AS A VEHICLE OE SOUND. 
191 
the shadow suffering most. (On the Map the condensations and rarefactions of the 
direct waves are shown by circular lines of varying closeness.) Here, then, we have one 
cause of the decay of the sound in the neighbourhood of the acoustic shadow. Another 
cause is the interference of the direct waves with those reflected from C and from other 
portions of the cliff. The remarks here applied to the sound-shadow west of the Fore- 
land are also applicable to that upon the other side. 
On July 25th a gradual improvement in the transmissive power of the air was observed 
from morning to evening ; but at the last the maximum range was only moderate. The 
fluctuations in the strength of the sound were remarkable, sometimes sinking to 
inaudibility and then rising to loudness. A similar effect, due to a similar cause, is 
often noticed with church-bells. The acoustic transparency of the air was still further 
augmented on the 26th: at a distance of 9^ miles from the station the whistles and 
horns were plainly heard against a wind with a force of 4 ; while on the 25th, with a 
favouring wind, the maximum range was only 6^ miles. Plainly, therefore, something 
else than the wind must be influential in determining the range of the sound. 
§ 5. Rotation of Horn. 
Thus far I have confined myself to the salient point or points of each day’s observa- 
tions, omitting numerous details. The observations of July 1 and 3 are, I think, of a 
character to bear a fuller treatment. Tuesday, July 1, was devoted to the further 
investigation of the sound-range, and also to obtaining additional information as to how 
the sound diminishes in intensity as we depart from the axis of the horn. It is obvious 
that for this purpose, instead of carrying the ship round the horn, we may cause the 
horn to rotate round its vertical axis while the ship maintains a fixed position. 
On this occasion the steam- whistle from the United States was mounted, and 
sounded at different times during the day. The whistle is 12 inches in diameter, 
and it was blown at 70 lbs. pressure. The resonant bell of the whistle cannot be moved, 
a fixed distance existing between the circular ring from which the steam issues and the 
cutting-edge. 
We steamed to a point on the axis which bore S.S.W. from the station. Wind calm 
on sea; on shore N.N.W., with a force of 3. We halted at a distance of 5J miles, and 
received a very good sound from the horn when it was pointed towards us. Forty-five 
degrees right and left of this, the sound was also good, but it became feebler as the 
departure from the axis was augmented. 
To attempt applying numerical estimates to the sounds would be mere guesswork ; 
still it will fix the ideas if I give a sample of the method by which the day’s observations 
were conducted. A circle was drawn, the circumference of which was divided at eight 
points into equal parts, the radius drawn from the centre to those points representing 
the direction of the horn. Opposite to each point was placed a number indicative of 
the strength of the corresponding sound. Thus, assuming the intensity when the 
