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Venice, he proceeded to Rome, where he was soon noticed head of Goliah, before the monogram of Marc Antonio was by Raffaelle, who employed him in engraving from his de- added, a copy of which produced 451. at the sale of the late signs, and, it is said, in some instances even traced the Sir M. M. Sykes, Bart., in 1824; the Madonna lamenting outlines on the plates, that the correctness of the drawing over the dead body of Christ, called 'La Vierge au bras might be more perfectly preserved. His first plate after nud,' from the circumstance of having one arm naked, a print Raffaelle was the Death of Lucretia, which is neatly en of much value, a copy of which feiched at the same sale 251., graved, but is not one of his best works. His next print, whereas the other print of the same subject, which has the after the same master, was a Judgment of Paris, executed arm draped, engraved also by Marc Antonio, produced only in a more bold and spirited style. These were followed by 21.; the Massacre of the Innocents, with the chicot-tree, many

more, and amongst them the Murder of the Innocents, the Martyrdom of St. Lawrence, a subject mentioned above, after Raffaelle, who was so perfectly satisfied with the efforts a first impression with the two forks, of the estimated value of the engraver, that he sent many, specimens of his works of a hundred guineas, a very inferior copy of which, as to as complimentary presents to Albert Dürer himself, by whom condition, produced at the above sale 461.; the Pest, a proof they were thought well worthy of acceptance.

taken before the letters were engraved, of which only ihree After the death of Raffaelle, which occurred in 1520, Giulio copies are known to exist; and the Dance of Cupids, a small Romano engaged Marc Antonio to engrave from his designs. plate, which, if in good condition, is valued at 601., a copy of Amongst these works are a set of disgusting plates of sub- which sold at the sale before alluded to for 577. jects for which Aretino composed the verses, and which so There does not appear to be any certainty as to the exact greatly excited the indignation of Pope Clement VII., that time of the birth of Marc Antonio. M. Heinecken observes he ordered the engraver to be thrown into prison, from which that Vasari alone has given us any notice of him. Borgo he was only released at the earnest intercession of some of hini, Malvasia, and Baldinucci, as well as all the later writers, the cardinals and of Baccio Bandinelli. Moved by grati- draw from Vasari all that they say. None of them mention tude for the services of Bandinelli, Marc Antonio engraved either the year of his birth or death. If however the piece his celebrated print of the Martyrdom of St. Lawrence from representing Apollo and Hyacinth was engraved by him ir. a picture by him, which, besides greatly conducing to the 1506, at the age of nineteen years, as the signature would engraver's high reputation, procured him not only the seem to indicate, for the nine only is legible in the impression entire pardon of the pope, but his active protection and in the Imperial collection at Vienna, we may infer that he support. On the sacking of Rome by the Spaniards, in was born at Bologna in 1487 or 1488. (Heinecken, Dict. 1527, he was obliged to tly, having lost all he had acquired des Artistes; Biographie Universelle ; Strutt's and Bryan's by his art. He returned to his native place, where he con- Dictionaries ; Bartsch, Le Peintre Graveur.) tinued to engrave until the year 1539, which is the date RAIN. The antients appear to have been very imperaffixed to his last plate, representing the Battle of the La- fectly acquainted with the constitution of the atmosphere; pithæ, after Giulio Romano He is said by Malvasia to and Descartes is probably the first who, in attempting to have been assassinated by a Roman nobleman for having, refer meteorological phenomena to their causes, has apcontrary to his engagement, engraved a seeond plate of the proached near the hypotheses now generally received ; for Murder of the Innocents, from the design of Raffaelle. he ascribes the formation of clouds, snow, rain, and hail to

