SIXTH, a musical interval, a concord, the ratio of which is 5 : 3. [CONCORD; Hannosr.) Of the Sixth there are three kinds : the Minor Sixth, the Major Sixth, and the Extreme Sharp Sixth. The first (e, c). is composed of three tones and two semitones; the second (c, a), of four tones and one semitone; the third (c, a:), of four tones and two semitones. Ex..— span so wide as to allow the stream to pass under it without being diverted ; or by building the arch square with the stream, and of sufficient length to allow the upper passage to take an oblique course over it ; but either of these is a clumsy expedient, although well adapted for some situations. The arches or tunnels by which the North-Western railway is conducted under the Hampstead Road and Park Street, near the London terminus, are instances of the latter kind of construction ; the length of the arches being such that they present faces square with the line of railway, notwithstanding the oblique direction of the roads over them. A similar case occurs at Denbigh
Hall, on the same line, where the railway crosses over the London and Holyhead road at such an angle that the difference of direction is only 25°. In this case a long gallery is constructed under the railway, eon eisting of iron ribs ur girders, resting upon walls built parallel with the road ; the ribs, and consequently the faces of the bridge, being at right angles with it. This gallery is about two hundred feet long and thirty-four feet wide ; and by its adoption, the necessity of building an oblique arch of eighty feet span was avoided. The necessity of increasing the span of an arch according to ite.degree of obliquity, by which the expense and difficulty are materially increased, is illustrated by fig. 3, the ground-plan of an oblique arch across a