flower, a genus of the Syngenesia Polygamia Frustranea class and order. Natural order of Compositæ Oppositifolia. Corymbiferæ, Jussieu. Essential character: calyx imbri. cate, somewhat squarrose; down twoleaved; receptacle chaffy, flat. There are twelve species. These are hardy herbace ous plants, most of them tall and large, all perennial excepting two, viz. H. annus and H. indicus. They are all natives of America.

HELICOID parabola, or Parabolic Spiral, is a curve arising from the supposition that the common parabola is bent or twisted, till the axis comes into the circumference of a circle, the ordinates still retaining their places and perpendicular positions with respect to the circle, all these still remaining in the same place.

HELICONIA, in botany, a genus of the Pentandria Monogynia class and order. Natural order of Scitamineæ. Musæ, 'Jussieu. Essential character: spathes; perianth none; corolla three-petalled; nectary two-leaved; pericarpium tricoccous; seeds solitary. There are three species, natives of the West Indies and South America.

HELICTERES, in botany, a genus of the Gynandria Decandria class and order. Natural order of Columniferæ. Malvaceæ, Jussieu. Essential character: pentagynous; calyx one-leafed, oblique; petals five; nectary of five leaflets; capsule five-twist ed. There are nine species, shrubs or trees, natives of both Indies, mostly tomentose; leaves alternate; peduncles axillary, few. flowered.

HELIOCARPUS, in botany, a genus of the Dodecandria Digynia class and order, Natural order of Columniferæ. Tiliacea, Jussieu. Essential character: calyx fourleaved; corolla four-petalled; styles sim

ple; capsule two-celled, compressed, lon. gitudinally radiated on both sides. 'There is only one species, viz. H. americana, American heliocarpus. It is found growing wild about La Vera Cruz, in New Spain.

HELIOMETER, the name of an instrument for measuring with particular exactness the diameters of the heavenly bodies, and especially those of the sun and moon. This instrument is a kind of telescope, consisting of two object-glasses of equal focal distance, placed one of them by the side of the other, so that the same eye-glass serves for both. The tube of this instrument is of a conical form, larger at the upper end, which receives the two object-glasses, than at the lower, which is furnished with an eye-glass and micrometer. By the construction of this instrument two distinct images of an object are formed in the focus of the eyeglass, whose distance, depending on that of the two object-glasses from one another,may be measured with accuracy; nor is it necessary that the whole disc of the sun or moon should come within the field of view, since, if the images of only a small part of the disc be formed by each object-glass, the whole diameter may be easily computed by their position with respect to one another: for if the object be large, the images will approach, or perhaps lie even over one another, and the object glasses being moveable, the two images may always be brought exactly to touch one another, and the diameter may be computed from the known distance of the centres of the two glasses. Besides, as this instrument has a common micrometer in the focus of the eye-glass, when the two images of the sun or moon are made in part to cover one another, that part which is common to both the images may be measured with great exactness, as being viewed upon a ground that is only one half less luminous than itself; whereas, in general, the heavenly bodies are viewed upon a dark ground, and on that account are imagined to be larger than they really are. By a small addition to this instrument, provided it be of a moderate length, M. Bonguer, the inventor, thought it very possible to measure angles of three or four degrees, which is of particular consequence in taking the distance of stars from the moon. With this instrument he found that the sun's vertical diameter, though somewhat diminished by the astronomical refraction, is longer than the horizontal diameter; and, in ascertaining this phenomenon, he also found, that the upper and lower edges of the sun's

disc are not so equally defined as the other parts; on this account his image appears somewhat extended in the vertical direction. This is owing to the decomposition of light, which is known to consist of rays differently refrangible in their passage through our atmosphere. Thus the blue and violet rays, which proceed from the upper part of the disc at the same time with those of other colours, are somewhat more refracted than the others, and therefore seem to us to have proceeded from a higher point; whereas, on the contrary, the red rays proceeding from the lower edge of the disc, being less refracted than the others, seem to proceed from a lower point; so that the vertical diameter is extended, or appears longer, than the horizontal diame


HELIOCENTRIC latitude of a planet, the inclination of a line drawn between the centre of the sun and the centre of a planet, to the plane of the ecliptic.

