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bles are driven out, and the ice is much clearer. Ice is considerably lighter than water, namely, about one-eighth part; and this increase of dimensions is acquired with prodigious force, sufficient to burst the strongest iron vessels, and even pieces of artillery. It does not arise from the extrication of the gases; for the refractive power of ice is less than that of water, as Dr. Hooke long ago shewed, and has since been confirmed by Wollaston. M. Prévost observes, that congelation takes place much more suddenly than the opposite process of liquefaction; and that, of course, the same quantity of heat must be more rapidly extricated in freezing, than is absorbed in thawing; that the heat thus extricated being disposed to fly off in all directions, and little of it being retained by the neighbouring bo. dies, more heat is lost than is gained by the alteration: so that, where ice has once been formed, its production is in this manner redoubled. This circumstance must occur whenever it freezes, that is, on shore, in latitudes above 35 degrees; and it appears, from 30 degrees to the pole, the land is somewhat colder than the sea, and the more as it is farther distant from it; and nearer the equator the land is warmer than the sea; but the process ef congelation cannot, by any means, be the principal cause of the difference, and it is probable that the different capacity of earth and water for heat is materially concerned in it. Since the atmosphere is very little heated by the passage of the sun’s rays through it, it is naturally colder than the earth’s surface ; and for this reason the most elevated tracts of land, which are the most prominent, and the most exposed to the effects of the atmosphere, are always colder than situations near the level of the sea. The northern hemisphere is somewhat warmer than the southern, perhaps be: cause of the greater proportion of land that it contains, and also in some measure on account of the greater length of its summer than that of the southern ; for although, as it was long ago observed by Simpson, the different distance of the sun compensates precisely for the different velocity of the earth in its orbit, with respect to the whole quantity of heat received on either side of the equinoctial points, yet M. Prévost has shewn that, in all probability, the same quantity of heat must produce a greater effect when it is more slowly applied "because the portion lost by radiation from the heated bo
dy is greater as the temperature is higher. Since, therefore, on account of the eccentricity of the earth's orbit, the north pole is turned towards the sun seven or eight days longer than the south pole, the northern winters must be milder than the southern; yet the southern summers, though shorter, ought to be somewhat warmer than the northern ; but, in fact, they are colder, partly perhaps from the much greater proportion of sea, which in some degree equalizes the temperature, and partly for other reasons. The comparative intensity of the southern summer and winter is not exastly known; but in the island of New Georgia, the summer is said to be extremely cold. The northern ice extends about 9° from the pole; the southern 18° or 20°; in some parts even 30°; and floating ice has occasionally been found in both hemispheres as #: as 40° from the poles, and sometimes, as it has been said, even in latitude 41° or 42°. Between 54° and 60° south latitude, the snow lies on the ground, at the sea-side, throughout the summer. The line of perpetual congelation is three miles above the surface at the equator, where the mean heat is 84°, at Teneriffe, in latitude 28°, two miles; and in the latitude of London, a little more than a mile; and in latitude 80° north only 1,200 feet. At the pole, according to the analogy deduced by Mr. Kirwan, from a comparison of various observations, the mean temperature should be 31°. In London, the mean temperature is 50°; at Rome, and at Montpelier, a little more than 60°; in the island of Madeira 70°; and in Jamaica 80°. Ice-house, a building contrived to preserve ice for the use of a family in the summer season. Ice-houses are more generally used in warm countries than with us, particularly in Italy, where the meanest person, who rents a house, has his vault or cellar for ice. However, as ice is much more used in England than it was formerly, it may not be amiss to give some direction for the choice of their situation, for the manner of building them, and for the management of the ice. As to the situation, it ought to be placed upon a dry spot of ground, because, wherever there is moisture, the ice will melt; therefore, in all strong lands which retain the wet, too much pains cannot be taken to make drains all round them. The place should also be elevated, and and as much exposed to the sun and air as possible. As to the figure of the building, that may be according to the fancy of the owner; but a circular form is most proper for the well in which the ice is to be preserved; which should be of a size and depth proportionable to the quantity to be kept; for it is proper to have it large enough to contain ice for two years consumption, so that if a mild winter should happen, in which little or no ice is to be had, there may be a stock to supply the want. At the bottom of the well there should be a space of about two feet deep left, to receive any moisture that may drain from the ice, over this space should be placed a strong wooden grate, and from thence a small drain should be laid under ground to carry off the wet. The sides of the well should be built with brick, at least