SKYLIGHT. In the arts, the word skyli' 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, skylight is understood to mean the view of a portion of the atmosphere itself, or, in other words, an uninterrupted view of the portion of apace 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, ANGRY.
Skylights, 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 sides. 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 trations of the use of lantern lights; the Pantheon at Rome. and the Pantheon in Oxford Street, London, are illustrations of the use of simple skylights; the other dames of skylights are principally used in commercial or manufacturing establishments, although the double ones are susceptible of being treated so as to produce very. striking architectural effeets. In mediaeval 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 walla.
The practical advantages of the respective kinds of skylights seem to be. as follows :—Iu lantern lights, it is easy to provide for effecting — — — ventilation at the rams time and by the same means as those used for the admission of light. The windows in there cases are on the vertical aides of the lantern. and the to 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 circumstances, on the vertical glass of the sides, and It can 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 should be the object of the architect to keep them within the width susceptible of being covered by one sheet of glass; because if the length of the payee be such as to require the use of two sheets, the drops of water arising from condensation are likely to accumulate at the line of junction, and either to fall from thence or to remain under the lap, and if a frost should occur whilst water is it is very probable that the expansion of the water passing into, ice may crack the glass. The
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 must, it 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 modern buildings it is found that the lanterns are invariably placed over their centres, whilst the skylights let into the sides of roofs are disposed in the panels or segmental divisions in such wise as to cause the light to fall in the desired manner on the aide walla. 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 from above. it would be preferable to introduce lanterns; whilst in picture galleries plane skylights 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 affects 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 3u 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 arca, 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 Descrizione del Tempio Vaticano; states that an examination of the most important buildings lighted by lanterns showed that the beat 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.