308 Pointed Lanterned Domes. Therefore the drum to carry such a dome without ties must have a slope of 16° instead of 12° as it was without the lantern. L represents any other weight or force as well as the lantern, and in fact it must include the leverage of the wind upon the lantern, which acts as weight on the side farthest from the wind. The effect of either wind or snow on an unlanterned dome is inconsiderable, for a dome must practically have an enormous superfluity of strength beyond what could be affected by any possible snow on the top or wind on the side. A dome is in fact the strongest against wind of all structures of the same height weight and size except a cone, of which the stability is limited by nothing but the compressibility of the stones. I calculated a similar table to the former for a pointed dome of 60° with a lantern, which is as follows; only it must be remembered that M (for reasons of calculation) is still the weight of a hemispherical dome on the same base, which bears the proportion to the taller equilateral dome of 1 to 1372; and I to 1177 of a dome of 70°. But as I have given L in tons also that does not affect the result. Showing that the weight of lantern again increases with the cube of the thickness very nearly, though the required thickness is much less than in a hemisphere. St. Paul's Dome. 309 Therefore Sir C. Wren was mechanically quite right in setting the heavy stone lantern at the top of St. Paul's on a hollow cone like a glass blower's chimney, and making the inner dome within that, both having a common base, and the outer one only a wooden framework; and the cone itself is tied with a chain at the bottom, where alone a cone wants tieing. Besides that, you may observe that the dome, and therefore the cone within it, stands so much within the outside of the drum, that the pressure is really carried down obliquely, just as if the drum itself were coned or sloped in, exactly as I said it might be. Inside, the slope is not concealed, but visible enough, and perhaps looks better than if the walls were upright, as upright walls at a great height are apt to look bursting outwards. The same thing had been done before on a smaller scale in the baptistery at Pisa, where the lantern at the height of 175 feet is carried on a cone 59 ft. wide at the base, and the stone dome, Mr. Fergusson says, was long afterwards built round it. A dome with an aisle all round the drum would afford the very best facilities for resisting the thrust by flying buttresses, either within the roof as described at p. 237, or outside as in most cathedrals. The drum might then be reduced to arches and pillars as in our four round churches (all of which have aisles round them) and the baptistery at Nocera (Fergusson, p. 511) or an apse with a periapse. And then also a great domed church might have supports occupying a much less proportion of the area covered than St. Peter's or St. Paul's, where the supports occupy twice as much as in some of our Gothic churches. I warn everybody against expecting any good architectural effect from small domes, though large ones are grand. 310 Polygonal Domes. 6 Most of those in London have been the subject of much more ridicule than admiration, from the national pepper-boxes' in Trafalgar Square, back to the caricature which I remember, of that curious little architect (Nash) sitting on his egg'-the little dome of Buckingham Palace, in the time of George IV. Polygonal domes.-Very nearly the same considerations apply to octagonal or other many-sided domes as to round ones (i.e. round horizontally), so that it is not worth while to distinguish them. So far as there is a difference it is in favour of the round ones, inasmuch as polygonal ones are not so independent of cement, and the middle of each side might fall inwards, which is impossible with a round dome of any kind or shape, until other parts burst outwards. The domes of St. Peter's and Florence, which last is octagonal and pointed, of about 70° (St. Peter's is rather less pointed), are composed of double shells joined in various places by ribs. Some persons have fancied that that construction has something of the strength of bones and quills and hollow iron pillars; but that is altogether a false analogy; for their strength arises from one side always resisting extension while the other resists compression; but masonry has practically no power of resisting tension directly tending to separate the stones. The only value of the double shell is that it takes firmer hold of the bottom of the lantern so as to resist the leverage of wind upon it. Otherwise the double dome merely increases the weight and thrust at the bottom for nothing. Though it is the fashion to condemn what is called the sham construction of the dome of our St. Paul's, there is a good deal to be said for it, as we have already St. Peter's being of brick covered with lead has seen. St. Paul's Dome. 311 no visible superiority over St. Paul's of which the outer dome is wood covered with lead. And when we look at the lantern of St. Paul's, proportionately larger than St. Peter's or Florence, standing with absolute safety on that thin cone of only 18 inches, with the other equally thin dome within, and compare them with the huge mass of masonry in the Italian ones, or with the three shells of the smaller dome of the Invalides at Paris of only 92 feet, where both the outer dome and the lantern are of wood, I cannot help pronouncing in favour of what every one admits to be the grandest looking dome externally, and which is managed in the most scientific way. We must also give Wren due credit for the science displayed in bringing down the cone so far below the base of his external dome that its slope is 24°, which diminishes the thrust immensely, and yet leaves the inner dome, which springs from the base of the cone, quite high enough above the ground. Indeed with all the knowledge that we have now of what can be done in dome building by the aid of hoop iron, we could do no better than copy the very same construction of two domes and a cone to carry the lantern, except that one would perhaps like to make the outer dome of stone, which could easily be done. According to the best authorities, including some measurements made by Mr. Donaldson for large drawings of his own, the internal diameters of all the masonry domes in the world above 90 feet are as The 138 is the smaller diameter of the octagon at Florence. The Malta and Milan domes are both of this century, as may be seen in Mr. Fergusson's Handbook, where they are fully described. The Milan one is very ugly, but the other handsome, though designed by a builder who could neither draw nor write, and received two shillings a day. The Gol Gomuz is 10 feet thick, and stands upon a square, whose corners and their turrets make an enormous abutment for the dome, though it would be hardly stable (apart from cement) if it stood upon a round drum only as wide as the square. The peculiar construction of the vaulting of that and other Indian domes is described by Mr. Fergusson: but I do not assent to his theory of the mechanical conditions of domal stability, which very much underrates the importance of the weight of the upper part of a dome; but he does not profess to have investigated it mathematically. A question was asked at the R.I.B.A. as to the possibility of building domes entirely without scaffolding, beyond a mere radial pole travelling about to regulate the position of each stone as it is put on. For a considerable height the stones will evidently stand by friction and cement until each ring is successively closed in, and then it cannot fall; after which the mortar should have time to set, or the ring will be squeezed too small and sink a little by the addition of more weight. As soon as the inclination becomes too great for the stones to stand alone, they may be notched or stepped so as to hang on the preceding ring; and the Maltese dome was actually built so without any centering otherwise there must be as much centering as will carry each ring in succession when you have reached the height at which the stones will not stick separately. |