SKYLliiHT. 



sl.XNI'KH. 



ruth. On the top of high mounUini, or in balloon* at great eleva- 

 Uoo>. the proportion of blue ray* which, after reflection in the atmo- 

 sphere enter the eye u very great ; and the blueneas at Irngth become* 

 a deep black ground on which the (tan appear to xlum- at all time* 

 with 03 much lustre a* at midnight on the earth, during the alwenoe of 

 the moon. It U hence evident that if it ,!- n <t for the innumerable 

 reflection* of the lulu from tin- nun or m i, whii-h take place in the 

 atmosphere, total darkuea* would prevail from the instant of Kim-net 

 to that of *un-rise ; and even during the day, darkne** would ensue, 

 no that the itan would become vuible, every time that the nun U 

 obscured by a cloud of mifflcient denaity. 



That the blue tint of the aky i* caused by light reflected in the 

 atmoiphere, U abundantly evident from the fact that the light of the 

 iky U found to be polarized, this quality in light being a result of it* 

 reflection. Sir David firewater, who first made the observation, has 

 moreover ascertained that the light of the aky conaist* of two parts, 

 one blue and the other nearly without colour ; and he discovered that 

 tha*e light* are polarize 1 in ilirt'.-rent directions. (' Treatise on New 

 Philoaophical Instruments,' p. 349.) 



The hypothesis that the azure colour of the sky is caused by re- 

 flection* of blue rays, wan at one time objected to on the ground that 

 the shadow* of opaque object*, placed on white (taper and exposed to 

 the sun'* light, should always appear to be blue, since the part of the 

 paper which is in shadow can only be visible by the light of the sky 

 reflected from thence. This phenomenon is, in fact, frequently 

 obserred ; but H. de Saussure, while admitting that he has often per- 

 ceived the shallow* of object* to be bluish in the mornings and 

 evening* on the general surface of the earth, states that in Alpine 

 region*, where the sky i* intensely blue, the shadows of objects never 

 appear to be so : he adds that, of fifty-nine observations made for the 

 purpose of ascertaining the colours of shadows on the mountains, 

 thirty-four showed them to be a pale vi let, eighteen showed them to 

 be black, six a pale blue, and once they appeared to be yellowish. It 

 may be inferred, therefore, that shadows cast by opaque objects are 

 RO much affected by the colours of the neighbouring objects that a 

 right judgment can scarcely be formed of the colour which they 

 receive from the light of the sky. [ACCIDENTAL COLOURS.] To the 

 like interference must be ascribed the variously-coloured shadows 

 which were observed by M. Bouguer (' Essai d'Uptique;' and M. 

 Burton, ' Me'moires de I'Anadrfmie des Sciences,' 1743.) 



Between the tropics the transparency of the atmosphere is far 

 grevterthan it is, in general, in regions beyond them towards the north 

 or south ; hence the sky there is almost always serene and intensely 

 blue, while the clouds near the setting sun arc brightly tinted with 

 the prismatic colours. The skies of the south of Europe and some 

 part* of North America are distinguished fur their serenity and 

 beauty ; but, in the.<e respects, they are said to be inferior to the 

 skies over the islands in the Pacific Ocean. 



For the description of an instrument invented in order to measure 

 the intensity of the blue colour in the sky, see CYANOJJETKR. 



8KYLIOHT. In the arts, the word uti/li ht is sometimes used to 

 express the frame or the window by which the direct light from the 

 upper regions of the atmosphere is allowed to enter a room ; at other 

 times, and especially in cases connected with architectural jurispru- 

 dence, li-ylujht is understood to mean the view of a portion of the 

 atmosphere itself, or, in other words, an uninterrupted view of the 

 portion of space to which a house proprietor is entitled. In this 

 notice, attention will principally be directed to the former meaning of 

 the word ; for the latter meaning, and the various conditions attaching 

 to it. see the articles EASEMENT, and LIGHTS, ANCIENT. 



Skylight*, in the ordinary sense, may be divided into those which 

 are placed in the roof or covering of the building, and into those which 

 are placed in lanterns rising above the line of the roof ; the former 

 transmitting the light directly, the latter by means of vertical openings 

 in their side*. In some cases, also, a building may be lighted by means 

 of inclined skylights on the respective sides of the inclined roof over 

 the flat ceiling of the room, and by a horizontal skylight in that ceiling ; 

 or, in other words, by a double skylight, as at the Madeleine at Paris ; 

 and in the enormous rooms of the spinning-factories a species of 

 imperfect lantern skylight is used, in which one vertical glazed side 

 and three obscure sides of a half pyramid, are introduced. Small 

 vertical windows, or, as they are technically called, " dormers," are 

 occasionally used for the purpose of lighting domical or other buildings 

 from the sky ; but, strictly speaking, they must come under the cate- 

 gory of windows. The hall of the Middle Temple of London, the 

 dome of St. Paul's, and the Law Courts at Westminster, are illus- 

 tration* of the use of lantern lights ; the Pantheon at Rome, and the 

 Pantheon m Oxford Street, London, are illustrations of the use of 

 simple skylight*; the other classes of skylights are principally used in 

 commercial or manufacturing establishments, although the double 

 one* are susceptible of being treated so as to produce very striking 

 architectural effects. In medieval structures, such as Westminster 

 Hall, a portion of the light is obtained from the sky by means of 

 dormers ; but clerestory windows are much more frequently resorted 

 to in the buildings of this period, when all the light cannot be 

 obtained from the lower side walls. 



