TRUMPET

, Listening or Hearing, is an instrument invented by Joseph Landini, to assist the hearing of persons dull of that faculty, or to assist us to hear persons who speak at a great distance.

Instruments of this kind are formed of tubes, with a wide mouth, and terminating in a small canal, which is applied to the ear. The form of these instruments evidently shews how they conduce to assist the hearing; for the greater quantity of the weak and languid pulses of the air being received and collected by the large end of the tube, are reflected to the small end, where they are collected and condensed; thence entering the ear in this condensed state, they strike the tympanum with a greater force than they could naturally have done from the ear alone.

Hence it appears, that a speaking Trumpet may be applied to the purpose of a hearing Trumpet, by turning the wide end towards the sound, and the narrow end to the ear.

Speaking Trumpet, is a tube of a considerable length, from 6 to 15 feet, used for speaking with to make the voice be heard to a greater distance.

This tube, which is made of tin, is straight throughout its length, but opening to a large aperture outwards, and the other end terminating in a proper shape and size to receive both the lips in the act of speaking, the speaker pushing his voice or the sound outwards, by which means it may be heard at the distance of a mile or more.

The invention of this Trumpet is held to be modern, and has been ascribed to Sir Samuel Moreland, who called it the tuba stentorophonica, and in a work of the same name, published at London in 1671, that author gave an account of it, and of several experiments made with it. With one of these instruments, of 5 1/2 feet long, 21 inches diameter at the greater end, and 2 inches at the smaller, tried at Deal-Castle, the speaker was heard to the distance of 3 miles, the wind blowing from the shore.

But it seems that Kircher has a better title to the invention; for it is certain that he had such an instrument before ever Moreland thought of his. That author, in his Phonurgia Nova, published in 1673, says, that the tromba, published last year in England, he invented 24 years before, and published in his Mesurgia. He adds, that Jac. Albanus Ghibbisius and Fr. Eschinardus ascribe it to him; and that G. Schottus testifies of him, that he had such an instrument in his chamber in the | Roman college, with which he could call to, and receive answers from the porter.

But, considering how famous the tube or horn of Alexander the Great was, it is rather strange that the Moderns should pretend to the invention. With his stentorophonic horn or tube he used to speak to his army, and make himself be distinctly heard, it is said, 100 stadia or furlongs. A figure of this tube is preserved in the Vatican; and it is nearly the same as that now in use. See Stentorophonic.

The principle of this instrument is obvious; for as sound is stronger in proportion to the density of the air, it follows that the voice in passing through a tube, or Trumpet, must be greatly augmented by the constant reflection and agitation of the air through the length of the tube, by which it is condensed, and its action on the external air greatly increased at its exit from the tube.

It has been found, that a man speaking through a tube of 4 feet long, may be understood at the distance of 500 geometrical paces; with a tube 16 2/3 feet, at the distance of 1800 paces; and with a tube 24 feet long, at more than 2500 paces.

Although some advantage in heightening the sound, both in speaking and hearing, be derived from the shape of the tube, and the width of the outer end, yet the effect depends chiefly upon its length. As to the form of it, some have asserted that the best figure is that which is formed by the revolution of a parabola about its axis; the mouth-piece being placed in the focus of the parabola, and consequently the sonorous rays reflected parallel to the axis of the tube. But Mr. Martin observes, that this parallel reflection is by no means essential to increasing the sound: on the contrary, it prevents the infinite number of reflections and reciprocations of sound, in which, according to Newton, its augmentation chiefly consists; the augmentation of the impetus of the pulses of air being proportional to the number of repercussions from the sides of the tube, and therefore to its length, and to such a figure as is most productive of them. Hence he infers, that the parabolic Trumpet is the most unfit of any for this purpose; and he endeavours to shew, that the logarithmic or logistic curve gives the best form, viz, by a revolution about its axis. Martin's Philos. Brit. vol. 2, pa. 248, 3d edit.

But Cassegrain is of opinion that an hyperbola, having the axis of the tube for an asymptote, is the best figure for this instrument. Musschenb. Intr. ad Phil. Nat. tom. 2, pa. 926, 4to.

For other constructions of Speaking Trumpets, by Mr. Conyers, see Philos. Trans. numb. 141, for 1678.

TRUNCATED Pyramid or Cone, is the frustum of one, being the part remaining at the bottom, after the top is cut off by a plane parallel to the base. See FRUSTUM.