ing, and, when the shower has ceased, to measure the depth of the water in the vessel by a scale of inches. But, unless the depth were ascertained immediately, a portion of the water would be carried off from such a vessel by evaporation, and the measure would be less than it ought to be. The difficulty also of ascertaining the true amount of a small depth of water would render the instrument of no practical use. For the purpose therefore of obtaining a more correct estimate of the quantity of rain, it has always been the practice to receive the water in a second vessel, or in a tube, the area of whose horizontal section is less than that of the first, so that the height of the column may be greater. And, since the heights of equal quantities of water in two prismatical or cylindrical vessels are inversely proportional to their bases, it is easy to perceive how a rod may be graduated so as to show, in inches, the depth of water in the upper vessel, and consequently the depth which would have Lain on the ground if no absorption had taken place.

Originally this instrument, which has been called indifferently udometer (vdwp and μirpov), pluviometer (pluvia), and ombrometer (õubpos, rain), was nothing more than a prismatical box, having a square base, open at the top and communicating with a prismatical box, placed vertically under it, by means of a pipe open at both ends; the area of a horizontal section of the lower box being, for the reason above given, less than that of the upper box. But it is evident that a prismatical or cylindrical vessel must retain, by adhesion to its sides and bottom, a sensible portion of the water which enters it; and consequently the depth measured in such vessel must indicate a quantity of rain less than that which has really fallen; it has therefore been customary of late to make the upper part of the vessel in the form of a funnel, or inverted

will be expressed by a space aqual to four inches on the length of the rod; and, each of such spaces being divided into 100 parts, the depth of water at the said section will be indicated in hundredths of an inch. The height of the cylindrical vessel below the funnel may be from 25 to 30 inches.

For the sake of diminishing the evaporation and of measuring small quantities of rain with greater precision, the diameter of the cylinder is sometimes reduced to two inches, and the collected water is, by means of a small pipe, inserted in the bottom of the cylinder, and furnished with a cock made to pass into a glass tube whose interior diameter is half an inch. In this case, the diameter of the upper extremity of the funnel being the same as before, the area of the surface which receives the rain from the atmosphere will be to the area of a horizontal section of the glass tube as 576 to 1. Consequently a shower of rain whose depth on the ground might be one-hundredth part of an inch would be indicated by 5.76 inches in the tube.

The funnel of the cylinder may be of tin or copper, and, however the instrument be constructed, it is evident that it should be placed in a vertical position in some place where no object may interfere with the free descent of the rain into the funnel. It is usual to observe the quantity of water in the vessel every morning, if rain has fallen during the preceding twenty-four hours; but, as some evaporation will take place, it would be advantageous to make the observations more frequently.

The sum of all the depths observed during any period of time, as a day, month, or year, will give the whole quantity of rain which has fallen in that time at the place. It is supposed that the rain falls uniformly over the tract of ground lying within the limits of the shower, and consequently that the quantity which passes through the circular area at the upper surface of the cone is equal to that which falls upon an equal area of ground anywhere within those limits.

A rain-gauge can never serve further than to give an approximation to the quantity of rain which may have fallen, since some of the water will always adhere to the sides of the vessel, but the following method of ascertaining the allowance to be made for the quantity thus lost has been recommended:-Let a sponge be made damp, yet so that no water can be squeezed from it, and with this collect all the water which adheres to the funnel and cylinder after as much as possible has been drawn off; then, if the sponge be squeezed and the water from it be received in a vessel which admits of measuring its quantity, a near estimate may be made of the depth due to it; and this being added to the depth given by the instrument would probably show very correctly the required depth of rain.

RAINBOW, a circular arch of variously coloured light which is visible in the heavens when the sun or moon is shining, and when, at the same time, a shower of rain is falling on the opposite side of the spectator. When the rain is abundant, a second bow is commonly seen on the exterior, and concentric with the first; their common centre being in a line drawn from the luminary through the eye of the spectator and produced towards the opposite part of the heavens. Both bows consist of concentric bands of the different prismatic colours arranged as they appear in the solar spectrum, but the order in which they are disposed in the first bow is inverted in the second. The lower edge of the interior bow is violet and the upper edge is red; on the contrary, the lower edge of the exterior bow is red and the upper edge is violet.

