388 ECHO ered with soft velvety spines, living on sandy shores, where they lightly bury themselves, only discoverable after storms or in still days when they slowly change their places. In the globular forms the eye specks, which are at the end of the rays in the star fish, are drawn close together at the summit; at the same place is a curious little sieve or madreporic body, through which water is admitted to the five principal tubes of the interior. Sea ur- chins are able to bore holes in the hardest rocks, in which they lodge, enlarging the cav- ity as they increase in size, the opening remain- ing the same, so that they are prisoners for life. It is generally believed that they thus pene- trate and excavate rocks by the constant mo- tion of the microscopic vibratile cilia which cover their spines. The heart-shaped sea ur- chins, the spatangoids, bury themselves in mud Mellita quinquefora. and sand. Sea urchins are found from the up- per Silurian to the present time ; they were very abundant in the mesozoic age, especially in the chalk epoch. ECHO (Gr. iixb, from fotf, a noise), a sound reflected by an opposing surface and repeated to the ear of a listener. The nature of those pulses that are propagated through the air to the ear and produce the sensation of sound is fully explained under the latter title, where also it is shown that the sound pulse is partly reflected when it encounters in its path a me- dium of different elasticity or density ; if the re- flected sound reach the observer so long after the original sound as to constitute a distinct noise, it is called an echo. Simple echoes, and even those that repeat several times in succes- sion the words of the human voice or other sounds, are familiar to all. Of more remark- able cases we may instance the following. A writer in 1766 states that at the Simonetta palace near Milan there was an echo that re- peated 60 times the sound of a pistol ; this echo does not now exist. Sir John Herschel quotes the case of an echo at Woodstock park that repeats 17 syllables in the daytime and 20 at night. Dr. Page (1839) notices one at Belvi- dere, Allegany co., N. Y., that repeats three syllables distinctly 11 times, the observer standing between two barns. Most remark- able, however, are the phenomena of harmonic echoes, or those that repeat in a different key the direct sound. Dr. Page observed such an echo in Fairfax co., Va., which repeats 13 syl- lables three times; and if 20 notes be played on a flute, they are returned with perfect dis- tinctness, but some of them are raised in pitch by a third, a fifth, or an octave, the effects varying with the state of the atmosphere. Tyndall describes the echoes of the Alpine horn rebounding from the rocks of the Wetter- horn or Jungfrau as dying away in successive reflections, gradually becoming more soft and flute-like. Dr. Brewer in his work on " Sound and its Phenomena" (1864) instances the fol- lowing cases : Not far from Coblentz an echo is found that makes 17 repetitions at unequal intervals, some loud, some soft; some to the right, others to the left of the observer ; some in unison with the direct sound, others a third, fifth, or tenth of the fundamental. At the lake of Killarney is an echo that renders an excellent second to any simple air played on the bugle. Some distance from Glasgow there was formerly a remarkable case in which eight or ten notes of a trumpet were repeated a third lower, and again a second and third time, each time lower still. The Hon. J. W. Strntt, now Lord Kayleigh, observed (" Nature," 1873) the sound of a woman's voice echoed from a plan- tation of firs, but its pitch raised an- octave ; with a man's voice the phenomenon could not be produced. The various interesting peculi- arities of echoes depend upon the conditions under which the reflection takes place, and the position of the observer with respect to the reflecting surfaces ; these conditions may be so varied as to more or less affect the intensity and the quality of the original sound, and wo will separately consider these two peculiarities. The intensity of a sound varies, other things being equal, directly as the solid angle included at the origin of the sound between the extreme limits of the wave that enters the observer's ear. If the reflecting surface be plane and so small that it subtends a smaller angle than the tympanum of the ear, the intensity of the ob- served echo will be diminished in proportion to the size of the reflector. If the effective surface be larger than this limit, and nothing of intensity be lost in the act of reflection, the echoes will be as loud as the direct sounds that have passed over the same length of path. There is however always in the act of reflec- tion an actual loss, depending on the relative density and elasticity of the air and the reflect' ing body ; this principle is applied to deaden the echoes within a poorly contrived audito- rium, by encasing the bare walls in wooden panels or draping them with any light material. Two planes may be so placed that the reflected sounds proceeding from each reach the ob'