This engraver may be considered one of the most eminent variations of temperature in the upper regions of the air. artists in that branch that has ever appeared. His outlines He supposes that when the coldness in any portion of those are pure; the character and expression of his heads beauti- regions becomes intense, the subtle matter disseminated ful; whilst the exact and correct drawing of his works, par- | among the particles of vapour becoming too weak to keep ticularly in the extremities of his figures, evidence that he those particles at a distance from one another, the latter was in all respects a complete master both of drawing and must rush together, and either form small spicular filaments design. He was of the first Italian engravers of or spherical drops of ice. The superficies of these filaments distinction. The high reputation of Raffaelle, and the or drops being supposed to be considerable when compared happy chance which conduced to the engagement of Marc with their volumes, he conceives that the resistance of the Antonio as the engraver of his chief works, contribute as air may be great enough to prevent them from descending well to his reputation as to the high value which is ever set by their weight, and that thus a great assemblage of them upon his engravings, and the great price they always obtain. may remain suspended in the form of a cloud above the Berghem paid sixiy florins for an impression of his Murder earth. The filaments becoming by an accession of heat of the Innocents; and one of Saint Cecilia was sold at the partially liquefied, it may happen that many of them will sale of St. Yves for six hundred and nineteen francs. M. adhere together, and thus form flakes of snow, which, at Ponce has given the date of his death as 1546; but M. Hei- length acquiring sufficient weight to overcome the resistnecken seems to consider that the date upon the Battle of ance of the air, descend to the ground. In order to explain the Lapithæ was about the period at which he ceased to the origin of rain and hail, he supposes that the flakes, on wor Some of his prints are marked with an A. and an arriving near the surface of the earth, may pass through a M. joined, and others with M. A. F. also joined, the F. being warmer region than that in which they were formed, and used in consequence of the cognomen La Francia having there dissolving, they assume the figure of spherical or been given to him from his successful study under Raibolini; spheroidal drops of water. Again, if in the descent the and some are marked with the tablet mentioned by M. latter should meet a current of cold air, they become gloHeinecken.

bules of ice. (Meteora, cap. v., vi.) The works of Marc Antonio are exceedingly numerous. The diffusion of the electric fluid through the earth and A very copious catalogue of them is given in the work of atmosphere has led some meteorologists to believe that the the last-named author, which extends to a hundred and variations in its quantity or intensity in particular regions twenty-five pages. Mr. Bryan observes, that in the prints may be the cause of the formation of snow, rain, and hail. of this eminent artist great attention sbould be paid io the The electrical particles, being endowed with a great repulsive different impressions of the plates, which have been greatly power, are supposed to keep in general the particles of vapour retouched and altered by the different printsellers through asunder; and when, from any cause, some given volume whose hands they have passed. The best impressions are of air is deprived of its natural quantity of electricity, these without the name of any publisher. After the plates were particles unite by their mutual attractions, and thus form taken from the stock of Tommaso Barlacchi, they came into drops of rain or ice. From the showers which accompany a the possession of Antonio Salamanca; afterwards they thunder-storm, there is no doubt that electricity co-operates passed through the hands of Antonio Laferri, from thence in some measure in the production of rain; and it may be to Nicholas van Aelst, and lastly became the property of remarked in support of the above hypothesis that rain is Rossi, or De Rubeis, at a time when they were nearly worn most abundant among inountains, their elevated summits out.

being favourable for receiving and discharging, electricity; In the Print Room of the British Museum there is a very while in some regions where thunder is little known there fine collection of the works of Marc Antonio. They amount is also little rain. to about five hundred, the whole of his labours enumerated But the theory first proposed by Dr. Hutton of Edinburgh by Bartsch being six hundred and fifty-two; but it must be (Phil. Trans., Edin., 1784) is that which appears to correrecollected that the works of two of his principal pupils, spond most satisfactorily to the observed phenomena of the Agostino Veneziano and Marco da Ravenna, are counted atmosphere; and accordingly it has been adopted by nearly with them. Amongst those of the greatest rarity are the every distinguished meteorologist since that time. This Transgression of Adam and Evc; David cutting off the theory will be briefly described.

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The atmospaere surrounding the earth is known to con- ( varying also with the temperature, there would arise by sist of air and aqueous gas or vapour, both of which are evaporation at the equator a current tending from thence to elastic; and, according to the experiments of M. Gay-Lussac, the poles, and this, being condensed in its course, would return the elasticity of the vapour is equal to that of the air at an from the poles towards the equator in the form of water. The equal temperature, both when the vapour exists alone, and condensation thus going on would cause the atmosphere to when it is combined with the air; hence it is inferred that be constantly charged with clouds and rain. Unless how in the atmosphere the vapour and air are in mechanical ever the excess of temperature at the equator were mainunion only, and also that the particles of the former have tained by some foreign power, as solar radiation, the tempethe power of moving freely in the intervals between those of rature over the whole sphere would by degress become the latter. The atmosphere is supplied with humidity by equalised; the equatorial parts becoming cooled by evapoevaporation from the waters of the earth, and its power to ration, while the polar regions would become warmed by hold the water in solution depends on its temperature, an the condensation. increase of the latter augmenting that power, and a decrease Mr. Daniell afterwards contemplates an atmosphere condiminishing it: but in the theory of Hutton, the diminution sisting of a permanently elastic fluid mixed with aqueous of the power takes place in a higher ratio than the diminu- vapour, and surrounding a sphere of water of uniform tempetion of the temperature.

