The organ pedals are connected to the pallets by an action similar to that of the keys. The pedal stops are generally of deep tone, 32-foot and 16-foot, as they have to sustain the bass part of the musical harmonies. By means of _couplers_ one or more of the keyboard stops may be linked to the pedals. SEPARATE SOUND-BOARDS. The keyboard of a very large organ has as many as five _manuals_, or rows of keys. Each manual operates what is practically a separate organ mounted on its own sound-board. [Illustration: FIG. 142.] [Illustration: FIG. 143.--General section of a two-manual organ.] The manuals are arranged in steps, each slightly overhanging that below. Taken in order from the top, they are:--(1.) _Echo organ_, of stops of small scale and very soft tone, enclosed in a "swell-box." (2.) _Solo organ_, of stops imitating orchestral instruments. The wonderful "vox humana" stop also belongs to this manual. (3.) _Swell organ_, contained in a swell-box, the front and sides of which have shutters which can be opened and closed by the pressure of the foot on a lever, so as to regulate the amount of sound proceeding from the pipes inside. (4.) _Great organ_, including pipes of powerful tone. (5.) _Choir organ_, of soft, mellow stops, often enclosed in a swell-box. We may add to these the _pedal organ_, which can be coupled to any but the echo manual. VARIETIES OF STOPS. We have already remarked that the quality of a stop depends on the shape and construction of the pipe. Some pipes are of wood, others of metal. Some are rectangular, others circular. Some have parallel sides, others taper or expand towards the top. Some are open, others stopped. The two main classes into which organ pipes may be divided are:--(1.) _Flue_ pipes, in which the wind is directed against a lip, as in Fig. 138. (2.) _Reed_ pipes--that is, pipes used in combination with a simple device for admitting air into the bottom of the pipe in a series of gusts. Fig. 144 shows a _striking_ reed, such as is found in the ordinary motor horn. The elastic metal tongue when at rest stands a very short distance away from the orifice in the reed. When wind is blown through the reed the tongue is sucked against the reed, blocks the current, and springs away again. A _free_ reed has a tongue which vibrates in a slot without actually touching the sides. Harmonium and concertina reeds are of this type. In the organ the reed admits air to a pipe of the correct length to sympathize with the rate of the puffs of air which the reed passes. Reed pipes expand towards the top. TUNING PIPES AND REEDS. [Illustration: FIG. 144.--A reed pipe.]