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To Mothers.

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PI ^pftore^cem Iracteria.

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Diamonds and X-Rays.

The Mystery of the Echo

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The Mystery of the Echo Hie sconce of acoustics is, as yet, in its. tIfall". and men nave much to learn before they ( -,ii solve the mystery of the echo or predict her fleeting moods, writes Miss Gertrude Baeor in Pood Wordt Helated in general terms the explanation of echoed simple and easy to understand. Soutie,, as we fcnow, is conveyed to us by vibrations of the air, which spread around from the source of sound exactly as waves of "iter spread in ever widening jings when a pebble 18 brown into a still lake. Very frequently it happens that these waves oi IOUnd, in their outward course, strike against sores Mtrface of such a nature that they are, by it, deflected bac\ again without being broken and teattered. And when it occurs that these ware» -.e returned /uch an angle as to strike the eat Of a listener, we have what we call an echo. Often own than one reflection goes to the making up ol an echo, the sound-waves being thrown from one tttsfSaee to another in thair. passage to the ear-juie M a billiard ball will rebound from cushion f cushion on ita way round the table. This, roughta I0 the cav-jse of the phenomenon. But so ending Me the v ^riations of circumstances and wriron- aent, and the effects they produce so far-reaching and hard to foresee, that we are continually being taken unawares. Sometimes fiie echo returns so ipickly that it cannot be distinguished from tht anginal sound; and yet its undetected prso- aace is enough to affect seriously the penetration Of a foice in a church or theatre, Sometimes one NOULD will produce several echoes in differswt directions, which return and return again at different time intervals, to the great dittmettam of the hearers- Again the surface of the reieotfau; Object has a great deal to do with the nature el the echo returned. Certain substances seem to have a tendency to absorb the sound-waves, aid others to eflect them more readily. Another curiooa property of sound-waves, exemplified in many wall* fcrtown buildings, is the tendency 1 the WST« to mm round a ourrtd apse or galles?*, much an a wove of the see, striking aslant 611 a shallow toy, Wlii mn round the tihw. This is the explanottaP familiar acoustic ewiosities, aoteMV Wtoapoimg SeWwy ei St F«*l'o.