rature; and he observes that, since the evaporation would be Now the quantity of moisture in the atmosphere will at slow, the small quantity of water precipitated would be almost all times be nearly equal to the greatest quantity that can immediately dissolved by the superior temperature of the be maintained in it in a state of vapour at the existing tem- stratum below, into which it would tend to fall; therefore perature. Therefore if two volumes of air thus saturated this atmosphere would be free from clouds. But in the with moisture, but of different temperatures, become by any event of the temperature of the sphere increasing from the means mixed together, a mean degree of heat results from the poles to the equator, the evaporation in the latter region union ; but the whole quantity of moisture in the sum of the would destroy the regular gradation of temperature in the volumes of air will, agreeably to the theory, be greater than atmosphere from the surface of the sphere upwards; the that which is due to the mean temperature, and the excess evaporated water rising to the middle regions would there, will of course be condensed or precipitated. The vapour so in consequence of the dininished temperature, give out its condensed forms a cloud; and if this be specifically heavier latent heat, and become condensed ; then descending, it than the air in wbich it is formed, it will begin to sink. would acquire from below a new portion of heat, with which Should the atmosphere near the earth be less dense than it would rise till it was again forced to part with its caloric. the cloud, the latier will continue to descend till it touches This process may be supposed to continue till those regions the ground, when the aqueous particles, if small, will form of the atmosphere become saturated with vapour, and at what is called a mist; or if large, and particularly if the con the same time rarified by the heat. The rarefaction of densation of the vapour has been rapid and copious, they the air would diminish its resistance to the general will descend by their gravity in rain, snow, or hail

, accord movement of the vapour towards the poles, and thus the ing to the temperature of the region through which they vapour would rush with force in those directions; but on pass. It may happen however in the descent that a cloud arriving in latitudes at which the temperature is too low to arrives in a warmer region than that in which it was formed; allow the air to hold it in solution, condensation would take in this case the condensed moisture may again become place, and clouds would be formed. vapour, and the cloud may re-ascend to a region at which The circumstances just mentioned correspond nearly to new condensation takes place. But though it be true that those which would take place about the earth if local and some precipitation must follow, whatever be the difference other accidental circumstances did not interfere with the between the temperatures of the two volumes of air, yet general process. In its actual condition, when a column of unless the mean of the two quantities of vapour should be air vertically over any place is from any cause heated more greater than the quantity necessary for complete saturation than the neighbouring columns, it begins to ascend by its at the mean of the two temperatures, the precipitation will diminished specific gravity, the colder air of the vicinity Dot be perceptible in the form of rain.

flows in to fill up the void, and thus the relation between the In order to illustrate the general subject of clouds and temperature and humidity at the place is deranged. Then, rain, Mr. Daniell, in his • Essays on the Constitution of the agreeably to the general iheory of Dr. Hutton, a precipitaAtmosphere,' supposes, first, that the earth is a sphere of tion of the vapours takes place. uniform temperature, and surrounded by an atmosphere of In proportion to the density of the vapour, the magnitudes dry and permanently elastic fluid; and he shows that on of the condensed particles of water are greater; in the upper this supposition the density of the air would diminish in a regions of the air the cloud assumes a light appearance, but geometrical progression at elevations increasing by equal below it is more dark. After their formation, the clouds increments. He observes also that the temperature would are driven about by the winds, receiving new accessions of decrease with the densities, and that the atmosphere would precipitated vapour till the air is no longer capable of supbe constantly in equilibrio. This would continue to be the porting them, and then their substance descends in rain, case if the general temperature of the sphere were to be in- snow, or hail. creased, provided that increase were uniform at all points on On the supposition that the surface of the earth is withits surface. Now, if the temperature of the sphere, instead out inequalities, and that the temperature gradually dimiof being uniform, were supposed to increase from the poles nishes from the equator towards either pole, it should towards the equator, the unequal densities produced in ver- follow that the rarefaction of the air and the evaporation of tical columns of the air by the differences of temperature at the water, and consequently the quantity of rain, must equal heights above the surface of the sphere, would give diminish according to some law with the distances of places rise to lateral pressures which, in the lower strata, would from the equator. Now the mean temperature in any latiproduce currents tending from the poles towards the equator; | tude being known, the quantity of moisture in the atmobut the elasticity of the air, which is constant near the sur spherical column at that latitude can be found, since it face of the sphere, varies with the height above that surface, depends on the temperature; hence knowing {lso the according to such a law that, beyond a certain elevation, it variations to which the temperature of the atmosphere at would produce lateral pressures exceeding those which arise | the place is subject in the course of the year, the mean from the density in the neighbouring columns at equal annual depth of rain in that latitude may be computed. On altitudes, and thus there would arise a current in the such principles Humboldt has determined that the mean upper regions flowing continually from the equator towards annual depth of rain should be, at the equator, 96 inches; the poles.