HELIOCENTRIC place of a planet, in astronomy, the place of the ecliptic wherein the planet would appear to a spectator placed at the centre of the sun.

HELIOPHILA, in botany, a genus of the Tetradynamia Siliquosa class and order. Natural order of Siliquosæ, Cruciformes. Cruciferæ, Jussieu. Essential character: nectaries two, bowed back towards the bladder of the calyx. There are ten species. These plants are all natives of the Cape of Good Hope.

HELIOSCOPE, in optics, a sort of telescope, peculiarly fitted for viewing the sun, without hurting the eyes. See TE


HELIOTROPE, in mineralogy, a species of the flint genus. It is of a green colour, and occurs massive, in angular and rolled pieces; it is commonly translucent on the edges; the specific gravity from 2.6 to 2.7. It is found in rocks, and is said to be the connecting link between jasper and chalcedony. In Asia, it is found in Bucharia, Persia, and Siberia; and in Europe, in Iceland and in Upper Saxony. From the beauty of the colour, and its great hardness, it is reckoned of great value among lapidaries, and that which has the greatest degree of translucency, and most numerous red spots, is of most value.

HELIOTROPIUM, in botany, a genus of the Pentandria Monogynia class and order. Natural order of Asperifolia. Borragineæ, Jussieu. Essential character: co

rolla salver-shaped, five-cleft, with teeth interposed; throat closed with arches. There are twenty-four species, of which H. peruvianum, Peruvian turnsole, or Heliotrope, is a small shrubby plant, from two to three feet in height; the leaves are long, hairy, and much veined, of an ash-colour on their under side, on short foot-stalks; the flowers are produced at the ends of the branches, in short reflex spikes, growing in clusters; the peduncles divide into two or three, and these again into smaller ones, each sustaining a spikelet of pale blue flowers, which have a strong, sweet odour, somewhat resembling bitter almonds. It grows naturally in Peru; it flowers with us great part of the year, and those flowers which come out early in the summer, are succeeded by ripe seeds in autumn.

HELIX, in geometry, the same with SPI. RAL, which see.

HELIX, in natural history, the snail, a genus of the Vermes Testacea class and order. Animal a limax; shell univalve, spiral, subdiaphanous, brittle; aperture contracted, semi-lunar or roundish. Of this genus more than three hundred species have been enumerated; they are separated into divisions; A. whorls, with a carinate acute margin; B. umbilicate, the whorls rounded; C. rounded imperforate; D. tapering; E. ovate, imperforate. Of the species, we shall notice H. cornea, the shell of which above is umbilicate, flat, blackish, with four round whorls. It is found in Europe, and on the coast of Coromandel, from a single line to an inch in diameter; shell chesnut, brown, rufous, whitish, yellowish, or blueish, polished and very fine striate transversely; whorls four or five, rarely turned contrary; the inhabitant is black, with dirty-grey tentacula, and produces a scarlet, but not very durable dye. H. formatia, a snail with five spires remarkably ventricose, slightly umbilicated, fasciated with a lighter and deeper brown: this is found in the woods of the southern counties of England; it is said to have been introduced into this country by Sir Kenelm Digby, for medical purposes. These are confined to the southern counties, attempts having been made, but without success, to bring them into Northamptonshire. This snail is used in many parts of Europe as food, particularly at Rome during the weeks of Lent: here they are fattened, and grow to a very large size. It is oviparous, very tenacious of life, and, towards winter, covers its aperture with a calcareous lid. H. hortensis,

