AS A VEHICLE OE SOUND. 
227 
were heard in the middle of Folkestone, while in the opposite direction they failed to 
reach the South Sand Head light-vessel. On the 29th the limits of range were East- 
ware 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 the one direction 
three times as far as in the other*. 
It may be worthy of note that within twenty yards or so of the gun the sound was so 
intense as to render it necessary, prior to each shot, to remove a barometer which hung 
on the side of the wooden shed containing the horns of Mr. Holmes. On one occasion, 
when this precaution was neglected, the barometer was broken by the concussion. Neither 
horns nor syren appear to affect the instrument perceptibly. Still, notwithstanding this 
initial ms viva , the gun-sound is often overmatched at a distance by both syren and 
horns. 
§ 17. Influence of Pitch and Pressme. 
On October 18 experiments were made in which the steam-pressure was varied with 
a constant pitch, and others in which the pitch was varied with a constant pressure. At 
a distance of 3 miles from the shore the intensities corresponding to 40 lbs. and 80 lbs. 
pressure respectively did not differ from each other as much as might have been expected. 
With a pressure of 40 lbs. the sound was very fine ; with a 50-lbs. pressure it was also 
very fine, and perceptibly harder ; with a pressure of 60 lbs. it differed but little from 
that of 50; with a pressure of 70 lbs. the sound was harder and firmer than the last; 
the difference between 70 and 80 lbs. was scarcely perceptible. 
* In vol. i. of the f Ann ales de Chemie’ for 1816, p. 176, Aeago introduces a memoir by De la Roche, then 
-recently deceased, in these words : — “ L’auteur arrive a des conclusions, qui d’abord pourront paraitre paradox- 
ales, mais eeux qui savent comhien il mettait de soins et d’exacitude dans toutes ses recherches se garderont 
sans doute d’opposer une opinion populaire a des experiences positives.” De la Roche’s paper was “ On the 
Influence exerted by the Wind on the Propagation of Sound and the strangeness of his 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, hut that the range at right angles to the wind is the greatest of all. 
The only attempt to account for De la Roche’s results theoretically is due to Professor Stokes. In a short 
but exceedingly able communication presented to the British Association in 1857, this eminent physicist points 
out a true 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 
therefore increases from the ground upwards. This difference of velocity throws the sound-wave upwards in a 
direction opposed to the wind, and downwards in a direction coincident with the wind. In this latter case 
■the direct wave is reinforced by the wave reflected from the earth. Now the reinforcement is greatest in the 
direction in which the direct and reflected waves inclose the. smallest angle — that is, at right angles to the 
direction of the wind; hence the greater range in this direction. It is not therefore, according to Professor 
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 ; hut on communicating with Mr. Coxwell, who has earned for himself so 
high a reputation as an aeronaut, I learned with regret that the experiment was too dangerous to be carried out. 
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