200 
PROFESSOR TYNDALL ON THE ATMOSPHERE 
than before. In the case of the syren, moreover, the reinforcement of the direct sound 
by its echo was distinct. About a second after the commencement of the syren-blast, 
the echo struck in as a new sound. This first echo, therefore, must have been flung 
back by a body of air not more than 600 or 700 feet in thickness. The few detached 
clouds visible at the time were many miles away, and could clearly have had nothing 
to do with the effect. 
This mingling of the echoes with the direct sound was much more distinctly heard 
in the case of the syren than in the case of the horn. With the horn, indeed, the echo 
was first plainly heard after the direct sound had ceased. 
As we descended towards St. Margaret’s Bay, where the horn and syren were well 
shaded by the hill, it was difficult to say when the direct sound ceased and the echoes 
began, so near in point of sensible intensity were the echoes to the direct sound. 
On the 10th of October I was again at the Foreland listening to the echoes, with 
results similar to those just described. On the 15th I had an opportunity of remarking 
something new concerning them. To the late Mr. Daboll, of the United States, 
belongs the credit of bringing large trumpets into use as fog-signals. At Dungeness 
one of his horns had been erected under his own superintendence ; and, wishing to make 
myself acquainted with its performance, we steamed thither on the 15th. The horn is 
worked by a caloric engine. Like the horns at the South Foreland it is vertically 
mounted, and like them bent near its extremity so as to present its mouth to the sea. 
It rotates automatically through an arc of 210°, halting at four different points on the 
arc and emitting a blast of 6 seconds duration, these blasts being separated from each 
other by intervals of silence of 20 seconds. The pressure in each case when the sound 
began was 8^ lbs., and it sank during the blast to 5f lbs. per square inch. 
I listened to the sound during several successive rotations of the horn : the augmen- 
tation was distinct when the axis of the horn was turned towards me. From the beach 
and from the lighthouse-tower I listened to the echoes l they were feeble compared with 
those of the syren, but were nevertheless very musical, and of 4 seconds duration. They 
were moreover purely aerial, as the heaven was quite free from clouds. 
The ‘ Galatea ’ at first sent us a startling echo from her side. Being afterwards 
turned with her bow facing in, this echo almost entirely disappeared, leaving the con- 
tinuous and gradually fading aerial echoes behind. Some ships were in sight, and when 
the horn accidentally pointed towards one of them, in the midst of the aerial echoes 
one would suddenly cease, thus breaking the uniform continuity of the sinking sound : 
the dying out would then continue. In the words of Admiral Collinson, who was at 
my side, the aerial sound, instead of ceasing suddenly and abruptly, “ tapered away.” 
The new point observed was that as the horn rotated the echoes were always 
returned along the line in which the axis of the horn pointed. Standing either behind 
or in front of the lighthouse-tower, or closing the eyes so as to exclude all knowledge 
of the position of the horn, the direction of its axis when it sounded could always be 
inferred from the direction in which the aerial echoes reached the shore. Not only 