in lat. 45°, 29 inches; and in lat. 60°, 17 inches. The cir He supposes next that the sphere is covered with water cumstances however which render the temperatures in everywhere of equal temperature, and is surrounded by an different latitudes, and even on the circumference of the atmosphere of pure aqueous vapour; and he shows not same parallel, irregular, must produce irregularities in the only that the density of this vapour would diminish upwards quantities of rain which fall at different places; yet the according to the law before mentioned, but that the atmo- results of observation show that, in proceding from the sphere would in this case also be in equilibrio and trans- equator towards the north pole, there is in reality a diminuparent even when the general temperature of the sphere tion in the mean annual quantities of rain. experiences a uniform increase. But if the temperature of From an average of the observations made during fourthe sphere were to increase as before, from the poles teen years (1810 to 1823 inclusive), the mean annual depth towards the equator, the density and elasticity of the vapour' of rain on the Malabar coast is 123.5 inches, and the mean

annual temperature is 80.4° (Fahrenheit), but the annual | during the second half year than during the first. The depths of rain are very irregular and differ considerably. means of observations continued during 40 years at London From a mean of observations for seven years (1817 to 1823 give, for the depth of rain from January to July inclusive, inclusive), the mean annual depth of rain at Bombay was 8.539 inches, and from July to December inclusive, 12:147 only 85.24 inches. From a mean of observations during inches. seventeen years (1802 to 1818 inclusive), Mr. Dalton makes In general the lowest stratum of air about the earth conthe mean annual depth of rain at Manchester equal to tains the greatest quantity of water in solution; and hence 33.596 inches, the mean annual temperature being 47.6°; it might be expected that more rain should fall on low level and here also the annual quantities of rain vary very irregu- plains than in elevated countries. The contrary however is larly. The same meteorologist estimates the average of the the fact: and this may be accounted for by the variety of annual quantity of rain in England to be 31.3 inches; the currents among mountains, and by clouds resting frequently greatest quantity being at Keswick in Cumberland (767.5 on the summits of hills without descending to the plains. inches), and the least at Upminster in Essex (=19.5 inches); While the average annual depth of rain at Keswick is 67.5 but it is supposed that this estimate of the mean quantity inches, in the interior of the country and on the sea coast it is higher than the truth, because too many of the observa- is but 25 inches: and while the average depth on the St. tions were made in the maritime counties, where the atmo- Bernard is 63:13 inches, that at Paris is 20 inches only. sphere may be expected to be the most humid. In regions Yet, from the observations of Dr. Heberden, Mr. Howard, where the trade-winds blow constantly, rain seldom falls; and M. Arago, it appears that the depth of rain on the and the reason may be, that both the temperature and the level of the ground is greater than at the top of a building. currents of air being there nearly uniform, the vapours The first of these philosophers found that the annual depth raised from the ocean are carried about the earth without at the top of Westminster Abbey was 12.099 inches, while suffering those partial accumulations by which condensation at a lower level, on the top of a house in the neighbourhood, and precipitation might be produced. But elsewhere the it was 18°139 inches; and on the ground, in the garden of irregular distribution of land and water, the existence of the house, it was 22-608 inches. M. Arago observed, from mountain-chains, and even the various capacities of different observations during twelve years, that on ihe terrace of the parts of the earth's surface for absorbing or communicating Observatory at Paris the annual depth was 50•471 centiheat, independently of variations in the electricity of the metres (19988 inches), wbile in the court of that building, air, are to be considered as the most frequent causes of per- which is 28 metres (30 yards) lower, the annual depth was turbation in the general currents of the atmosphere, and 56 371 centimetres (22-21 inches). consequently of the fall of rain.

