236 



AR( HITECTL'KE. 



are produced, the one by the weight of the portion of 

 the chain In-low tlu- link, tin- other by tin- sime 

 weight increased by tliat of the link itself, both of 

 tin-in aci'ng originally in a vertical din c; ion. Now, 

 supping tin- chain inverted, so as to constitute an 

 nrcli of the same form ami weight, the relative situa- 

 tions of the forces will be the same, only they will act 

 n contrary directions, so that they are compounded 

 in a similar manner, and balance each other on the 

 same conditions. The arch thus formed is denomi- 

 nated a catenary arch. In common cases, it ditlcrs 

 but little from a circular arch of the extent of about 

 one third of a whole circle, and rising from the abut- 

 ments with an obliquity of about 30 degrees from a 

 perpendicular. But though the catenary arch is the 

 ix-st form for supporting its own weight, and also all 

 additional weight which presses in a vertical direction, 

 it is not the best form to resist lateral pressure, or 

 pressure like that of fluids, acting equally in all direc- 

 tions. Thus the arches of bridges and similar struc- 

 tures, when covered with loose stones and earth, are 

 pressed sideways, as well as vertically, in the same 

 manner as if they supported a weight of fluid. In this 

 case, it is necessary that the arch should arise more 

 perpendicularly from the abutment, and that its gen- 

 eral figure should be that of the longitudinal segment 

 of an ellipse. In small arches, in common buildings, 

 where the disturbing force is not great, it is of little 

 consequence what is the shape of the curve. The 

 outlines may even be perfectly straight, as in the tier 

 of bricks which we frequently see over a window. 

 This is, strictly speaking, a real arch, provided the sur- 

 faces of the bricks tend towards a common centre. 

 It is the weakest kind of arch, and a part of it is ne- 

 cessarily superfluous, since no greater portion can act 

 in supporting a weight above it, that can be in- 

 cluded between two curved or arched lines. Besides 

 the arches already mentioned, various others are in 

 use. The acute or lancet arch, much used in Gothic 

 architecture, is described usually from two centres 

 outside the arch. It is a strong arch for supporting 

 vertical pressure. The rampant arch is one in which 

 the two ends spring from unequal heights. The 

 horse shoe or Moorish arch is described from one or 

 more centres placed above the base line. In this 

 arch, the lower parts are in danger of being forced in- 

 ward. The ogee arch is concavo-convex, and there- 

 fore fit only for ornament. In describing arches, the 

 upper surface is called the extrados, and the inner, 

 the intrados. The springing lines are those where 

 the intrados meets the abutments, or supporting walls. 

 The span is the distance from one springing line to 

 the other. The wedge-shaped stones, which form an 

 arch, are sometimes called voujsoirs, the uppermost 

 being the key-stone. The part of a pier from which 

 an arch springs is called the impost, and the curve 

 formed by the upper side of the voussoirs, the archi- 

 volt. It is necessary that the walls, abutments, and 

 piers, on which arches are supported, should be so 

 firm as to resist the lateral thrust, as well as vertical 

 pressure, of the arch. It will at once be seen, tliat 

 the lateral or sideway pressure of an arch is very con- 

 siderable, when we recollect that every stone, or por- 

 tion of the arch, is a wedge, a part whose force acts to 

 separate the abutments. For want of attention to this 

 circumstance, important mistakes have been commit- 

 ted, the strength of buildings materially impaired, and 

 their ruin accelerated. In some cases, the want of 

 lateral firmness in the walls is compensated by a bar 

 of iron stretched across the span of the arch, and 

 connecting the abutments, like the tie-beam of a rof. 

 This is the case in the cathedral of Milan, and sou e 

 other Gothic buildings.* In an arcade, or continuiv- 



CadelTc Jour, through Canuola and Italy, vol. ii. p. 77. 



tion of arc-lies, it is only necessary that the outer sup- 

 ports of the terminal arches should be strong enough 

 to resist horizontal pressure. In the intermediate 

 arches, the lateral force of each arch is counteracted 

 by the opposing lateral force of the one contiguous to 

 it. In bridges. ho\\e\cr, where individual arches are 

 liable to be destrojed by accident, it is desirable that 

 each of the pier- should po^i^s sufficient horizontal 

 strength to resist the lateral pn-sMin- of the adjoining 

 arches. The vault is the lateral continuation of an 

 arch, serving to cover an area or passage, and bear- 

 ing the same relation to the arch that iiie walls do to 

 the column. A simple vault is constructed on the 

 principles of the arch, and distributes its pivvMire 

 equally along the walls or abutments. A complex or 

 groined vault is made by two vaults intersecting each 

 other, in which case the pressure is tlmmn upon 

 springing points, and is greatly increased at tho-<- 

 points. The groined vault is common in Gothic 

 architecture. The dome, sometimes called i-npnlu, 

 is a concave covering to a building, or part of it, 

 and may be cither a segment or a sphere, of a 

 spheroid, or of any similar figure. When built of 

 stone, it is a very strong kind of structure, even more 

 so than the arch, since the tendency of each part to 

 fall is counteracted, not only by those above and be- 

 low it, but also by those on each side. It is only ne- 

 cessary that the constituent pieces should have a 

 common form, and that this form should be somewhat 

 like the frustum of a pyramid, so that, when placed 

 in its situation, its four angles may point toward the 

 centre, or axis, of the dome. During the erection of 

 a dome, it is not necessary that it should lie supported 

 by a centring, until complete, as is done in the arch. 

 Each circle of stones, when laid, is capable of sup- 

 porting itself without aid from those above it. It 

 follows that the dome may be left open at top, with- 

 out a key-stone, and yet be perfectly secure in this 

 respect, being the reverse of the arch. The dome of 

 the Pantheon, at Rome, has been always open at top, 

 and yet has stood unimpaired for nearly 2000 years. 

 The upper circle of stones, though apparently the 

 weakest, is nevertheless often made to support the 

 additional weight of a lantern or tower above it. In 

 several of the largest cathedrals, there are two domes, 

 one within the other, which contribute their joint 

 support to the lantern, which rests upon the top. In 

 these buildings, the dome rests upon a circular wall, 

 which is supported, in its turn, by arches upon mas- 

 sive pillars or piers. This construction is called build- 

 ing upon pendentives, and gives open space and room 

 for passage beneath the dome. The remarks which 

 have been made in regard to the abutments of the 

 arch, apply equally to the walls immediately support- 

 ing a dome. They must be of sufficient thickness and 

 solidity to resist the lateral pressure of the dome, 

 which is very great. The walls of the Roman Pan- 

 theon are of great depth and solidity. In order that 

 a dome in itself should be perfectly secure, its lower 

 parts must not be too nearly vertical, since in this 

 case, they partake of the nature of perpendicular walls, 

 and are acted upon by the spreading force of the parts 

 above them. The dome of St Paul's church, in Lon- 

 don, and some others of similar construction, are bound 

 with chains or hoops of iron, to prevent them from 

 spreading at bottom. Domes which are made or 

 wood depend, in part, for their strength, on their in- 

 ternal carpentry. The Halle du Bled, in Paris, had, 

 originally, a wooden dome more than 200 feet in 

 diameter, and only one foot in thickness. This has 

 since been replaced by a dome of iron. The roof is 

 the most common and cheap method of covering 

 buildings, to protect them from rain and other effects 

 of the weather. It is sometimes flat, but more fre- 

 quently oblique, in its sliape. The flat or platform 



