Iron is, however, fit for window-bars, and there seems no constructive reason why we should not have iron traceries, as well as iron pillars, iron churches, and iron steeples. But I have, in the “Seven Lamps,” given reasons for not considering such structures as architecture at all.
The window-bars must, therefore, be of stone, and of stone only.
§ V. The purpose of the window being always to let in as much light, and command as much view, as possible, these bars of stone are to be made as slender and as few as they can be, consistently with their due strength.
| Fig. XLV. |
Let it be required to support the breadth of glass, a, b, [Fig. XLV.] The tendency of the glass sustaining any force, as of wind from without, is to bend into an arch inwards, in the dotted line, and break in the centre. It is to be supported, therefore, by the bar put in its centre, c.
But this central bar, c, may not be enough, and the spaces a c, c b, may still need support. The next step will be to put two bars instead of one, and divide the window into three spaces as at d.
But this may still not be enough, and the window may need three bars. Now the greatest stress is always on the centre of the window. If the three bars are equal in strength, as at e, the central bar is either too slight for its work, or the lateral bars too thick for theirs. Therefore, we must slightly increase the thickness of the central bar, and diminish that of the lateral ones, so as to obtain the arrangement at f h. If the window enlarge farther, each of the spaces f g, g h, is treated as the original space a b, and we have the groups of bars k and l.
So that, whatever the shape of the window, whatever the direction and number of the bars, there are to be central or main bars; second bars subordinated to them; third bars subordinated to the second, and so on to the number required. This is called the subordination of tracery, a system delightful to the eye and mind, owing to its anatomical framing and unity, and to its expression of the laws of good government in all fragile and unstable things. All tracery, therefore, which is not subordinated, is barbarous, in so far as this part of its structure is concerned.
§ VI. The next question will be the direction of the bars. The reader will understand at once, without any laborious proof, that a given area of glass, supported by its edges, is stronger in its resistance to violence when it is arranged in a long strip or band than in a square; and that, therefore, glass is generally to be arranged, especially in windows on a large scale, in oblong areas: and if the bars so dividing it be placed horizontally, they will have less power of supporting themselves, and will need to be thicker in consequence, than if placed vertically. As far, therefore, as the form of the window permits, they are to be vertical.