garden-snail, shell imperforate, globular, pale, with broad interrupted brown bands: this species inhabits the garden and or chard in most parts of Europe; it abounds with a viscid slimy juice, which it readily gives out by boiling in milk and water, so as to render them thick and glutinous, and the compound, especially with milk, is reckoned efficacious in consumptive cases. Snails are very destructive to wall-fruit: lime and ashes sprinkled on the ground will keep them away, and destroy the young brood. Fruit, already bitten, should not be taken off the tree, for they will not touch the other, till they have wholly eaten this, if left for them. The eyes of snails are lodged in their horns, one at the end of each horn, which they can retract at pleasure. The manner of examining these eyes, which are four in number, is this: when the horns are out, cut off nimbly the extremity of one of them, and, placing it before the microscope, you may discover the black spot at the end to be really a semiglobular eye.

The dissection of this animal is very curious; for, by this means, the microscope not only discovers the heart beating, just against the round hole near the neck, which seems the place of respiration; but also the liver, spleen, stomach, and intestines, with the veins, arteries, mouth, and teeth, are plainly observable. The intestines of this creature are green, from its eating herbs, and are branched all over with fine capillary white veins; the mouth is like a hare's or rabbit's, with four or six needle-teeth, resembling those of leeches, and of a substance like horn. Snails are all hermaphrodites, having both sexes united in each individual; they lay their eggs with great care in the earth, and the young ones are hatched with shells completely formed. Cutting, off a snail's bead, a little stone appears, which is supposed to be a great diuretic, and good in all nephritic disorders: imme. diately under this stone, the heart is seen beating; and the auricles are evidently distinguishable, and are membranous, and of a white colour, as are also the vessels which proceed from them. So small an animal as the snail, is not free from the plague of supporting other smaller animals on its body; and, as in other animals, we find these secondary ones either living on their surface, as lice, &c. or only in the intestines, as worms; it is very remarkable, that the snail is infested in both these manners, lice being found sometimes on the surface of its body, and worms sometimes within its in

testines. There is a part of the common garden snail, and of other of the like kinds, commonly called the collar; this surrounds the neck of the snail, and is considerably thick, and is the only part that is visible when the animal is retired quietly into its shell: in this state of the animal, these insects which infest it are usually seen in considerable numbers, marching about very nimbly on this part; besides, the snail, every time it has occasion to open its anus, gives them a place by which to enter into its intestines, and they often seize the opportunity.

HELLEBORUS, in botany, English hellebore, a genus of the Polyandria Polygynia class and order. Natural order of Multisilique. Ranunculaceæ, Jussieu. Essential character: calyx none; petals five, or more; nectary two-lipped, tubular; capsules manyseeded. There are seven species. The hellebores are all hardy, herbaceous perennials, with compound leaves, digitate, pedate, palmate, or ternate; the flowers have only a single cover; they grow either several together, at the ends of the stalk and its subdivisions, with a single bracte to each pedicle; or single on a scape, naked, or with a leaf for an involucre. They appear early in the spring, and often in the winter. The root of this plant is tuberous; at first it has no taste, but in a short time, a strong acridity becomes sensible to the mouth and throat. By distillation, an oil is obtained which is extremely poisonous : a one similar in its effects may be obtained from many plants.

HELM, in naval architecture, a long and flat piece of timber, or an assemblage of several pieces, suspended along the hind part of a ship's stern-post, where it turns upon hinges to the right or left, serving to direct the course of the vessel, as the tail of a fish guides the body. The helm is usually composed of three parts, viz. the rudder, the tiller, and the wheel, except in small vessels, where the wheel is unnecessary. As to the form of the rudder it becomes gradually broader in proportion to its distance from the top, or to its depth under the water. The back, or inner part of it, which joins to the stern post, is diminished into the form of a wedge throughout its whole length, so as that the rudder may be more easily turned from one side to the other, where it makes an obtuse angle with the keel. It is supported upon hinges, of which those that are bolted round the sternpost to the after extremity of the ship are