two bricks thick; for the thicker it is, the less danger there will be of the well being affectcd by any external cause. When the well is brought up within three feet of the surface, there should be another outer arch or wall begun, which should be carried up to the height of the intended arch of the well ; and if there be a second arch turned over this wall, it will add to the goodness of the house : the roof must be high enough above the inner arch to admit of a door-way to get out the ice. If the building is to be covered with slates or tiles, reeds should be laid considerably thick under them, to keep out the sun and external air; and if these reeds are laid the thickness of six or eight inches, and plastered over with lime and hair, there will be no danger of the heat getting through them. The external wall may be built in what form the proprietor pleases; and as these ice-houses are placed in gardens, they are sometimes so contrived as to have a handsome alcove seat in front, with a small door behind it, through which a person might enter to take out the ice ; and a large door on the other side, fronting the north, with a [..." wide enough for a small cart to ack in, in order to shoot down the ice near the mouth of the well, which need not be more than two feet in diameter, and a stone so contrived as to shut it up in the exactest manner; all the vacant space above and between this and the large door should be filled up with straw. The building, thus finished, should have time to dry before the ice is put into it. It is to be observed, that upon the wooden grate, at the bottom of the well, there should be laid some small faggots, and if upon these a layer of . is placed smooth for the ice to lie upon, it will be better than straw, which is commonly used. As to the choice of the ice, the
thinner it is, the easier it may be broken into powder; for the smaller it is broken, the better it will unite, when put into the well. In putting it in, care must be taken to ram it as close as possible; and also to allow a vacancy of about two inch. es, all round, next the side of the well, to give passage to any moisture occasioned by the melting of some of the ice. When the ice is put into the well, if a little saltpetre be mixed with it at every ten inches or a foot in thickness, it will cause it to unite more closely into a solid mass.
ICH DIEN, the motto of the Prince of Wales's arms, signifying, in the high Dutch, “I serve.” It was first used by Edward the Black Prince, to shew his subjection to his father king Edward III.
ICHNEUMON, in natural history, a so of insects of the Hymenoptera or
er: mouth with a straight horny mem. branaceous bifid jaw, the tip rounded and ciliate; mandibles curved sharp ; lip cylindrical, membranaceous at the tip, and emarginate ; feelers four, unequal, filiform, seated in the middle of the lip; antennae setaceous, of more than thirty articulations; sting exserted, inclosed in a cylindrical sheath, composed of two valves, and not very pungent. There are more than five hundred species enumerated by different authors. These are separated into two families. A. scutel, white or yellow ; antennae annulate with white. B. scutel, white or yellow; antennz entirely black. C. scútel, the colour of the thorax ; antennae annulate. D. scutel, the colour of the thorax ; antennae black. E. antennae yellow. F. minute; antenna filiform; abdomen sessile, ovate.
The whole of this singular genus have been denominated parasitical, on account of the very extraordinary manner in which they provide for the future support of their offspring. The fly feeds on the honey of flowers, and when about to lay her eggs, perforates the body of some other insect, or its larva, with its sting or instrument, at the end of the abdomen, and there deposits them. These eggs in a few days hatch; and the young larva, which resemble minute white maggots, nourish themselves with the juices of their foster parent, which however continues to move about and feed till near the time of its change to a chrysalis, when the larva of the ichneumon creep out, by perforating the skin in various places, and each spinning itself up in a small oval silken case, thanges into a chrysalis, and after a certain period they emerge in the state of complete ichneumons. .
I. glomeratus may be given as an example of this process. The caterpillar of the common white or cabbage butterfly, which, in the autumnal season, may be observed to creep up some wall, &c. in order to undergo its own ehange into a chrysalis : but in the space of a day or two, a numerous tribe of small maggots will be seen to emerge from it, and immediately proceed to envelope themselves in distinct, yellow, silken cases, the whole forming a group round the caterpillar. These are the ichneumons glomeratus: they are black, with yellow legs, and they usually make their appearance in about three weeks from the time of their spinning themselves up. Some of the ichneumon genus pierce the skins of newly-changed chrysalises of butterflies and moths, in which their larva remain during their own incomplete state. Others are so minute, that the female pierces even the eggs of moths and butterflies, and deposits her own in each. I. seductor, has a yellow scutel; tip and petiole of the abdomen and crenate band on the fore-part yellow ; legs mostly yellow. This insectis found chiefly in Pavia; it forms a nest of cemented clay, in chimaneys and windows, divided into cylindrical cells, in each of which is contained a cylindrical, brown, lucid follicle, and in this the larva, with frequently the carcase of a spider, in which the insect had deposited her eggs.