The rainbow is a phenomenon which appears at all times to have been understood to depend upon the light of the sun or moon and the drops of falling rain, but the first complete explanation of the circumstances connected with it is due to Newton (Optices, lib. i., p. 2, prop. 9). In the beginning of the sixteenth century no better notion was entertained of the cause of the phenomenon than that the interior bow was a distorted reflection of the sun's image from the surface of a cloud, and that' the exterior bow was a reflected image of the first. But the reflection of light is not capable of producing different colours, and it is said that Fleischer of Breslau (1571) was the first who entertained the idea that the particles of light from the sun entered into the drops of rain. His opinion was that a ray of light suffered one refraction on entering and another on leaving a drop; and that it entered the eye of the spectator after

reflection from the surface of a second drop. It appears that Kepler, in a letter to Harriot (1606), suggested that the particles of light, in a ray which is a tangent to some part of the surface of a drop f rain, might enter the drop by refraction, and that this ray, being subsequently reflected at the interior surface of the latter, might enter the eye of the spectator after being again refracted on .eaving the drop. The hypothesis is worthy of Kepler's sagacity, and, as far as it goes, it differs from the fact only in the manner in which the incident ray is supposed to fall on the drop. Newton ascribes the first idea of the true explanation to Antonio de Dominis, bishop of Spalatro, whose work, De Radiis visûs,' was published in 1611, but is said to have been composed in 1590; the work however appears to have been so obscurely written and to betray so much ignorance of the laws of optics, that it is doubtful whether or not the author had any more than a vague conception of the cause of the colours. (See Montucla, Histoire des Math., tom. ii.)

Descartes is certainly the first who has distinctly explained the causes by which the two bows are produced, and he states (Meteora, cap. viii.) that he detected those causes on observing the phenomena presented by a glass globe filled with water, which he placed in various positions with respect to the sun. He shows that the interior or primary bow is produced by rays from the sun falling upon the drops of rain near their upper surfaces, where, being refracted, they pass to the side of the drop which is farthest from the sun and spectator; from thence they are reflected towards the lower surface, and, on quitting the drop, they suffer a second refraction. He shows also that the exterior or secondary bow is produced by rays from the sun falling upon the drops of rain near their lower surfaces, where, being refracted,' they pass, as before, to the farther side of the drop; from thence they are reflected towards the upper surface, and there they suffer a second reflection. After this they pass to the side of the drop which is nearest to the sun, and from thence they emerge after a second refraction. Now it is not sufficient that the pencils of light which are incident on the drops of rain should be so refracted and reflected; it is moreover necessary that each pencil on emerging from the drop should consist of parallel rays of light, that, when it enters the eye of the spectator, it may produce in the mind the perception of brightness; and Descartes determined by computation the positions of the incident and emergent rays so that this effect may be produced.

Thus, let SI (fig. 1) be a very slender pencil of rays of some one colour incident on a spherical drop of water at the

A

after crossing at X and being reflected from B may pass from B to C in parallel directions; then, after a second reflection, crossing at Y and being refracted at K, they will emerge in parallel directions as they entered at I, so that if KE be the direction of the emergent pencil, the angle DKE will be equal to AIS: the angle made by the lines SI and EK was found by Descartes to be about 52 degrees. If the angle AIS were varied, the rays of the pencil would leave the drop in a divergent state.

Now let A, B, C, D (fig. 3) be four globules of rain in a

Fig. 1

K

E

F

B

angle AIS, and let this angle be such that the rays in the pencil may, by the laws of refraction in water, converge at B; then, though many rays will pass through the drop at that point and be dispersed, yet many will be reflected from thence as from a radiant point, and will emerge at K in parallel directions, as they entered at I, so that if KE be the direction of the emergent pencil, the angle CKE will be equal to AIS: the angle made by the lines SI and EK produced was found by Descartes to be about 42 degrees. If the angle AIS were varied, the rays of the pencil would leave the drop in a divergent state, and then the impression which they would make on the eye might be too feeble to produce the sensation of brightness. Again, let SI (fig. 2) be a very slender pencil of rays of some one colour incident on a spherical drop of water at the angle AIS, and let this angle be such that, by the laws of refraction in water, the rays

cloud covering a considerable part of the heavens on one side of the horizon. Let E be the eye of the spectator, and, on account of the remoteness of the sun, let the rays of light which proceed from his disk be considered as parallel to one another. Let SE be a line drawn from the sun through the eye of the spectator, and let it be produced towards O; also let SA, SB, &c. be very slender pencils of parallel rays (supposed at present to be of one colour) falling upon the globules of water. Let the refraction and reflection of these pencils in A and B be similar to those which are shown in fig. 1; and the refraction and reflection in C and D be similar to those in fig. 2: also from the points of emergence suppose lines to be drawn to E. It is evident, on account of the parallelism of the lines SO, SA, &c., that if the angle AEO or BEO were nearly equal to 42°, and if the angles CEO or DEO were nearly equal to 52°, the eye would be affected by the sensation of brightness as explained above; therefore if the lines AE, BE, &c. were to revolve conically about EO as an axis, all the globules of rain upon the conical surfaces so described would send pencils of parallel rays to the eye, and two concentric arches of bright light would be seen in the heavens. This hypothesis accounts satisfactorily for the existence of two concentric bows of bright light, but it affords no indication of the bands of colours of which they consist. Descartes however very sagaciously refers their cause to the decomposition of light on entering and quitting the drops of rain; observing that the convex surfaces of the latter must produce effects similar to those which take place when light is made to pass through the plane faces of a triangular prism of water. But when Newton had discovered the different degrees of

