230 
PEOFESSOE TYNDALL ON THE ATMOSPHEEE 
pressure or thereabouts: the trumpets have been worked with compressed air; and 
our experiments have proved that a pressure of 20 lbs. yielded sensibly as loud a 
sound as higher pressures. The possibility of obtaining a serviceable sound with this 
low air-pressure may render the employment of caloric engines available with trumpets : 
if so, the establishment of trumpets on board light-vessels would be greatly facilitated. 
The signals at present existing onboard such vessels are very inefficient, and may, I 
think, be immeasurably improved upon. There are, I am told, practical difficulties as 
to the introduction of steam on board light-ships ; otherwise I should be strongly 
inclined to recommend the introduction among them of the Canadian whistle. The 
syren would probably be found too large and cumbrous for light-vessels. 
The form of the syren which has been long known to scientific men is worked with 
air, and it would be worth while to try how the fog-syren would behave supposing 
compressed air to be substituted for steam. Compressed air might also be tried with 
the whistles. Such experiments, to render them comparable with our previous ones, 
ought to be made at the South Foreland. 
No fog-signal hitherto tried is able to fulfil the condition laid down by Dr. Robinson, 
in the very able letter already quoted in § 2, namely, “ihat all fog-signals should he 
distinctly audible for at least 4 miles , under every circumstance .” Circumstances may exist 
to prevent the most powerful sounds from being heard at half this distance. What may 
with certainty be affirmed is, that in almost all cases the syren may certainly be relied 
on at a distance of 2 miles ; in the great majority of cases it may be relied upon at a 
distance of 3 miles, and in the majority of cases to a distance greater than 3 miles. 
Happily the experiments thus far made are perfectly concurrent in indicating that at 
the particular time when fog-signals are needed, that is during foggy weather, the air 
in which the fog is suspended is in a highly homogeneous condition ; hence it is in the 
highest degree probable that in the case of fog we may rely upon these signals being 
effective at far greater distances than those j ust mentioned. 
I say “ probable,” while the experiments seem to render this result certain. Before 
pronouncing it so, however, I should like to have some experience of warmer fogs than 
those in which the experiments have hitherto been made. That the fog-particles them- 
selves are not sensibly injurious to the sound has been demonstrated; but it is just 
possible that in warm weather the air associated with the fog may not be homogeneous. 
It will probably be found so, but I would recommend the experiment to be made on 
some of the fogs of the early summer. 
I am cautious not to inspire the mariner with a confidence which may prove delusive. 
When he hears a fog-signal he ought, as a general rule (at all events until extended 
experience justifies the contrary), to assume the source of sound to be not more than 2 or 
3 miles distant, and to take precautions accordingly. 
Once warned, he may, by the heaving of the lead or some other means, be enabled to 
check his position. But if he errs at all in his estimate of distance, it ought to be on 
the side of safety. 
Unless strong practical reasons be adduced in its favour, I should deprecate a 