Mr. Howard has observed that, in this country, when the The dense mists which rest on the ocean near Newfound- moon has south declination there falls but a moderate land are precipitations caused by inequalities in the tempe- quantity of rain, and that the quantity increases till she has rature of the ocean in the line of the Gulf-stream. In ihe attained the greatest northern declination; and on some year 1821, in consequence of very strong winds between the such results of cbservation the popular opinion that there is tropics during the summer having caused an extraordinary a connection between the alternations of rain and fair weadifference between the levels of the waters in the Gulf of ther and the changes of the moon may be founded. Our Mexico and those of the Atlantic Ocean, the stream of knowledge of the variations to which the temperature of the warm water was found to extend eastward of the Azores; air is subject, is however yet too imperfect to allow much. and it deserves to be remarked that this unusual circum- dependence to be placed on predictions relating to the stance was attended, both in France and in England, by a weather which are formed from the moon's phases, or even very hot and damp winter, together with an excessive fall of from variations in the state of the barometer or hygrometer. rain. (Sabine, Experiments on the figure of the Earth, RAIN, FOSSIL. Singular as may appear the notion 1825.) The rains which frequently deluge the tropical that the impressions of rain should be recognisable and be islands are in part produced by ihe volumes of air which are recognised on the surfaces of stratified rocks, the opinion intermingled by the sea and land breezes; and those which is held by some eminent geologists, on the evidence of specifall at the time of the summer solstice in Africa may be mens of new red-sandstone taken from the Siorelon ascribed to the immediate precipitation of the vapours which Quarries near Liverpool. In March, 1839, Mr. Cunningflow from the seas to supply the place of the rarefied air ham, to whose researches in the Storeton quarries we are above the heated lands: while the drought which prevails indebted for much of our knowledge of the foot-prints of in the sandy deserts that quarter of the earth is explained Cheirotheria and other antient animals, communicated a by the level character of those deserts, over which the cur- paper on the subject to the Geological Society of London. rents of air may be supposed to flow nearly without inter- In examining some of the slabs of stone extracted at the ruption.

depth of above 30 feet, Mr.Cunningham observed that their From April to October, the winds blowing from the south- under surface was thickly covered with minute hemispheriwest towards the coast of Malabar are accompanied by heavy cal projections, or casts in relief, of circular pits in the imrains, and the circumstance may be accounted for by the mediately subjacent layers of clay. The origin of these vapours of the ocean being brought from a warm region to marks, he is of opinion, must be ascribed to showers of rain, one which is less so, and consequently becoming there con- which fell upon an argillaceous beach exposed by the retirdensed and precipitated. On the other hand, the prevailing ing tide, and their preservation to the filling up of the inwinds on the coast of Peru, being from the south and south- dentations by sand. On the same slabs are impressions of the west, come from a cold to a warmer region; consequently a feet of small reptiles, which appear to have passed over the diminution of the degree of saturation must there take clay previously to the shower, since the foot-marks are also place, and the vapours remain suspended ; accordingly it is indented with circular pits, but to a less degree, and the diffound that rain seldom falls on that coast. The clouds ference Mr. Cunningham explains by the pressure of the which overhang the coast of Malabar during the monsoon animal having rendered these portions less easily acted above mentioned are arrested by the chain of the Ghauts, upon. If these impressions on the clay be really the marks and while it rains on the western side the fair season is of rain or hail (a specimen is before us, and it certainly enjoyed on the coast of Coromandel. Again, the currents resembles such impressions on clay), perhaps the easiest of air which pass over Peru, in crossing the chain of the way of comprehending the preservation of them is to suppose Andes, where the temperature is lower, become condensed by dry sand drifted by the wind to have swept over and filled the cold, and the rain is there precipitated in abundance. up the foot-prints, rain-pits, and hollows of every kind The vapours which come from the Atlantic ocean, and pass which the soft argillaceous surface had received. (Geologiover the south-western counties of England, must be more cal Proceedings, 1839.) abundant than those which arrive there from the continent RAIN-GAUGE, a vessel for measuring the quantity of of Europe; and from observations made at Penzance, the rain which falls on any particular part of the earth's rains which accompany the westerly winds at that place surface, the quantity being indicated by the depth of the exceed those produced by the easterly winds in the ratio of precipitated water which would cover the ground about the about three to one.