refrangibility in the different coloured rays which compose a pencil of white or compounded light, he was able to assign immediately the cause of the coloured bands in the rainbow, the order of their position, and the breadth which they must occupy. Thus, if the incident pencil SI (figs. 1 and 2) had consisted only of violet-coloured light (for example), the angle AIS must have had that particular value which alone would allow the rays of the emergent pencil to be parallel to one another; but if the incident pencil were supposed to consist of light of another colour, as red, it should have fallen nearer to the centre of the drop, in order that the angle AIS might have the particular value which would allow the rays of the emergent pencil to be parallel to one another. Since the red rays suffer less refraction than the violet rays, if KE be the direction in which the latter emerge from a drop, KF in both figures may represent the direction in which the former would emerge; and if the eye be situated so as to receive the pencil KE, it would have the impression of a violet colour; while, if situated so as to receive the pencil KF, it would have that of a red colour. We have mentioned, for simplicity, only the violent and red rays, which form the two extremes of the coloured spectrum; but it is easy to conceive that a like explanation might be given for rays of the intermediate colours. And since the pencils of all the different colours diverge from one another on quitting a rain-drop, it is evident that the spectator whose eye receives one of the pencils will be affected by the colour of that pencil only, the other pencils passing either above or below his eye.

Newton has determined by computation that when the angle AEO (fig. 3) = 40° 17', the violet rays alone, after two refractions and one reflection, will enter the eye of the spectator at E, the other rays falling below; and when Z BEO=42° 2′, the red rays alone will enter the eye, the violet rays passing above. Again, when ▲ CEO=50° 59′, the red rays only will enter the eye, after two refractions | and two reflections, the violet rays falling below; and when <DEO=54° 9', the violet rays alone will enter, the red passing above. If the interval between the drops A and B, and also between the drops C and D, were occupied by other drops, it may readily be imagined that the pencils of parallel rays which come from them to the eye would be of all the prismatic colours between the red and violet, and that thus there would appear in the heavens two narrow spectra: the length of that between A and B would be 1° 45', and of that between C and D would be 3° 10'. Therefore, if all the lines drawn to E from the drops in the two spectra were to revolve conically about EO as an axis, the drops on these lines would be in situations to send to the eye rays of their own proper colours, and thus there would exist the appearance in the heavens of two concentric bands of variously coloured light.

But it has been here supposed that the pencils SA, SB, &c. come from the centre only of the sun's disk, whereas each point of the disk produces two bows similar to those which have been described: therefore the lower extremity of the interior bow will be a violet band whose breadth is equal to half the diameter of the sun (suppose 15′), and which is situated immediately below the violet line formed by the centre of the disk; and in like manner the upper extremity of the interior bow will be a red band whose breadth is also 15', and which is situated immediately above the red line formed by the centre of the disk: consequently the whole breadth of the interior bow is about 2° 15'. Similarly 30' (the measure of the sun's diameter) must be added to the breadth of the outer bow, as before determined, which thus becomes about 3° 40'. In both bows, the colours between the violet and red are less distinct than those two colours, because of the interference of the coloured light from all parts of the disk.

On account of the two reflections which take place in the interior of the drops which give rise to the outer bow, while there is but one reflection in those which produce the inner bow, there must be a greater quantity of light lost by transmission through the drops in the former case than in the latter; and hence the outer bow is always fainter than the other. The interval between the primary and secondary bow has occasionally been observed to be occupied by an arch of coloured light; but this, which is not always concentric with the others, has been ascribed to some reflection of one of those bows.

A rainbow can never be greater than a semicircle, if the spectator be not on elevated ground; for if it were, the P. C., No. 1201.

|

centre of the bow would be above the horizon, and the sun, which is in a line drawn through that centre and the eye, would then be below the horizon; but, in this case, the sun could not shine on the drops of rain, and consequently there would be no bow. When the rain-cloud is of small extent, there is seen only that portion of the bow which the cloud can form; yet the bow is sometimes seen against the blue sky, when there exist in the air vapours which are not dense enough to be visible in the form of a cloud; and a portion of a bow has occasionally been seen in an inverted position on the ground by the refraction of the light in drops of rain adhering to the grass or the leaves of trees. It may be added that a coloured bow similar to that which is produced by rain may be observed in the spray from a fountain when the jet of water is agitated by the wind, and also in the mists which at times lie upon low grounds.

