. Annual report of the Board of Regents of the Smithsonian Institution. Smithsonian Institution; Smithsonian Institution. Archives; Discoveries in science. 210 THE EEFRACTION OF SOUND. deflection presented the only satisfactory explanation of the familiar fact of observation that sound is usually heard many times farther in the direction of a wind than in a direction opposed to its action. His explanation of this phenomenon is as follows : "If we imagine the whole massof airin the neighbourhood of the source of disturbance divided into horizontal strata, these strata do not all move with
. Annual report of the Board of Regents of the Smithsonian Institution. Smithsonian Institution; Smithsonian Institution. Archives; Discoveries in science. 210 THE EEFRACTION OF SOUND. deflection presented the only satisfactory explanation of the familiar fact of observation that sound is usually heard many times farther in the direction of a wind than in a direction opposed to its action. His explanation of this phenomenon is as follows : "If we imagine the whole massof airin the neighbourhood of the source of disturbance divided into horizontal strata, these strata do not all move with the same velocity. The lower strata are retarded by friction against the earth, and by various obstacles they meet with ; the upper by friction against the lower, and so on. Hence the velocity increases from the ground upward, conformably with observation. This difference of velocity disturbs the spherical form of the sound-wave, tending to make it somewhat of the form of an ellipsoid, the section of which, by a vertical diametrical plane parallel to the direction of the wind, is an ellipse meeting the ground at an obtuse angle on the side toward which the wind is blowing, and an acute angle on the opposite side. Now sound tends to propagate itself in a direction perpendicular to the sound-wave; and if a portion of the wave is intercepted by an obstacle of large size, the space behind is left in a sort of sound-shadow, and the only sound there heard is what diverges from the general wave after passing the obstacle. Hence, near the earth, in a direction contrary to the wind, the sound continually tends to be propagated upward, and consequently there is a continual tendency for an observer in that direction to be left in a sort of sound-shadow. Hence, at a sufficient distance, the sound ought to be very much enfeebled; but near the source of disturbance this cause has not yet had time to oper- ate, and therefore the wind produces no sensible effect, except what arises from the au
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