ON FOG SIGNALS. 107 
efficiency can only be imperfectly estimated in the absence of authentic data. 
The steam-whistle is the best-known of them, and is stated to act well. It 
is said that one used in the Bay of Fundy had been heard eight miles against 
the wind (the velocity of which, however, is not given), One witness thinks 
that in rough weather it is heard further than a gun. It is possible that 
some loss of sound may take place with it where the steam comes into con- 
tact with the air. 
* An air-whistle, by Wells, is reported as ‘ very feeble at 2,5, miles ;’ it was 
blown by bellows. Horns and trumpets are preferred by some: blown by 
men, they are said to be heard in Noya Scotia from two to three miles, and 
probably with steam or condensed air will be much further, The instru- 
ments of Holmes* (so well known by his electric light) and of Daboll are said 
to have great power. 
“This summary shows how little is known of the facts on which the 
efficiency of fog-signals depends, even of that which is the most important 
—the distance at which they can surely be heard under those circumstances 
where they are most necessary. 
“A. At the outset, it is obvious that, to make experiments comparable, we 
must haye some measure of the fog’s power of stopping sound, without 
attending to which the most anomalous results may be expected. It seems 
probable that this will bear some simple relation to its opacity to light, and 
that the distance at which a given object, as a flag or pole, disappears may be 
taken as the measure. It is easily ascertained, and should be noted both at 
the signal-station and the observing one: the fog may have many fluctua- 
tions of density between them; so that this will only be an approximate 
estimate, but one which will aid in insuring that the signals shall have suf- 
ficient power to pass the minimum of efficiency, That minimum is when a 
ship, putting down her helm on hearing the signal, can just come round clear 
of the danger. For very large steamers this cannot be done in less than 
two miles; and, allowing for sea and currents, that limit should at least be 
doubled ; so that it may be assumed as a law, that (except in harbour) all fog- 
signals should be distinctly audible for at least four miles under every eircwm- 
stance. 
« B. The range of sound depends on causes not thoroughly understood, and 
sometimes is very different from what might be expected. Some sounds which 
near at hand are very loud, as the explosions of fulminating compounds, reach 
buta very little way. Others fail from want of intensity, though the quantity 
of sound is enormous: thus thunder, however violent, is not heard at twenty 
miles’ distance, while heavy ordnance is said to haye reached two hundred. 
Experiment can alone decide. It should therefore be ascertained by trial, 
first, what source of sound (of course among those already mentioned as suitable 
to this particular object) has the greatest space-penetrating power in still and 
clear air. Secondly, besides the natural decay of sound due to distance alone, 
it is, in the case which interests us, stifled by other sounds near the listener. 
The movements of the crew, the noise of the engines, the rush of the vessel 
through the sea, the murmur of winds and waves are close at hand to prevent 
it from being noticed (though still of sufficient power to be heard), unless it 
have some peculiar character which prevents it from blending with them. 
* Holmes’s trumpet has a strong reed, and is blown by steam of about 20 lbs. pressure. 
He thinks low pitch is heard the furthest, and compound sounds still better. One at 
Dungeness has been heard in fog at five miles, when a bell of 8 cwt. was inaudible at little 
more than two. One larger, and an octave lower, was heard certainly at nine and a half 
miles. These require less steam than the whistle. 
