The _harmonium_ has a keyboard of five octaves compass when complete,
[musical notes], and a simple action controlling the valves, &c. The
necessary pressure of wind is generated by bellows worked by the feet
of the performer upon foot-boards or treadles. The air is thus forced
up the wind-trunks into an air-chamber called the wind-chest, the
pressure of it being equalized by a reservoir, which receives the
excess of wind through an aperture, and permits escape, when above a
certain pressure, by a discharge valve or pallet. The aperture
admitting air to the reservoir may be closed by a drawstop named
"expression." The air being thus cut off, the performer depends for
his supply entirely upon the management of the bellows worked by the
treadles, whereby he regulates the compression of the wind. The
character of the instrument is then entirely changed from a mechanical
response to the player's touch to an expressive one, rendering what
emotion may be communicated from the player by increase or diminution
of sound through the greater or less pressure of wind to which the
reeds may be submitted. The drawstops bearing the names of the
different registers in imitation of the organ, admit, when drawn, the
wind from the wind-chest to the corresponding reed compartments,
shutting them off when closed. These compartments are of about two
octaves and a half each, there being a division in the middle of the
keyboard scale dividing the stops into bass and treble. A stop being
drawn and a key pressed down, wind is admitted by a corresponding
valve to a reed or vibrator (fig. 1). Above each reed in the so-called
sound-board or pan is a channel, a small air-chamber or cavity, the
shape and capacity of which have greatly to do with the colour of tone
of the note it reinforces. The air in this resonator is highly
compressed at an even or a varying pressure as the expression-stop may
not be or may be drawn. The wind finally escapes by a small
pallet-hole opened by pressing down the corresponding key. In Mustel
and other good harmoniums, the reed compartments that form the scheme
of the instrument are eight in number, four bass and four treble, of
three different pitches of octave and double octave distance. The
front bass and treble rows are the "diapason" of the pitch known as 8
ft., and the bourdon (double diapason), 16 ft. These may be regarded
as the foundation stops, and are technically the front organ. The back
organ has solo and combination stops, the principal of 4 ft. (octave
higher than diapason), and bassoon (bass) and oboe (treble), 8 ft.
These may be mechanically combined by a stop called full organ. The
French maker, Mustel, added other registers for much-admired effects
of tone, viz. "harpe eolienne," two bass rows of 2 ft. pitch, the one
tuned a beat too sharp, the other a beat too flat, to produce a waving
tremulous tone that has a certain charm; "musette" and "voix celeste,"
16 ft.; and "baryton," a treble stop 32 ft., or two octaves lower than
the normal note of the key. The "back organ" is usually covered by a
swell box, containing louvres or shutters similar to a Venetian blind,
and divided into fortes corresponding with the bass and treble
division of the registers. The fortes are governed by knee pedals
which act by pneumatic pressure. Tuning the reeds is effected by
scraping them at the point to sharpen them, or near the shoulder or
heel to flatten them in pitch. Air pressure affects the pitch but
slightly, being noticeable only in the larger reeds, and harmoniums
long retain their tuning, a decided advantage over the organ and the
pianoforte. Mechanical contrivances in the harmonium, of frequent or
occasional employment, besides those already referred to, are the
"percussion," a small pianoforte action of hammer and escapement
which, acting upon the reeds of the diapason rows at the moment air is
admitted to them, gives prompter response to the depression of the
key, or quicker speech; the "double expression," a pneumatic balance
of great delicacy in the wind reservoir, exactly maintaining by
gradation equal pressure of the wind; and the "double touch," by which
the back organ registers speak sooner than those of the front that are
called upon by deeper pressure of the key, thus allowing prominence or
accentuation of certain parts by an expert performer. "Prolongement"
permits selected notes to be sustained after the fingers have quitted
their keys. Dawes's "melody attachment" is to give prominence to an
air or treble part by shutting off in certain registers all notes
below it. This notion has been adapted by inversion to a "pedal
substitute" to strengthen the lowest bass notes. The "tremolo" affects
the wind in the vicinity of the reeds by means of small bellows which
increase the velocity of the pulsation according to pressure; and the
"sourdine" diminishes the supply of wind by controlling its admission
to the reeds.
[Illustration: By courtesy of Metzler & Co.