228 
PROFESSOR TYNDALL ON THE ATMOSPHERE 
In determining the influence of pitch, the rate of revolution was varied from 1500 to 
2400 a minute. The sound in all cases was very fine, but that corresponding to the 
most rapid rate of revolution seemed the best and most penetrating. Though these 
experiments were made at distances of 2 and 3 miles from the shore, distinct and long- 
continued echoes followed every syren-blast, coming to us from a bearing directly opposed 
to that of the signal-station". 
I have already mentioned October 17 as our day of maximum acoustic transparency; 
the transparency continued to some extent during the 18th ; for on this day, while halting 
at a distance of 3 miles from the station, we heard the loud report, not of cannon, but 
apparently of musketry on shore. The sounds were afterwards traced to rifle practice on 
Kingsdown beach. The day was optically far less favourable than July 3, but each rifle 
made itself distinctly heard to twice the distance at which an 18-pounder failed to be 
heard on the 3rd of July. 
Arrangements having been made to enable the syren to be pointed in different direc- 
tions, November 21 and the two subsequent days were devoted to the investigation of 
both it and the gun, with reference to the direction of their axes. The sonorous waves 
surrounding the gun proved to be of almost equal intensity throughout, very little dif- 
ference being observed between the sound when the gun was pointed at us, and when it 
was pointed at right angles to the line joining it and us. In the syren, however, the 
intensity in the axis was markedly superior to the intensity at right angles to the axis. 
This is what might be expected; for the syren-trumpet being expressly intended to project 
the sound in a certain direction, it can only do this by withdrawing it from other direc- 
tions. 
§ 18. Concluding Remarks. 
A few additional remarks and suggestions will fitly wind up this paper. It has been 
proved that in some states of the weather the howitzer firing a 3-lb. charge commands a 
larger range than the whistles, trumpets, or syren. This was the case, for example, on the 
particular day, October 17, when the ranges of all the sounds reached their maximum. 
On many other days, however, the inferiority of the gun to the syren was demonstrated 
in the clearest manner. The gun-puffs were seen with the utmost distinctness at the 
Foreland, but no sound was heard, the note of the^ syren at the same time reaching us 
with distinct and considerable power. 
The disadvantages of the gun are these : — 
a. The duration of the sound is so short that, unless the observer is prepared before- 
hand, the sound, through lack of attention rather than through its own powerlessness, 
is liable to be unheard. 
b. Its liability to be quenched by a local sound is so great that it is sometimes obli- 
terated by a puff of wind taking possession of the ears at the time of its arrival. This 
point was alluded to by Arago, in his report on the celebrated experiments of 1822. 
By such a puff a momentary gap is produced in the case of a continuous sound, but not 
entire extinction. 