spot, supposing the ground to be horizontal and that the In tropical regions the quantities of rain which fall in water could neither flow off nor penetrate into the soil. different months of the same year are very unequal: at In order to ascertain the quantity of rain which has fallen Bombay the mean monthly depih in June was found to be during the continuance of a shower, it might suffice to place 24 inches, and in October, 1.26 inches. In temperate a prismatical or cylindrical vessel, open at the top, in a clicates the quantities differ much less, but more rain falls horizontal position on the ground or on the top of a build

ing, and, when the shower has ceased, to measure the depth will be expressed by a space jual to four inchos ou the of the water in the vessel by a scale of inches. But, unless length of the rod; and, each of such spaces being divided the depth were ascertained immediately, a portion of the into 100 parts, the depth of water at the said section will be water would be carried off froin such a vessel by evaporation, indicated in hundredths of an inch. The height of the and the measure would be less than it ought to be. The cylindrical vessel below the funnel may be from 25 to 30 difficulty also of ascertaining the true amount of a small inches. depth of water would render the instrument of no practical For the sake of diminishing the evaporation and of meause. For the purpose therefore of obtaining a more correct suring small quantities of rain with greater precision, the estimate of the quantity of rain, it has always been the diameter of the cylinder is sometimes reduced io two inches, practice to receive the water in a second vessel, or in a and the collected water is, by means of a small pipe, intube, the area of whose horizontal section is less than that serted in the bottom of the cylinder, and furnished with a of the first, so that the height of the column may be greater. cock made to pass into a glass tube whose interior diameter And, since the heights of equal quantities of water in two is half an inch. In this case, the diameter of the upper exprismatical or cylindrical vessels are inversely proportional tremity of the funnel being the same as before, the area of to their bases, it is easy to perceive how a rod may be the surface which receives the rain from the atmosphere graduated so as to show, in inches, the depth of water in the will be to the area of a horizontal section of the glass tube upper vessel, and consequently the depth which would have as 576 to 1. Consequently a shower of rain whose depth sain on the ground if no absorption had taken place. on the ground might be one-hundredth part of an inch

Originally this instrument, which has been called in- would be indicated by 5.76 inches in the tube. differently udometer (lowp and pétpov), pluviometer (pluvia), The funnel of the cylinder may be of tin or copper, and, and ombrometer (õubpos, rain), was nothing more than a however the instrument be constructed, it is evident ihat it prismatical box, having a square base, open at the top and should be placed in a vertical position in some place where communicating with a prismatical box, placed vertically no object may interfere with the free descent of the rain into under it, by means of a pipe open at both ends; the area the funnel. It is usual to observe the quantity of water in of a horizontal section of the lower box being, for the the vessel every morning, if rain has fallen during the prereason above given, less than that of the upper box. ceding twenty-four hours; but, as some evaporation will But it is evident that a prismatical or cylindrical vessel take place, it would be advantageous to make the observamust retain, by adhesion to its sides and bottom, a sen tions more frequently. sible portion of the water which enters it ; and conse The sum of all the depths observed during any period of quently the depth measured in such vessel must indicate time, as a day, month, or year, will give the whole quana quantity of rain less than that which has really fallen; tity of rain which has fallen in that time at the place. 'It is it has therefore been customary of late to make the upper supposed that the rain falls uniformly over the tract of part of the vessel in the form of a funnel, or inverted ground lying within the limits of the shower, and consecone.

quently that the quantity which passes through the circular The most general construction of a rain-gauge is shown area at the upper surface of the cone is equal to that which in the subjoined diagram, which represents a vertical sec- 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 heaveus 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

