Mr. Seymour concurred in this opinion, and immediately afforded the following explanation:--“An echo is nothing more than a reflected sound. When the aërial vibrations strike against any obstacle of sufficient magnitude, they are reflected back to the ear, and produce a repetition of the sound, which will appear to proceed from the point whence they are reflected, so that the apparent direction of the voice becomes completely changed by an echo. A considerable extent of level wall will sometimes produce it in great perfection; for a smooth surface reflects sounds much better than a rough one: but the circumstance which, perhaps, contributes more than any other to the perfection of an echo, is the form of the reflecting surface; a convex surface is a very bad reflector of sound, a flat one reflects very well, but a small degree of concavity is the form best adapted to the purpose.” “I believe,” observed the vicar, “that fluid bodies will also, under certain circumstances, so reflect sound as to produce echoes.” “Undoubtedly. The surface of water, especially at the bottom of a well; and sometimes even clouds will produce this effect.” “Do you mean to say, papa,” asked Tom, “that sound is reflected from an obstacle to the ear, in the same manner as my ball is reflected after striking the wall?” “Certainly: supposing, of course, that your ball is perfectly elastic; and in that case, you no doubt remember the direction it will follow.” “It will always make the angle of _reflection_ equal to the angle of _incidence_,”[58] said Tom. “Undoubtedly; and so it is with sound, since air, as you know, is perfectly elastic. If, therefore, the vibrations fall perpendicularly on the obstacle, they are reflected back in the same line; if obliquely, the sound returns obliquely in the opposite direction, the angle of reflection being equal to that of incidence. You will, therefore, readily perceive,” continued Mr. Seymour, addressing his conversation more particularly to Miss Villers, “that a person situated at an appropriate angle may hear an echo, as it is returned from the reflecting surface, without hearing the original sound which produced it. M. Genefay has described, as existing near Rouen, a curious oblique echo which is not heard by the person who emits the sound. A person who sings hears only his own voice, while those who listen hear only the echo.” “As a smooth and concave surface is capable of producing an echo, how does it happen that we so rarely meet with one in a room?” asked Louisa.