The construction of the organistrum ([Fig. 31]) requires but little explanation. A glance at the finger-board reveals at once that the different tones were obtained by raising the keys placed on the neck under the strings, and that the keys were raised by means of the handles at the side of the neck. Of the two bridges shown on the body, the one situated nearest the middle was formed by a wheel in the inside, which projected through the sound-board. The wheel which slightly touched the strings vibrated them by friction when turned by the handle at the end. The order of intervals was c, d, e, f, g, a, b-flat, b-natural, c, and were obtainable on the highest string. There is reason to suppose that the other two strings were generally tuned a fifth and an octave below the highest. The organistrum may be regarded as the predecessor of the hurdy-gurdy, and was rather a cumbrous contrivance. Two persons seem to have been required to sound it, one to turn the handle and the other to manage the keys. Thus it is generally represented in mediæval concerts.
The monochord was mounted with a single string stretched over two bridges which were fixed on an oblong box. The string could be tightened or slackened by means of a turning screw inserted into one end of the box. The intervals of the scale were marked on the side, and were regulated by a sort of movable bridge placed beneath the string when required. As might be expected, the monochord was chiefly used by theorists; for any musical performance it was but little
suitable. About a thousand years ago when this monochord was in use the musical scale was diatonic, with the exception of the interval of the seventh, which was chromatic inasmuch as both b-flat and b-natural formed part of the scale.
This ought to be borne in mind in examining the representations of musical instruments transmitted to us from that period.
As regards the wind instruments popular during the Middle Ages, some were of quaint form as well as of rude construction.
Fig. 31.—Organistrum.
The chorus, or choron, had either one or two tubes. There were several varieties of this instrument; sometimes it was constructed with a bladder into which the tube is inserted; this kind of chorus resembled the bagpipe; another kind resembled the pungi of the Hindus, mentioned on page 52. The name chorus was also applied to certain stringed instruments. One of these had much the
form of the cithara, page 84. It appears, however, probable that chorus or choron originally designated a horn (Hebrew, keren; Greek, keras; Latin, cornu).