The practical advantages of the respective kind* of skylights seem to 

 be a* follows : In lantern light*, it i* easy to provide for effecting 



ventilation at the same time and by the same means as those used 

 For the admission of light. The windows in there oases are nn tin- 

 vertical sides of the lantern, and the top is covered by an opaque 

 roof, usually formed of imperfectly conducting materials; any con- 

 densation of moisture from the internal atmosphere takes place, under 

 these circumstance*, on the vertical glass of the sides, and i 

 thence be easily removed. Skylights placed on the slope or at the top 

 of a roof are exposed to considerable inconvenience from the condensa- 

 tion thus alluded to; and if they must be resorted to, it rhoiild be the 

 jbjectof the architect to keep them within the width Bii>.-.-|.iil.l,- of 

 being covered by one sheet of gloss ; because if the length of the panes 

 be such as to require the use of two sheet*, the drops of water mi, n^ 

 from condensation are likely to accumulate at the line of junction, aipl 

 either to fall from thence or to remain under the lap, and if a frost 

 i-hould occur whilst water is there, it is very probable that tin- 

 expansion of the water passing into ice may crack the glass. Th, 

 introduction, by Sir J. Paxton, of the ridge and furrow system, has 

 enabled modern architects to execute large plane surfaces of skylight ; 

 and it has, moreover, the advantage of being equally applicable- to 

 lantern or to flat lights of this description. Lantern lights in 

 may be added, be placed upon the axis of the roof covering the room 

 so lighted ; skylights may be placed wherever it may be desired so to 

 do ; and in the large domes of modem buildings it is fouml tli.it th.- 

 lanterns ore invariably placed over their centres, whilst the skylight* 

 let into the sides of roofs are disposed in the panels or seginental 

 divisions in such wise as to cause the light to fall in the desired 

 manner on the side walls. Lanterns diffuse light more equally over 

 the areas they serve than lights let into the sides of the roof, but not 

 more so than central skylights, such as those of the Pantheon ; and 

 for this reason it seems that, in sculpture galleries lighted fnmi 

 above, it would be preferable to introduce lanterns ; whilst in picture 

 galleries plane skylight* on the slopes of the roof are the most 

 advantageous, provided always that they do not receive the direct rays 

 of the sun. 



There are, indeed, few positions in which it is desirable to admit the 

 sun's rays in rooms lighted from above ; and wherever it is possible so 

 to do, the light should be admitted from the north exclusively, because 

 the glare and the reflection of the sun's rays affect* the purity of colour 

 of the objects exposed to them. In factories, and in show rooms, this 

 law of excluding the direct sun's rays is carefully observed, and the 

 skylights formed over them are usually made of the form above 

 described; that is to say, as a section of a square pyramid, receiving 

 light from its base which faces the north. The proportion of the 

 surface for the transmission of light to the opaque part of the ceiling 

 should be at least as 1 : 20 ; for in the Pantheon the diameter of the 

 rotunda is about 141 feet, and that of the central light is 30 feet (or 

 in the ratio of their areas as 1 to 22 nearly), and the light in that 

 building would not be sufficient for the purposes of commerce or of 

 manufacture. The height of the room will, however, affect this con- 

 sideration ; for the pencil of rays admitted through the skylight must 

 be able to reach every portion of the inclosed urea, and in a well-lighted 

 room there should be no necessity for trusting to reflected rays. In 

 the Pantheon, the height from the floor to the under side of the sky- 

 light is precisely equal to the diameter of the rotunda ; and Fontana, 

 in his ' l)escrizione del Tempio Vaticano,' states that an examination of 

 the most important buildings lighted by lanterns showed that the best 

 proportions of the diameters of those structures to the diameters of 

 the cupolas on which they rested were as 1 to 6, and that the height 

 of such lanterns should be equal to half the diameter of the cupola ; 

 the whole of this height is not, however, devoted to the glazed part of 

 the structure. 



SLAKED LIME. [CALCIUM.] 



SLAN'DKK consists in the malicious speaking of such words as 

 render the party who speaks them in the hearing of others liable to 

 an action at the suit of the party to whom they apply. The mere 

 speaking of the defamatory words instead of the writing of them is 

 that which constitutes the difference between libel and slander. 

 [ LIBEL.] 



Slander is of two kinds : one, which is actionable, a* necessarily 

 importing some general damage to the party who is slandered ; the 

 other, which is only actionable where it has actually caused some 

 special damage. The first kind includes all such words as impute to 

 a party the commission of some crime or misdemeanor for which he 

 might legally be convicted and suffer punishment, as where one asserts 

 that another has committed treason, or felony, or perjury. It also 

 includes such words spoken of a party, with reference to his office, 

 profession, or trade, as impute to him malpractice, incompetence, or 

 bankruptcy ; as of a magistrate, that he is partial, or corrupt ; of a 

 clergyman, that " he preaches lies in the pulpit ;" of a barrister, that 

 " he is a dunce, and will get nothing by the law ;" and so on : or that 

 tend to the disherison of a party, as where it is said of one who holds 

 lands by descent, that he is illegitimate. Where a party is in posses- 

 sion of lands which he desires to sell, he may maintain an action 

 against any one who slanders his title to the lands ; as by stating 

 that he is not the owner. With respect to the second kind of slander, 

 the law will not allow damage to be inferred from words which are 

 not in themselves actionable, even although the words ore untrue and 

 spoken maliciously. But if, in consequence of such words being so 