called googings, and are furnished with a large hole on the after-part of the sternpost. The other parts of the hinges, which are bolted to the back of the rudder, are called pintles, being strong cylindrical pins, which enter into the googings and rest upon them. The length and thickness of the rudder is nearly equal to that of the sternpost. The rudder is turned upon its hinges by means of a long bar of timber called the tiller, which is fixed horizontally in its upper end within the vessel. The movements of the tiller, to the right and left, accordingly direct the efforts of the rudder to the government of the ship's course as she advances; which, in the sea language, is called steering. The operations of the tiller are guided and assisted by a sort of tackle, communicating with the ship's side, called the tiller-rope, which is usually composed of untarred rope-yarns, for the purpose of traversing more readily through the blocks or pulleys. In order to facilitate the management of the helm, the tiller-rope, in all large vessels, is wound about a wheel which acts upon it with the powers of a crane or


There are several terms in the sea lan guage relating to the helm; as, "bear up the helm;" that is, let the ship go more large before the wind: "helm a mid-ship," or "right the helm;" that is, keep it even with the middle of the ship: "port the helm," put it over the left side of the ship: "starboard the helm," put it on the right side of the ship.

HELMET, an ancient defensive armour worn by horsemen both in war and in tournaments. It covered both the head and face, only leaving an aperture in the front secured by bars, which was called the visor. It is still used in heraldry by way of crest over the shield or coat of arms, in order to express the different degrees of nobility by the different manner in which it is borne. Thus, a helmet in profile is given to gentlemen and esquires: to a knight, the helmet standing forward and the beaver a little open the helmet in profile and open, with bars, belongs to all noblemen under the degree of a duke: and the helmet forward and open, with many bars, is assigned to kings, princes, and dukes.

There is generally but one helmet upon a shield; but sometimes there are two, and even three if there be two, they ought to face each other; and if three, the middlemost should stand directly forward, and the other two on the sides facing towards it.

HELONIAS, in botany, a genus of the Hexandria Trigynia class and order. Natural order of Coronariæ. Junci, Jussieu. Essential character: calyx none; corolla six-petalled; capsule three-celled. There are two species, viz. H. bullata, spear-leaved helonias; and H. asphodeloides, grassleaved helonias; both natives of North America.

HEMEROBIUS, in natural history, a genus of insects of the order Neuroptera. Mouth with a short horny mandible, the jaw cylindrical, straight, cleft; feelers four, unequal, filiform; wings deflected, not folded; antennæ setaceous, projecting, longer than the thorax which is convex. There are nearly forty species, in two divisions; A. lip cylindrical, membranaceous, annulate: B. lip horny, rounded at the tip, vaulted. The insects belonging to this genus are, like the ephemeræ, very shortlived, and in every state of their existence prey, with unceasing avidity, upon plantlice. The larva is six-footed, generally ovate and hairy; the pupa mostly follicu late; the eggs are deposited in clusters on the leaves of plants, each placed on a small gummy pedicle. When touched many of them have an excrementious smell. The most common species is the H. perla, an insect of great beauty, seen chiefly in the middle, and towards the decline of summer; and is a slender-bodied fly, of a grass-green colour; with bright gold-coloured eyes; and four large, transparent, oval, wings, finely reticulated with pale-green veins. The eggs of this insect are supported on a delicate stem, of more than half an inch in length, which is attached to the surface of a leaf or twig. They may be seen most frequently on the lime-tree, and by some persons, unacquainted with their nature, they have been taken for a small species of the fungus. From the eggs are hatched the larva, which in a few days become fitted to undergo their change into the chrysalis state. For this purpose the animal draws a fine silk from the extremity of its body, and in a short space envelopes itself in a round ball, of the size of a small pea, affixed to a leaf or twig of the tree it frequents; and divesting itself of its skin commences a chrysalis; in about three weeks it becomes a complete insect. The hemerobius takes its name from the shortness of its life, as it seldom lives more than two or three days.

HEMEROCALLIS, in botany, English day lily, a genus of the Hexandria Mono

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