The width of the mouth, also, or the distance of the upper lip from the plate of wind, is of much importance. If this distance be too great, the pipe will be slow to speak, or will not speak at all. If the mouth be too nar row, an open pipe will speak•the octave, or even the double octave of the note which it is intended to give, and a stopped one the twelfth.
The proportion between the size of the tube and length of the pipe, though it has no effect on the acute ness or gravity, has a decided effect on the quality of the tone; and as it is essential to have all the notes or any one stop as nearly alike as possible in quality, organ builders have regular scales by which each pipe in any one stop is formed, from the largest up to the smallest. Stops are of very different scales for large church organs, and for chamber organs ; and stops of different scales are admitted for variety into the same organ.
All the pipes in one stop are to be regulated as equal ly as possible in respect of loudness and softness. This is chiefly effected by enlarging or diminishing thc aper ture of the foot d, which consequently regulates the quantity of wind admitted into the pipe.
The note which the pipe speaks depends on the length of the tube; a stopped pipc, therefore, ns Fig. 14. is tuned by means of the tompion, or plug. If this be push ed deeper hue, the tube, so as to shorten the column of air within the pipe, the note will be more acute; and vice versa. If the tompion were taken out altogether the pipe would give exactly the octave above what it would give if stopped at the very top AA. A stop of open pipes, thcrelore, in order to speak in unison with a stop of shut pipes, must be double the length of the latter. An open pipe is tuned by means of a thin plate of lead, Fig. 16. a, fastened at the upper end, and bent over the open tube more or less to depress or raise the pitch. Open pipes are also usually of larger diameter than shut or stopped pipes in unison with them. Thus, in the unison stops, called the open and stopped diapa sons, the unison in the stopped diapason to C of the open diapason, would be of the same diameter as the G above of the latter stop.
The quality or the tone of wooden pipes is influenced also by the material or wood used. Thus mahogany,
or wainscot pipes, have a clearer tone than pipes of fir or other soh wood : on the other hand, the latter are more mellow.
Afetal mouth pipes are made on the same prin ciple. Such a pipe consists of a cylindrical tube, Plate CCCCXLVI. Fig. 9. A. The mouth is formed by a por tion of the cylinder flattened, and retiring inwards b a; the upper lip a being cut a little shorter than the remaining part of the circumference. The foot B is a cone, open at the apex c, but closed at its base b) a piece of pretty thick metal d, Fig. 9. called the languette, (which cor responds to the block of a wooden pipe,) except a thin opening or plate of wind. Part of the conical surface of the foot, towards the mouth of the pipe, is flattened, and retires inwardly to meet the upper lip e a, and con sequently the languette is here cut to a straight edge. The edge of the languette, which, with the under lip, forms the filate of wind, is bevelled inwards and upwards to direct the wind duly on the upper lip; and the voic ing is performed by making notches on this bevelled surface parallel to each other, and at an angle to the axis of the pipe.
Metal pipes are either opcn or stopped, and what is said of wooden pipes is equally applicable to them. There is a third kind, called chimney-top pipes, Fig. 10. which have a small open tube a inserted in the middle of the top plate; the effect of which is to make the note considerably sharper than if it were quite stopped. This is also sometimes imitated by boring a small hole through the tompion of stopped wooden pipes.
Stopped metal and chimney-topped pipes have large ears f f, Fig. 10. one on each side the mouth, by means of which they are tuncd. Bending the ears towards the mouth flattens the note; and vice versa. The larger open metal pipes, that is, more than a foot in length, have also small ears f f, Fig. 9. which make them speak better. Open pipes arc tuned by lessening or enlarg ing the diameter of the open end.
The metal used is a mixture of tin and lead, in dif ferent proportions, to which a little bisniuth is some times added, or occasionally a very little copper. The metal is cast in thin plates, and thcn planed to the re quisite thickness.