hemisphere. It may seem paradoxical that it should be less there than at 52 ; but the explanation is that the tension bears a higher proportion to the weight in a thin dome than a thick one, and it was an infinitely thin one which had the tension of - 3 of its weight at 52; and such a dome cannot be carried lower without bands. In a dome of the required thickness ties would have very little to do
above 68°.As the tension at the bottom is rather more than a fifth of the weight, a dome of proper thickness would be stable standing on a conical drum, with a slope inwards of about 1 to 5, or 12, of which the tangent is 215, if the drum itself has foundations which cannot spread. The thickness requisite, and also the tension at the bottom, may evidently be greatly diminished by gradually tapering the dome upwards. If it is half as thick at the top as at the bottom, with the thickness increasing downwards as the height, it need only weigh -$ of the lightest uniform dome of the sime size, and only need be 20 inches thick at the bottom for 100 feet diameter.
Pointed domes are also much stronger than hemispheres, having lost the flat top which has the greatest bursting pressure. A dome generated by the revolution of an equilateral arch, or one of 60, requires a thickness only=0137 diameter, or 16/ inches for 100 feet; and one of 70 requires 20 inches. The tension at the bottom of a 60 dome is only 15 of its weight, which weight, how ever, is T372 of a hemisphere on the same base, their heights being as 1 73 to 1.
For the same reason pointed domes are fittest for carry ing a lantern, but they are not much benefited by tapering, having already lost the most oppressive part. The Florence dome, across the flat sides of the polygon, is about 70 of the circle of its curvature. It is shown in the R. I.B.A. paper that both in hemispherical and pointed domes the weight of the lantern they will carry varies practically as the cube of the thickness. Moreover a lanterned dome requires tieing much higher up than a plain one. In short, the cone is the only proper way of carrying a stone lantern. The cone at St Paul s has a great chain round the base, which is probably superfluous, as the drum below it seems thick enough to contain the requisite slope, and visibly leans in wards besides.
Ribs inside a dome weaken more than strengthen it, as some persons imagine, unless they are themselves deep enough to be stable as independent arches, or unless they decrease in width and weight upwards like a luue, as those in the Pantheon do, which also is so enormously thick at the haunches that it has superabundant stability. Some of the Indian domes are thick enough for arches, and they have neither eyes nor lanterns. Polygonal domes may be considered as composed of a small number of widish lunes, and only differ from round ones in being rather weaker for any given thickness and size.
Domes require no wooden centring to build them on as arches do, until you get near the top, i.e., so long as each stone laid on the ring of stones below it will not slide inwards. And if they are notched to prevent sliding the whole dome may be built without centring. The dome of Mouata in Malta was so built in this century by a common mason, who must, however, have been a man of genius. There would be no difficulty in building a dome of almost any size of bricks or stones, with the help of hoop iron in all the lower courses up to about 22 from the bottom, and then less up to 52, and higher if it has to carry a lantern. There is no masonry dome in the world wider than 142 feet. But there have been several larger iron ones, which are an easy piece of engineering, inasmuch as iron has enormous tensile strength, while stone has very little, and mortar practically none; and all the calculations above mentioned assume the domes to be composed of nar row lunes having no lateral bond or tie ; but on the other hand all the stones are assumed to go right through the thickness and not to be liable to crush at the edges. Building the lowest courses with horizontal beds, which some architects suggested, was shown to be exactly the opposite of what is mathematically required, as there would be nothing to prevent their sliding over each other, where as the essence of dome-construction is that the lower courses should confine the upper. It is not however prac tically expedient to make the beds lean inwards so much as to involve acute angles of the stones, as such angles in stone will bear very little pressure. Brick domes over wells or tanks, which should always be round for strength, are usually built on mere mounds of earth for centring, and they are always of flat section, or only about the upper half of a hemisphere, and are consequently stable with very little thickness, as the earth round them forms a strong abutment.
The following inside diameters of the largest domes in the world are given in Sir E. Beckett s Book on Building:—
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Vienna Exhibi tion, 1873.... 1862 Exhibition Albert Hall .... Pantheon iron Feet. 360 140 219x185 . 142 Florence 138i St Peter s. . . . 137i Feet. Bijapore, Gol Gomuz ... 137 Mousta 124 St Sopbia 105 Milan, S. Carlo 105 St Paul s 102 luvalid.es, Paris 92
DOMENICHINO, or Domenico, Zampieri (1581–1641), the celebrated painter, born at Bologna on 21st October 1581, was the son of a shoemaker. The diminutive form of Christian name by which he is known indicates his short stature. He was placed, when young, under the tuition of Denis Calvart ; but having been treated with great severity by that master, he left him, and became a pupil in the academy of the Caracci, under Agostino. To wards the beginning of the 17th century he went to Rome, at the invitation of his fellow- pupil and intimate Albano, and prosecuted his studies under Annibale Caracci. The faculty of Domenichino was slow in its development. He was at first timid and distrustful of his powers ; while his studious, unready, and reserved manners were misunder stood by his companions for dulness, and he obtained the nickname of " the Ox " (Bue). But Annibale Caracci, who observed his faculties with more attention, predicted, that the apparent slowness of Domenichino s genius would in time produce what would bo an honour to the art of painting. When his early productions had brought him into notice, he studied with extreme application, and made such advance as to raise his works into a comparison with those of the most admired masters of the time. From his acting as a continual censor of his own works, he became distinguished amongst his fellow-pupils as an accurate and expressive designer ; his colours were the truest to nature ; Mengs, indeed, found nothing to desire in his works, except a somewhat larger proportion of elegance. That he might devote his whole powers to the art, Domenichino shunned all society ; or, if he occasionally sought it in the public theatres and walks, it was in order better to observe the play of the passions in the features of the people those of joy, anger, grief, terror, and every affection of the mind and to commit them vividly to his tablets ; thus, says Bellori, it was that he succeeded in delineating the soul, in colouring life, and calling forth heartfelt emotions, at which all his works aim. In personal character he is credited with temperance and modesty ; but, besides hia want of sociability, he became somewhat suspicious, and jealous of his master.
