if
1612
THEORY AND PRACTICE OF ENGINEERING. Book II.
executed about the middle of the fourteenth century; the centre of the head of the window,or rather the nucleus to the tracery, is an octagon, six sides of which are retained, the othertwo being suppressed, to allow of a better combination with the three centre divisions ofthe lower part.
Th equilateral triangle also definedthe form and magnitude of the severalmullions, as shown by the d agram,constructed upon measurement of thewindows of the clerestory of the naveat Winchester: a line drawn fromthe apex of one mullion to the otheris the base of the triangle, andthe space inclosed by the two isdivided into ten other equilateraltriangles, two of which agree indimensions and form with each mul-lion. Of the twelve equilateral tri-angles embracing two half mullions,ten are given to the day or space toadmit the light, and two, or one-sixth of the whole, is comprised bythe mullion; such appears to havebeen the manner of proportioning theparts of windows in the middle ages.
Fig. 3019.
Rose Windows in the West Transept of the Church of St. Ouen at Rouen is 29 feet 6 inchesin diameter, and composed of seven equal circles, one of which occupies the centre : each otthose, which surround it, are again subdivided by others ; two only of the outer six are pre-served in the figure, and form the quatrefoils, whilst the intersections of the others serve ascentres to the rest of the design.
ST. OUEN AT ROUEN.