Page:Popular Science Monthly Volume 6.djvu/719

Rh us with surprising power, piercing through the whistling and moaning of the wind, which blew through Dover toward Folkestone. The sounds w r ere heard at 6 miles from the Foreland on the Folkestone road, and, had the instruments not then ceased sounding, they might have been heard much farther. At the South Sand Head light-vessel, 3$3/4$ miles on the opposite side, no sound had been heard throughout the day. On the 28th, the wind being north by east, the sounds were heard in the middle of Folkestone, 8 miles off, while in the opposite direction they failed to reach 3$3/4$ miles. On the 29th the limits of range were Eastware Bay on the one side, and Kingsdown on the other; on the 30th the limits were Kingsdown on the one hand, and Folkestone Pier on the other. With a wind having a force of 4 or 5, it was a very common observation to hear the sound in one direction three times as far as in the other.

This well-known effect of the wind is exceedingly difficult to explain. Indeed, the only explanation worthy of the name is one offered by Prof. Stokes, and suggested by some remarkable observations by De la Roche. In vol. i. of the "Annales de Chemie" for 1816, p. 176, Arago introduces De la Roche's memoir in these words: "L'auteur arrive à des conclusions, qui d'abord pourront paraître paradoxales, mais ceux qui savent combien il mettait de soins et d'exactitude dans toutes ses recherches se garderont sans doute d'opposer une opinion populaire a des expériences positives." The strangeness of De la Roche's results consisted in his establishing, by quantitative measurements, not only that sound has a greater range in the direction of the wind than in the opposite direction, but that the range at right angles to the wind is the maximum.

In a short but exceedingly able communication presented to the British Association in 1857, the eminent physicist above-mentioned points out a cause which, if sufficient, would account for the results referred to. The lower atmospheric strata are retarded by friction against the earth, and the upper ones by those immediately below them; the velocity of translation, therefore, in the case of wind, increases from the ground upward. This difference of velocity tilts the sound-wave upward in a direction opposed to, and downward in a direction coincident with, the wind. In this latter case the direct wave is reenforced by the wave reflected from the earth. Now, the reënforcement is greatest in the direction in which the direct and reflected waves inclose the smallest angle, and this is at right angles to the direction of the wind. Hence the greater range in this direction. It is not, therefore, according to Prof. Stokes, a stifling of the sound to windward, but a tilting of the sound-wave over the heads of the observers that defeats the propagation in that direction.

This explanation calls for verification, and I wished much to test it by means of a captive balloon rising high enough to catch the deflected wave; but, on communicating with Mr. Coxwell, who has