icinogiophy, in perspective, the
view of any thing cut off by a plane parallel to the horizon, just at the base of it. Ichnography, in architecture, a description or draught of the platform or round-work of a house, or other building. Or it is the geometrical plan or platform of an edifice or house, or the groundwork of an house or building, delineated upon paper, describing the form of the several apartments, rooms, windows, chimneys, &c. Ichnogh Aphy, in fortification, denotes the plan or representation of the length and breadth of a fortress, the distinct parts of which are marked out, either on ibe ground itself, or on paper. ICHTHYOCOLLA. See Is INGLAss. ICHTHYOLOGY, is that part of natural history that treats of fishes. And fishes are animals having a heart with one auricle and one ventricle, with cold red blood, which inhabit water, and breathe by means of gills. Most of the species
are likewise distinguished by fins and scales. Different naturalists have given different systems; we shall briefly notice some of the principal, because we find them perpetually referred to in works of importance. Aldrovandus, about two hundred years ago, distributed the fishes according to the nature of their residence. His first book treats of those that frequent rocks; the second is devoted to those found near the shores, called littoral; the third, pelagian, &c. Willoughby formed his system from his observation on the anatomy and physiology of fishes; he was followed by Ray, who fixed a series of genera. Artedi, the friend and countryman of Linnaeus, has the merit of having first traced the outlines of that classification of fishes, which is now almost generally adopted. For, independently of the cetaceous tribes, which are now generally classed with the mammalia, his method consisted of four orders, viz. 1. The malacoterygian, or those which have soft fins, or fins with bony rays, but without spines. 2. The acanthopterian, those with spiny fins. 3. The branchiostegous, corresponding to the amphibia nantes of Linnaeus. 4. The chondropterygian, or those
which have not true bones, but only car
tilages, and the rays of whose fins hardly differ from a membrane. At first Linnaeus adopted this method entirely, but he af. terwards improved upon it; and now, accerding to his system, the orders have been instituted from the situation, presence, or absence of the ventral fins. 1. Such as are entirely destitute of these fins are termed pisces apodes, apodal or footless fishes. 2. The jugulares, or jugular, are those which have ventral fins, placed more forward than the pectoral fins, or under the throat. 3. The thoracici, or thoracic, include those whose ventral fins are placed immediately under the pectoral fins, or on the breast. 4. The abdominales, or abdominal, comprise those whose ventral fins are situated behind the pectoral fins, or on the abdomen. 5. There still remains a particular tribe, denominated cartilaginei, which, as their name imports, have a cartilaginous instead of a bony skeleton. This tribe was by Linnaeus separated from the rest, on the mistaken idea, that the individuals which compose it were furnished both with lungs and gills, and should be ranked in the class of amphibious animals. The genera *I. to the preceding orders are determined by the number of rays in the branchiostegous membrane, the condition of the teeth, the figure of the body, and of other remarkable parts. The characters of the species are taken chiefly from the number of rays in the fins, which differs in the different species. But as the precise enumeration of these rays is sometimes a matter of difficulty, and as they are likewise subject to variation, it is necessary to have recourse to other marks, and to adopt, as subsidiary characters, the form and situation of particular fins, the proportion of the head to the body, the condition of the lateral line, the number of the vertebræ and ribs, &c. Mr. Pennant describes fishes under the three great divisions of cetaceous, cartilainous, and bony. The latter, which is by #. the most numerous, he subdivides into four sections, entitled, agreeably to the Linnaean orders, apodal, thoracic, jugular, and abdominal. The shape of the body of fishes is subject to considerable varieties. It is said to be compressed, when the diameter, from side to side, is less than from back to belly; and depressed, on the contrary, when the diameter, from side to side, is greater than from back to belly. It is cylindrical, when it is circular in the greater part of its length; ensiform, or sword-shaped, when the back and belly terminate in a sharp edge, or when the body gradually tapers from the head to the tail; cultrated, or knife-shaped, when the back is somewhat flat, and the angle below acute; carinated, or keel-shaped, when the back is rounded, and the under part of the belly, acute, through its length; oblong, when the longitudinal diameter is much longer than the transverse; oval, when the longitudinal diameter not only exceeds the transverse, but the base is circular, and the apex more acute; orbicular, when the longitudinal and transverse diameters are nearly equal; cuneiform, or wedge-shaped, when the body gradually flattens towards the tail: conical, when it is cylindrical, and grows gradually more slender towards the tail; ventricose, when the belly is very prominent; gibbous, when the back presents one or more protuberances; annu. lated, when the body is surrounded by rings, or elevated lines; articulated, when it is covered with connected and bony plates; trigon, tetragon, pentagon, and hexagon, when the sides are plain, with three, four, or six longitudinal angles; if the number of these angles exceed six, it is termed a polygon. The surface of the body of fishes is termed naked, when it is destitute of
