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CIVIL ARCHITECTURE. 



P/tctfe*. Sophia at Constantinople, to the finishing of St Paul's 

 ~~*~ cathedral at London, the dome* are all of the surmount- 

 ed kind, and approach, in a certain degree, to the pro- 

 portion* of *pire* or towers, o much affected in the 

 middle ages. Since the labour* and taste of Mr Stewart 

 and other* have revived the legitimate Grecian architec- 

 ture, the contour of the ancient Roman dome ha* been 

 also restored, especially in cases where the structure il 

 'ornamented with any of the order*. 



In the interior of the large towers of our Gothic ca- 

 thedrals, over the interjections of the cross, we find 

 dome*, riling from a iquare base, generally pierced with 

 two window* over each wall, and forming beautiful 

 groins, by their intersection with the interior domic 

 ceiling. 



Equilibri- Though the equilibrium and pressure of domes are 

 ,.'.' very different from those of ordinary arches, yet they 



. ::..>. have some common properties, as will appear from the 

 following comparison : If, in their cylindrical or cylin- 

 droidal vaulting of uniform thickness, the tangent to the 

 arch at the bottom be perpendicular to the horizon, the 

 vault cannot stand ; neither can it be built with a concave 

 contour in the whole, or in any part ; and to make the 

 arch in equilibrium, whether its section be circular or 

 elliptical, supposing the intrados to be given, the ex- 

 tremes must be loaded vastly high, between the extrados 

 of the curve, which runs upward, and the tangent to the 

 arch, which is an asymptote, rising vertically from each 

 foot or extreme of the arch. In like manner, in thin 

 domical vaulting of equal thickness, if the curved surface 

 rise perpendicular from the base, the bottom will burst, 

 let the contour be as it may. 



Notwithstanding this agreement, dome vaulting differ* 

 in other particulars very essentially from the common 

 sort ; for instance, to bring the figure of a dome into 

 proper equilibrium, after the convexity has been carried 

 to its full extent of equilibrium around, and equidistant 

 from the summit on the exterior, the curvature may al- 

 ter into a concavity ; for, since the interior circumference 

 of the courses is less than that of the exterior, the stones 

 cannot fall inwardly, whatever be the outward pressure, 

 unless they be squeezed into a less compass, which is 

 supposed impossible ; consequently, they must be crush- 

 ed to powder before such a vault can give way. For the 

 tame reason, a vault may be constructed, that shall be 

 convex within, and concave outwardly, and yet be suffi- 

 ciently firm. The strongest form, however, of a circular 

 vault, intended to bear a load at the top, is that of a 

 truncated cone, similar to Sir Christopher Wren's con- 

 trivance for supporting the stone lanthorn and exterior 

 dome of St Paul's. In this kind of vault, the pressure 

 is communicated in the sloping right line of the sides of 

 the cone perpendicular to the joints, consequently the 

 conic sides have no tendency to bend to one side more 

 than to the other, unless it be from the gravity of the 

 materials tending towards the axis, which is counteract- 

 ed by the abutting vertical joints : A form so strong, a* 

 to be adequate to sustain or repel any force acting oa it* 

 tummit that we can possibly conceive. 



In dome vaulting, on the contrary, the contour being 

 convex, there is a certain load, which, if laid on the 

 apex of the dome, must cause it to burst outwardly. 

 The power of this load will be greater or less, accord- 

 ing to the approximation of the contour towards, or its 

 recession from, the chords of the arches of the two sides, 

 or to a conic vaulting on the same base, carried up to 

 the same altitude, and ending in the ame circular course. 

 In esemplification of this, if we begin at the keystone, 

 and proceed downwards, from course, to course, supposing 



a horizontal line to be a tangent at the vertex, we shall Praciir*. 

 discover, that every successive couring-joint may be *" *V*' 

 made to dope 10 much, and, consequently, the pressure 

 of the architoii'.s of any course towards the axis may be 

 so great, as to be more thin sufficient to resist the 

 weight of all the part above ; hence it is evident, that a 

 certain degree of curvature may be given to the contour, 

 which will be just sufficient to prevent the stones, in any 

 succeeding course, from being forced outwardly. 



A circular vault, thus balanced, is what is called an 

 equilibrated dome ; but it is the weakest of all vaults, be- 

 tween that of its own contour, and that of a cone upon 

 the same base, rising to the same height, and ending 

 in a key-stone, or finished with an equal circular course. 



From these data we may conclude, that the equilibra- . 

 ted dome has the boldest contour, but is only the limit contour . 

 of an indefinite number of inscribed circular vaults, all 

 stronger than itself. 



In other respects circular vaulting differs from the 

 straight, in being built with courses in circular rings ; 

 so that the stones in each course being of equal length, 

 and pressing equally towards the axis, cannot slide in- 

 wardly. Hence circular vaults may be left open at the 

 top, and even the equilibrated dome may carry a lantern 

 of equal gravity with the part that would have been ne- 

 cessary to complete the whole. But domes of a more 

 flat contour may carry more, according as they approach, 

 nearer to a cone, as already remarked ; and those circu- 

 lar vaults that are cither straight or concave on the sides, 

 may be loaded without limit, and can never fail till the 

 materials are crushed, provided they be hooped at the 

 bottom. For a more complete investigation of this im- 

 portant subject, see DOME. 



We have already observed, that, from the earliest pe- ^.,-j^, 

 riods of Roman architecture, ARCHES have been of im- 

 portance, and have been extensively used in the construc- 

 tion of most sorts of edifices ; but, as their nature and 

 principles have been treated of at some length under the 

 term BRIDGE, we shall at present only observe, that they 

 are introduced to cover openings to which lintels are in- 

 adequate, as drains and inverted arches in foundations, 

 in vaulting over cellars, halls, and passages ; also over 

 doors, windows, and recesses. They are, indeed, be- 

 come so generally useful, and are capable of so nice and 

 accurate adjustment, that we cannot too earnestly press 

 upon the young architect, the propriety of rendering 

 their principles and practice perfectly familiar to hi* 

 mind. 



BALUSTERS are sometimes of real use, as in stairs, _ , 

 windows, terraces, and on the sides of passages open on 

 one or both sides. At other times they are merely orna- 

 mental, as when terminating the upper part of the front 

 of a building, as a screen to conceal the whole or part of 

 a roof, or as a finish to insulated triumphal arches or 

 gateways. They consist of short pillars, of a peculiar 

 outline, standing upon a socle or plinth, and covered 

 with a small cornice. No remains of balusters have been 

 found in any ancient buildings. In the theatres and 

 amphitheatres of the Romans, the pedestals of the up- 

 per orders were always continued through the arcades, 

 and served as parapets. The lower seats, next to the 

 arena in the amphitheatres, and those next the orchestra 

 in the theatres, were guarded by a parapet called the 

 podium. The top of the monument of Lysicratcs at 

 Athens is finished with a sort of parapet or ledge, com- 

 posed of honey suckles, solid behind, and open between 

 every pier. Each plant is bordered with a curved head, 

 and the bottom of every interval with an inverted curve. 

 By impressions met with on coins, we arc led to suppose, 