The lunar rainbows appear in general white; and when they are coloured, they differ from those produced by the sun only in the colours being much more faint.

The circle of light which is occasionally seen surrounding the sun or moon at some distance from the disk of the luminary, is called a halo or a corona, and is caused by the refractions of light in particles of ice which float in the air. This phenomenon having some resemblance to that which has been just described, a brief explanation of it may be with propriety introduced in this place.

The cause of the halo was first investigated by Des cartes, who observes (Meteora, cap. ix.) that this phenomenon differs from the rainbow, inasmuch as the latter is seen only while rain is falling, whereas halos are never seen at such times; and he ascribes their formation to refractions of light in star-shaped crystals of ice, which he remarks are thicker in the middle than at the edges, and are therefore proper to produce refractions.

Sir Isaac Newton also ascribes the halo to refraction in floating hail or snow; but it appears that Mariotte (in 1686) was the first who considered it to be produced by refraction in the small equilateral prisms of ice which abound in the air in a separate state before they unite together and form the flakes which descend during severe frosts; and Dr. Young, without being aware of Mariotte's hypothesis, entertained and developed the same idea.

According to this philosopher, there may be in the air an immense number of prismatic particles whose transverse sections are equilateral triangles, the planes of the sections deviating but little from one passing through the sun or moon and the spectator Now, by the laws of refraction in water, when a pencil consisting of parallel rays of light, as SI (fig. 4), is incident on a face of such prisms, and makes

the angle of incidence SIP equal to about 41° 50', the axis KE of the emergent pencil will make an equal angle EKQ on the other face, and the angle of deviation SAF, or the angle between the incident and emergent ray will be 23° 40. Therefore if the line FE produced to the spectator's eye were to revolve conically about a line joining the sun and spectator as an axis (which line, since the sun is very remote, may be considered as parallel to SA), all the prisms similarly situated on such conical surface would transmit to the eye pencils of parallel rays, if the light were of one colour, and thus there would be produced the perception o. a bright circle in the heavens, having the sun for its centre; its radius subtending at the eye an angle of about 23° 40′. The angle SAF varies very slowly, the variation amounting only to about 30, when the angle SIP varies as much as 30° consequently there may be innumerable prisms in the air in such positions that the angle SIP, for pencils incident upon them, does not vary more than 15° on either side of that which has been above supposed; and these will transmit to the eye light in such abundance as to produce the appearance of an annulus about 3° broad. This is the appearance of the common halo. Dr. Young supposes further, that when there is a very great number of particles of ice VOL. XIX.-2 N

The parhelia which sometimes appear above or below the true sun, are supposed to be produced by the refractions of light in equilateral prisms of ice, when such prisms happen to be very short, so as to have the form of thin triangular plates, and when the flat sides are in or near a vertical plane passing through the sun and spectator. A great number of such prisms would, by refracting the light as above shown, give rise in the halo to the appearance of a bright spot resembling the true sun, and in a vertical line passing through it; and, if the prisms in such positions have a certain length, the image of the sun would be distorted, and might assume the appearance of being winged. The horizontal parhelia are accounted for in a similar manner. Sometimes false suns (anthelia) appear on the side of the heavens which is opposite the true sun; and these are supposed to be produced by two refractions and two internal reflections in such prisms of ice. In this case the images ought to be 60° beyond the halo; that is, they ought to be about 83° from the true image of the sun.

Subjoined is a sketch of the double halo with parhelia, which was observed by Sir Henry Englefield at Richmond in 1802. (Young's Nat. Phil.; and Journal of the Royal Institution, vol. ii ›

Fig. 5.

RAISIN, or RAISEN, MARKET. [LINCOLNSHIRE.] RAISINS. The dried fruits of several varieties of the vine are called raisins, a term derived from the French, raisin in that language being a general name for grapes, the dried fruit being distinguished as Raisins secs ou passés. Raisins are named after the countries where they are produced, or the places whence they are imported; as Malaga, Valencia, and Smyrna. The peculiar small and generally seedless grapes, formerly called Corinths, are now better known as the dried or Zante currants of the shops. Other denominations by which different kinds of raisins are distinguished, arise from the variety of grape employed, or from the mode of preparation; as muscatels, blooms, sultanas, raisins of the sun, and lexias.