scales; scaly, when provided with them; smooth, when the scales are without angles, furrows, roughness, or inequalities; lubricous, or slippery, when invested with a mucous or slimy humour; tuberculated, or rough, when covered with prominent warts or tubercles; papillous, when covered with fleshy points; spinous, when the asperities are elongated, and pointed at their extremities; loricated, or mailed, when the body is enclosed in a hard, callous, or bony integument, or in scales so closely united as to seem but one ; fasciated, or banded, when marked with transverse zones from the back to the belly; striped, when marked with very narrow, scattered and coloured streaks; vittated, when marked with longitudinal zones along the side, from the head to the tail; reticulated, or chequered, when marked with lines forming the appearance of network; pointed or dotted, when marked with points, either longitudinally disposed, or without order; and variegated, when of different colours. The head is always placed at the anterior part of the body, and reaches from the extremity of the nose to the gills. The head contains the mouth, nose, jaws, lips, teeth, tongue, palate, nostrils, eyes, branchial opercules, the branchiostegous membrane, the aperture of the gills, and the nape. The branchial opercules are scaly or bony processes, situated on both sides of the head, behind the eyes, closing the aperture of the gills, and sustaining the branchial membrane. The branchial, or branchiostegous membrane, is a true fin, formed of cartilaginous crooked bones, joined by a thin membrane, lurking id: the opercula, to which it adheres, and is capable of being folded, or expanded, as necessity requires. The trunk is that part of the body which extends from the nape and branchial aperture to the o of the tail. It comprehends the gills, throat, thorax, back, sides, abdomen, lateral line, anus, tail, and scales. The gills, or branchio, consist, for the most part, of four crook: ed, parallel, unequal bones, furnished on the outer, or convex part, with small soft appendages, like the beards of a feather, and generally of a red colour. The fins consist of several rays, connected by a tender film or membrane, and they are raised, expanded, or moved, in various directions, by means of appropriate muscles. The rays of the fins are either jointed and flexible small boness whose extremity is often divided into two parts, or hard and prickly, without division at the extremity. In some cases, those on the back of the fish are furnished with membranaceous appendages, simple, or palmated, and adhering to the apex, or sides. The fins, according to their position, are denominated dorsal, pectoral, ventral, anal, or caudal. The skeleton of a fish is the assemblage of bones which constitutes the framework of its body. The number of these bones is not uniform in each individual, but varies according to age and species. They may be conveniently divided into * of the head, thorax, abdomen, and ns. The muscles are an assemblage of small bundles of fleshy fibres, partly red, and partly whitish, enveloped in a common membrane. The first of these is called the fleshy portion of the muscle; the second the tendon. Each muscle thus composed is susceptible of contraction and dilatation. The former is accompanied by a visible swelling, hardening, wrinkling, and shortening of the muscle, and the latter by its elongation, expansion, and recovery of its former softness and flexibility. Its force, in general, depends on the quantity of fibrous matter which enters into its composition, and its moving power on the length and size of the fibres. The brain of fishes is a very small organ relative to the size of the head. It is divided into three equal lobes, of which the two anterior are contiguous; the third being placed behind, and forming the cerebellum. These three lobes are surrounded by a frothy matter, resembling saliva. In this region, the optic and olfactory nerves are easily discovered. The swimming, or air bladder, or sound, is an oblong, white, membranous bag, sometimes cylindrical, sometimes elliptical, and sometimes divided into two or three lobes, of different lengths. It is usually situated between the vertebræ and the stomach, and included within the peritonaeum. In some fishes it communicates with the stomach, and in others with the oesophagus. . The flat fishes are unprovided with this organ. The intestines, which in man are placed transversely, have a longitudinal position in fishes, and are all connected with the substance of the liver. They are in general very short, making only three turns, the last of which terminates in a common outlet or vent. The appendices, or secondary intestines, are very numerous. composing a group of worm-like processes, all ultimately terminating in two large canals, opening into the first intestine, inVOL. VI.
ICONOCLASTS, in church history, an appellation given to those persons, who, in the eighth century, opposed imageworship; and is still given by the Church of Rome to all Christians who reject the use of images in religious matters. ICOSAHEDRON, in geometry, a regular solid, consisting of twenty triangular pyramids, whose vertexes meet in the centre of a sphere, supposed to circumscribe it; and, therefore, have their height and bases equal ; wherefore the .#. of one of those pyramids multiplied by twenty, the number of bases, gives the solid content of the icosahedron. See Body. ICOSANDRIA, in botany, the name of the twelfth class in the Linnaean system, consisting of plants with hermaphrodite flowers, furnished with twenty or more stamina, that are inserted into the inner side of the calyx, or petals, or both. By this last circumstance, and not by the number of stamina, is this class distinguished from the class polyandria, in which the number of stamina is frequently the same with that of the plants of the class icosandria, but they are inserted, not into the calyx or petals, but into the receptable of the flower. The icosandria furnishes the pulpy fruits that are most esteemed, such as apples,plumbs, peaches, cherries, &c. whereas the polyandria are mostly poisonous, as the aconife, columbine, larkspur, hellebore, and othersThe species of the icosandria have a hollow flower-cup, composed of one leaf, to the inner side of which the petals are fastened by their claws. In this class G g