ECHINUS (from extrog, a word denoting the prickly cover of a Chesnut) a convex moulding in the form of a conic section, generally carved into ornaments representing trun cated spheroids, or eggs, with the upper ends cut off, the upper part of the axis projecting, and the lower part receding. Each truncated spheroid is surrounded with a border, of an elliptic figure, in close contact, showing something more than a semi-ellipsis, the shorter axis being horizontal.
The projecting edge of the border is in the surface of the moulding, previously wrought, as is also the curvature or the upper part of the spheroid. Every two adjacent borders contain a space equal to the thickness of the border at the top, and gradually receding towards the bottom. In each recess, or space, is an anchor, or tongue ; the front edge of which comes in contact with the surface of the original moulding, and is in a vertical line cutting the surthce of the moulding at right angles.
In Grecian architecture, the front of each border, and also the front of each tongue, or anchor. is wrought to an angle, the section of which inclines equally to the surface. The bottom of the spaces on each side of the tongue, on the under side, is nearly in the same surface as the recess on the sides of the eggs.
In Boman architecture, the general contour is the segment or quarter of a circle, and the fronts of the surrounding bor ders are not brought to an angle, but remain as part of the moulding, either plain or with a hollow sunk between the edges, leaving a fillet next to each edge.
Its situation in an order, is in the entablature or capital, but never in the base.
In the original Doric order, the ovolo, which crowns the cornice, and that of the capital, are never carved. In the Ionic and Corinthian entablatures, it may either be carved or not, but in the antiques it is generally carved. The ovolo in the capitals of these orders, is, however, always carved into the ornaments we are describing. The French call this moulding, quart de road; the English, quarter-round, or boulting ; the Italians, ovolo; the Latins, ovum, from its being usually carved with the figures of eggs ; and the French, for the same reason, sometimes call it a'm..J: ECHO, (from the Greek, ytoc, sound, of the verb, nxeco, I sound.) the reverberation of sound, occasioned by the particular construction of a vault or wall, the section of which is most commonly of an elliptical or parabolical figure. The method of making artificial echoes, is taught by the Jesuit Illancani, in his Echometria, at the end of his book On the Sphere.
We are informed by Vitruvins, that in various parts of Greece and Italy, there were brazen vessels, ingeniously arranged under the seats of the theatres, to render the sound of the actors' voices more clear, and make a kind of echo ; by which means the prodigious multitude of persons pre sent at their spectacles were enabled to hear with ease and pleasure.
The distribution of sound in public edifices, so that the echoes may be most advantageously brought to strengthen the original sound, is a subject practically deserving much attention. In Sir J. Herschel's Treatise on Sound, the reader will find some sensible observations on the errors of architects in this respect. The inattention of the latter to the effect of the reverberation of sound, was curiously exemplified in the cathedral of Girgenti, where the con fessional was placed in a focus conjugate to another and unenclosed part of the church ; by which unlucky error the echo was instrumental in informing a husband of the infi delity of his spouse. In many of our public buildings, though professedly erected for purposes where the proper distribution of sound is of paramount importance, it is no uncommon occurrence, that one part of the audience pos sesses a monopoly, while the remainder witness the ceremony or performance in dumb show.
Sounds are reflected by certain configurations of bodies, like the reflection of light from polished surfaces; so that if a person situated before one of these bodies utter a word, he will in a short time after hear the echo, or repetition of the sound. The vibratory motion of the air, which constitutes sound, is reflected by hard bodies, and, in certain cases, even by fluids. Thus the sides of a hill, houses, rocks, banks of earth, the large trunks of trees, the surface of water, espe cially at the bottom of a well, and sometimes even the clouds, have been found capable of reflecting sounds. The confi guration of the surface of these bodies, is much more con cerned in the production of the echo than their substance. A smooth surface reflects sounds much better than a rough one. A convex surthce is a very bad reflector of sound ; a fiat one reflects very well ; but a small degree of concavity, par ticularly when the sounding body is in or near the focus of concavity, renders the surface a much better reflector, and the echo is heard considerably louder.