3. The most essential and prominent parts of this machine are the _wind-chest_, the _pipes_, and the _bellows_. The former of these is an oblong box, made perfectly air-tight, and placed in a horizontal position. The top of this chest is perforated with several rows of holes of different sizes, and into these are inserted the pipes. Those for the higher notes are of a cylindrical form, and are made of a mixture of metals, chiefly of tin and lead; but those designed for the expression of the lowest notes of the base are made of wood, in a square form. The dimensions of these pipes are regulated by a _diapason_, or _scale_. 4. There are as many of these rows of pipes, which are called _stops_, as there are kinds of tones in the organ; and to every row or stop is a plug, attached to a slide, which is denominated a _register_, and which is designed to regulate the admission of wind into the pipes. The pipes are also furnished with valves, which can be opened at pleasure, by means of keys similar to those of the piano-forte. Some organs have few, others have many stops; and, in order to regulate the force of sound, most church organs have two or three rows of keys, whereby a greater or less number of pipes may be filled, and the powers of the instrument may be controlled in what is called the _small organ_, or let loose, so as to become the _full organ_. 5. The fingering of an organ is similar to that of the piano-forte, so far as relates to the position of the keys; but, on account of the great number of holding notes in organ music, and the manner in which the sound is produced, the fingers are more kept down; whence it is considered injurious for performers on the piano-forte to practise on the organ, lest that lightness of touch, so necessary for the former instrument, be affected. It is hardly necessary to remark that, during the performance on the organ, the wind-chest is filled by means of the bellows. 6. The structure of the organ is lofty, elegant, and majestic; and its solemnity, grandeur, and volume of tone, have obtained for it a pre-eminence over every other instrument for the sacred purposes to which it has been applied. The largest organ known is in St. Peter's Church, at Rome. It has one hundred stops. 7. The church organ was probably suggested by the _water organ_ of the Greeks, which was invented five or six hundred years before our era. At what period, organs began to be employed in churches, cannot now be ascertained. By some, it is said that Pope Vitelianus caused them to be used in Rome in the seventh century. Others are of opinion, that they were not introduced until three hundred years later. But, be this as it may, the church organ was not in common use until the fourteenth century; and now it is very different in its construction from that of early times. It has received many additions and improvements since the beginning of the fifteenth century.