190 
PROFESSOR TYNDALL ON THE ATMOSPHERE 
§ 4. Influence of Sound- Shadow. 
On the 19th of May we noticed a phenomenon of grave import in connexion with the 
establishment of fog-signals. I refer to the rapid fall of intensity on both sides of the 
signal-station at the South Foreland. We had halted between the station and the South 
Sand Head light-ship, at a distance of 2| miles from the former. The trumpets and. 
whistles were sounded, but they were quite unheard. We moved nearer; but even at a 
mile distance, with the instruments plainly in view, their sound failed to reach us. A 
light wind, however, was here opposed to the sound. Abreast of the signal-station the 
trumpets were very powerful ; but on approaching the line joining the Foreland to the 
end of the Admiralty Pier the sound fell rapidly, though in this case the wind was 
favourable to the sound. Plainly, therefore, some other cause than the wind must be 
invoked to account for the phenomenon. 
On the 10th of June the same effect was very strikingly manifested. After our day’s 
work we steamed past the Foreland and towards the end of the Pier. At the distance 
of a mile the sounds fell with such rapidity that I thought something must have occurred 
to the whistles and horns. Happily the guns were there to test this surmise. At 2 miles 
distance we signalled for them. With a 3-lb. charge, though their puffs were clearly 
seen, not one of them was heard; with a 6-lb. charge the 18-pounder was barely heard, 
the howitzer was slightly better heard, while the. mortar was quite unheard. No pecu- 
liarities of the horns or whistles could therefore account for the phenomenon. 
On the 11th of June the effect was equally pronounced. On the line joining the 
Foreland and the Admiralty Pier, and at f of a mile from the station, the sound rapidly 
sank in power, and soon afterwards became inaudible. At 1^ mile distance we signalled 
for the guns; the report in each case was a low indistinct thud. A necessary requirement 
in fog-signals is stated to be that they should, under all circumstances, be heard to a 
distance of 4 miles. Now the gun was undoubtedly the signal of greatest range when 
this inquiry began, and here we find that conditions may exist which render even the 
gun ineffectual at less than half the distance deemed essential. 
The Map on Plate XIX., which consists of a portion of Plate XVII. enlarged, will 
help us to an explanation of these observations. Near the fog-signal station a projecting 
chalk cliff at C receives the impact of the sonorous waves and disperses them by reflec- 
tion. The whole sea space between the line A B and the cliffs under Hover Castle is in 
the sound-shadow. Within this line the instruments cannot be seen, without it they 
can ; and we have to account for the fact that the enfeeblement of the sound occurs not 
only inside but immediately outside the boundary, and while the instruments are in 
sight. A sudden subsidence of the sound is always observed on crossing the boundary 
towards the shore, and a correspondingly sudden augmentation on crossing it towards 
the sea ; but the stoppage of the sound on entering the shadow is by no means total. 
The whole of the shaded space is filled with sound of enfeebled intensity, produced in 
great part by the divergence into the shadow of the waves which abut against the 
boundary. Through this divergence the direct waves suffer, the portions nearest to 
