On the refraction of sound by the atmosphere

Abstract

My object in this paper is to offer explanations of some of the more common phenomena of the transmission of sound, and to describe the results of experiments in support of these explanations. The first part of the paper is devoted to the action of wind upon sound . In this part of the subject I find that I have been preceded by Professor Stokes, who in 1857 gave precisely the same explanation as that which occurred to me. I have, however, succeeded in placing the truth of this explanation upon an experimental basis; and this, together with the fact that my work upon this part of the subject is the cause and foundation of what I have to say on the second part, must be my excuse for introducing it here. In the second part of the subject I have dealt with the effect of the atmosphere to refract sound upwards, an effect which is due to the variation of temperature, and which I believe has not hitherto been noticed. I have been able to show that this refraction explains the well-known difference which exists in the distinctness of sounds by day and by night, as well as other differences in the transmission of sound arising out of circumstances such as temperature; and I have applied it in particular to explain the very definite results obtained by Professor Tyndall in his experiments off the South Foreland. The Effect of Wind upon Sound is a matter of common observation. Cases have been known in which, against a high wind, guns could not be heard at a distance of 550 yards, although on a quiet day the same guns might be heard from ten to twenty miles. And it is not only with high winds that the effect upon sound is apparent; every sportsman knows how important it is to enter the field on the lee side even when the wind is very light. In light winds, however, the effect is not so certain as in high winds; and (at any rate so far as our ears are concerned) sounds from a small distance seem at times to be rather intensified than diminished against very light winds. On all occasions the effect of wind seems to be rather against distance than against distinctness. Sounds heard to windward are for the most part heard with their full distinctness; and there is only a comparatively small margin between that point at which the sound is perceptibly diminished and that at which it ceases to be audible.