The views, however, of professor Leslie do not coincide with this theory. "The performance of the speaking-trumpet," he says, "does certainly not depend upon any supposed repercussion of sound; repeated echoes might divide, but could not augment the quantity of impulse." His idea is, that "the tube, by its length and narrowness, detains the efflux of air, and has the same effect as if it diminished the volubility of that fluid, or increased its density." "The organs of articulation," he adds, "strike with concentrated force; and the pulses thus so vigorously excited are, from the reflected form of the aperture, finally enabled to escape and spread themselves along the atmosphere." The experiments of Hassenfratz, a French philosopher, are cited in support of this theory. He tried the power of a speaking-trumpet, by measuring the distance at which the ticking of a watch could be heard through it, and found the effect the same, whether the metal tube were used simply, or wrapped round in such a way as to prevent vibration. It was also heard at the same distance when the inner surface was lined with linen, or woollen cloth, to diminish reflection; and the range of a cylindrical trumpet was the same as that of a conical one. In the ear-trumpet, used by persons partially deaf, and in the stethoscope, it appears to us that we have concentrations of sound into a focus, by repeated reflexions of the aërial wavelets from side to side of the tube; and we may ask, does not the murmuring sound which we hear, when we apply a conch shell to our ear, arise from the same kind of percussion? In echoes we have an instance of the reflection of atmospheric undulations, and this reflection, as in many natural situations, may be multiplied till the last faint repetitions die away; and when a succession of echoes is very rapid, a loud exclamation of the monosyllable, Hah! may be thrown back upon the ear like a boisterous laugh. The echoes of old halls and galleries, of vaults and dungeons, of caverns, rocks, and grottoes, have served as subject-matter for the writers of imaginative poetry and romance. It is not only to old romantic halls and galleries that echoes are confined; they are not uncommon in modern public buildings, so that the speaker hears his words repeated. "The distribution of sound in public edifices, so that the echoes (or reflections of aërial waves from the walls, etc.) may be most advantageously brought to strengthen the original sound, is a subject deserving of much attention." For some observations on the errors of architects on this point we may refer to sir J. Herschel's Treatise on Sound.