The most simple, and, when circumstances are favourable, the best mode of preparation is to dry the grapes, after being cut when fully ripe, by exposure to the heat of the sun on a floor of hard earth or of stone. Another method is to cut the stalk half-way through when the grapes are nearly ripe, and leave them suspended till the watery part is evaporated; the flow of sap is in a great measure prevented from entering the fruit, in consequence of the incision, and whilst evaporation continues to go on undiminished, desiccation must take place.

Some sorts are prepared by dipping the grapes in a ley, and afterwards drying them in the sun. This ley is formed of water, wood-ashes, and a small portion of oil of olives. The ashes of vine branches and tendrils are preferred. In Valencia, in addition to the ashes of rosemary and vine branches, a little slacked lime is used. Raisins so prepared are called lexias; whilst those prepared entirely by sunneat are denominated raisins of the sun.

A fourth method, only used for raisins of inferior quality, is to dry the grapes in an oven.

The currant-grapes are gathered in the end of August and beginning of September. Rains often spoil the crop when they occur at the time of gathering or drying. The fruit, when sufficiently dry, is separated from the stalks by smalı

The Malaga raisins are esteemed the finest; and the muscatels from thence exceed all others in price by at least one-third. The black Smyrna raisins are those of least value.

Of all the known varieties of grapes, the white muscat of Alexandria is that which furnishes raisins of the finest description. The berries are large, oval, white, rather firmfleshad, with a rich muscat flavour, superior in this respect to all others that have hitherto been fruited in this country. From the synonyms which it has obtained, its extensive cultivation and use as a raisin grape may be inferred; for example, it is called the muscat of Jerusalem, Malaga, Passé-Musquée, Passé-Longue Musquée, Muscat d'Espagne, &c. There is also a black muscat of Alexandria, and a red muscatel, both of which have a firmness of pulp which renders them fit for drying; for grapes, however rich they may be, and excellent in a fresh state, yet if they do not possess a certain degree of firmness, are unfit for drying, inasmuch as their substance would be too much dissipated in the process.

The variety of grape-vine that bears the small and generally seedless bunches of grapes, which, when dried, become the Corinths or Zante currants of the shops, belongs to vitis vinifera, and is not a distinct species, as it has been by some supposed. This has been proved by plants sent direct from Zante, and which have been fruited in the garden of the Horticultural Society. In a good season it is capable of being ripened against a south wall. The berries have the same size and character as those imported, being small and seedless, except occasionally one that acquires a somewhat larger size and contains a seed; such are even found amongst imported fruit. The variety is figured in the Hort. Soc. Transactions (2nd series), vol. i., p. 246. According to M. Beaujour, the first grapes of this variety that appeared in the great marts of Europe were brought at the beginning of the seventeenth century from the Gulf of Corinth, and hence were called Corinthian raisins. Latterly however the cultivation has become chiefly confined to the western territories of the Morea and the Ionian Islands, particularly those of Zante, Cephalonia, and Ithaca. The fertile island of Zante is the place where this variety of grape is produced in greatest abundance.

The chief employment of raisins in medicine is to flavour unpleasant mixtures, or for their demulcent properties. In the former point of view they are unimportant; in the latter, of considerable utility. Fres grapes are cooling, aperient, moderately nutritive, and demulcent. Their use in the south of France is thought to contribute greatly to the amielioration which consumptive persons experience there, and in some instances their effect is so striking as to have given rise to the term cure de raisins. The dried fruit is less acid, but more nourishing, and more demulcent. It possesses all the soothing qualities of jujube, and is much cheaper. It may be easily made into a conserve by removing the seeds and beating the pulp into a thick mass. For persons with irritable throats and liable to winter coughs, a portion of this put into the mouth before going into the open air is an excellent protective measure, and often prevents cough, which, when once excited, it is difficult to allay. An excellent demulcent drink is made from a compound of barley and raisins. Currants contain more acid than common raisins, and should be preferred where an aperient action is desired.

An oil exists in the seeds of the grape, in the proportion of 12 pounds of oil to 100 pounds of seeds. Though it is not obtained without difficulty, it is extracted in Italy in large quantity. When heat is used, it has a harsh taste, and is mostly used for burning; but when cold-drawn, it may be used for food.

Tannin of the purest kind may be obtained from the seeds of the grape.

Nearly all the raisins imported into this country are from Spain and Turkey. Of the total quantity imported, 99 per cent. is from these countries, namely, 64 per cent. from Spain, and 35 per cent. from Turkey. A small supply is received from Portugal, Italy, and the Cape of Good Hope.