l'he spectator and produced towards the opposite part of the tion of the instrument. The part CDE is a conical funnel, heavens. Both bows consist of concentric bands of the difopen both at top and bottom, and the lower extremity ferent prismatic colours arranged as they appear in the solar enters into the cylinder FG below, which thus receives spectrum, but the order in which they are disposed in the the rain from the funnel. The rod AB passes through first bow is inverted in the second. The lower edge of the a perforation in a bar CD (in the direction of a diameter interior bow is violet and the upper edge is red; on the conof the cone at its upper surface), and is attached, at B, trary, the lower edge of the exterior bow is red and the to a circular piston, which has nearly the same diameter upper edge is violet. as the interior of the cylinder: the weight of the piston The rainbow is a phenomenon which appears at all times and rod is such as to allow the former to float with its to have been understood to depend upon the light of the upper surface on a level with the surface of the water ; sun or moon and the drops of falling rain, but the first comand the graduations, which are numbered towards B, plete explanation of the circumstances connected with it is commence from a point a on a level with the upper sur- due to Newton (Optices, lib. i., p. 2, prop. 9). In the beface of the bar CD, when the piston B touches the bot-ginning of the sixteenth century no better notion was entertom of the cylinder. A rim, of a cylindrical form, rises a tained of the cause of the phenomenon than that the inlittle way above the upper extremity of the conical part of terior bow was a distorted reflection of the sun's image from the funnel, in order to prevent the rain-water, which would the surface of a cloud, and that the exterior bow was a strike the interior of the latter near that extremity, from reflected image of the first. But the retlection of light is being thrown out in consequence of the shock.

not capable of producing different colours, and it is said The diameter of the funnel at the top may be 12 inches, that Fleischer of Breslau (1571) was the first who entertained and that of the cylinder six inches; in which case the area the idea that the particles of light from the sun entered into of the horizontal' section on which the rain falls will be to the drops of rain. His opinion was that a ray of light sufthat of the cylinder in the ratio of four to one. Hence a fered one refraction on entering and another on leaving a depth of water equal to one inch at the horizor tal section drop; and that it entered the eye of the spectator after

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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

Fig. 2. refraction, and that this ray, being subsequently reflected at the interior surface of the latter, might anter 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

X 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 after crossing at X and being reflected from B may pass more than a vague conception of tbe cause of the colours. from B to C in parallel directions; then, after a second (See Montucla, Histoire des Math., tom. ii.)

reflectión, crossing at Y and being refracted at K, they will Descartes is certainly the first who has distinctly ex emerge in parallel directions as they entered at I, so that if plained the causes by which the two bows are produced, and KE be the direction of the emergent pencil, the angle DKE he states (Meteora, cap. viii.) that he detected those causes will be equal to AIS: the angle made by the lines SI and on observing the phenomena presented by a glass globe EK was found by Descartes to be about 52 degrees. If the filled with water, which he placed in various positions with angle AIS were varied, the rays of the pencil would leave respect to the sun. He shows that the interior or primary the drop in a divergent state. bow is produced by rays from the sun falling upon the Now let A, B, C, D (fig. 3) be four globules of rain in a 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

D the lower surface, and, on qunting the drop, they suffer a

Fig. 3. 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 tnence 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

B 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 pro

Red duce 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 i fig. 1) be a very slender pencil of rays of some one colour incident on a spherical drop of water at the

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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 Fig. 1

through the eye of the spectator, and let it be produced towards 0; 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 angle AIS, and let this angle be such that thé rays in the the angles CEO or DEO were nearly equal to 52°, the eye pencil may, by the laws of refraction in water, converge at would be affected by the sensation of brightness as explained B; then, ihough many rays will pass through the drop at above; therefore if the lines AE, BE, &c. were to revolve that point and be dispersed, yet many will be reflected from conically about EO as an axis, ali the globules of rain upon thence as from a radiant point, and will emerge at K in the conical surfaces so described would send pencils of paralparallel directions, as they entered at I, so that if KE be lel rays to the eye, and two concentric arches of bright ihe direction of the emergent pencil, the angle CKE will be light would be seen in the heavens. This hypothesis equal to AIS: the angle made by the lines ŠI and EK pro- accounts satisfactorily for the existence of two concentric duced was found by Descartes to be about 42 degrees. If the bows of bright light, but it affords no indication of the bands angle AIS were varied, the rays of the pencil would leave the of colours of which they consist. Descartes however very drop in a divergent state, and then the impression wbich sagaciously refers their cause to the decomposition of light they would make on the eye might be too feeble to produce on entering and quitting the drops of rain; observing that the sensation of brightness. Again, let si (fig. 2) be a the convex surfaces of the latter must produce effects similar very slender pencil of rays of some one colour incident on a to those which take place when light is made to pass spherical drop of water ai the angle AIS, and let this angle through the plane faces of a triangular prism of water. be such that, by the laws of refraction in water, the rays But when Newton had discovered the different degrees of

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