Page:Encyclopædia Britannica, Ninth Edition, v. 4.djvu/526

474 BUILDING [MASON-WOKK. each of the several periods of the style in England, and also in the variations of the style as practised here and in each country on the Continent. We insert an outline, fig. 22, of a window jamb at Sleaford church, Lincolnshire, to show the forms and combination of an example during the Decorated period. A full description of those used in each style would exceed our limits, nor, indeed, is it a subject within the scope of this article. They are sometimes set out with the compasses, and many often appear to have been drawn by eye. We must refer our readers to the works of Professor Willis, F. A. Paley, and of J. H. Parker, for any details re quired about them. A very curious treatise was pub lished by Professor Willis, called the Architectural Nomenclature of the Middle Ages, which goes at great length into the subject. A bead or astragal seems to have been called a bowtelle ; Working the mould- Arches. Fia. 22. Outline of Window Jamb at Sleaford Church. a torus, a grete bowtelle; a hollow or scotia, a casement; an ogee, a ressaunte, and so on. The methods of working mouldings in the stones at fa ti builder s command have been already noticed at the commencement of this section. The soft stones and marbles arc easily shaped into mouldings with the chisel and the mallet, and are sawn and the surfaces even finished with a plate having a fine edge. These stones will take mouldings of minuter character than the harder stones, which have to bo worked with force, and require pointed tools to form the faces. The hardest material, granite, again, has to be stunned with heavy picks to make an impression, so that only bold ornaments have a good effect ; much money has been thrown away in details more suitable for softer materials. Good effect is obtained by the contrast of axed and polished surfaces. A designer should visit the places where buildings in granite have been erected for a knowledge of a proper application of that material. The earliest arches were circular and, of course, easily set out. But as the Pointed styles came in, several methods were used for describing them. Pointed arches may be classed as 1st, lancet; 2d, equilateral; 3d, depressed; FIG. 23, Lancet Arch. Fi.i. 24. Equilateral Arch. and 4th, four-centred or Tudor. In the first the centres, as 1,2, in fig. 23, are outside the arch a b. At West- Depressed Arch. minster Abbey the arches of the choir arc so acutely pointed that the distances la and 26 are nearly two-thirds of the entire opening a b. In the nave at York the points are without the arch at a distance of about one-fifth the opening a b. In equilateral arches the centres are exactly on the points a b in fig. 24, so that the apex c, joined to a and b, will form an equilateral triangle. The nave arches at W ells are of this description, and also those at Lincoln (see vol. ii. Plate XVIII. fig. 1). In later times the arches were of lower pitch, as fig. 25, and then, of course, the centres 1 and 2 were within the arch a b. At Salisbury Cathedral the distance al is one-sixth of a b, while in the choir at Lincoln (vol. ii., Plate XVIII., fig. 2) it is as much as two-fifths. To describe arches which shall be similar to one another throughout a building, however the openings may differ, this principle must always be borne in mind, that the centres are to be always distant from the points a b by some aliquot portion of the whole opening. This is the more important, as the lines of tracery will not fall into their proper places except the arches arc set out upon some regular principle. If the arches are not equilateral, some distance from each point a b should be first determined on (say one-third the opening a b), and after this, whatever the span of the other arches may be, one-third its own opening is to be taken from the points a b, as the centres from which to strike its curves. The only exception is that, in mediseval buildings, the arches to the doorways are frequently some what flatter than those of the windows. In the Tudor Four-cei period the arches are very frequently drawn from four t f ed arc! centres instead of two. As there has been great mis apprehension as to four-centred arches, some persons treat ing them as parts of conic sections, whereas they are really parts of segments of circles, it is thought well to give two methods of describing these arches. ; First, when the width AB, fig. 26, of the arch, and the apex height 00, are given, and a tangent to the upper circle as CD. In this case draw AD perpendicular to AB, and .set out Al equal to AD ; draw 03 perpendicular to CD, and make CE equal to AD or Al ; join IE and Insect the same is shown by a perpendicular meet ing CE produced in 3 ; join 3 1 ml produce towards F, then 1 and FIG. 26. Four-centred Arch, 3 will lie tho centres for half the arch ; and, transferring the points across, 2 and 4 will 1m the centres for the other half. In the second case, when the width AB and the height 00, and the centres of the small circles 1, 2, are given. Make AD equal to Al, join CD

(which will be a tangent to the upper curve), draw 03 at right angles