186 
PEOFESSOB TYNDALL ON THE A TM OSPHEEE 
opinions regarding the deadening power of fog than the paper of Derham already referred 
to. In consequence of their a priori probability, his conclusions seem to have been 
transmitted unquestioned from generation to generation of scientific men. 
§ 3. Instruments and Observations. 
These extracts and references sufficiently indicate the uncertain state of the question 
when, on the 19th of May, 1873, this inquiry began. The South Foreland, near 
Dover, was chosen as the signal-station, steam-power having been already established 
there to work two powerful magneto-electric lights. The observations for the most 
part were made afloat, one of the yachts of the Trinity Corporation being usually 
employed for this purpose. Two stations had been established, the one at the top, 
the other at the bottom of the South-Foreland Cliff ; and at each of them trumpets 
and air- and steam-whistles of great size were mounted. The whistles first employed 
were of English manufacture; but intelligence having been received regarding a large 
United-States whistle, and also a Canadian whistle, of great reputed power, the Elder 
Brethren had them subsequently added to the list. 
On the 8th of October another instrument, which has played a specially important 
part in these observations, was introducd. During my recent visit to the United States, 
I was favoured by an introduction to General Woodruff by Professor Joseph Henry, 
of Washington. Professor Henry is Chairman of the Lighthouse Board, and General 
Woodruff is engineer in charge of two of the Lighthouse districts. I accompanied 
General Woodruff to the establishment at Staten Island, and afterwards to Sandy 
Plook, with the express intention of observing the performance of a steam-syren which, 
under the auspices of Professor Henry, has been introduced into the lighthouse system 
of the United States. Such experiments as were possible to make under the circum- 
stances were made ; and I carried home with me a somewhat vivid remembrance of the 
mechanical effect of the sound upon my ears and body generally. Hence my desire to 
see the syren tried at the South Foreland. The formal expression of this desire was 
anticipated by the Elder Brethren, while their wishes were in turn anticipated by the 
courteous kindness of the Lighthouse Board at Washington. Informed by Major Elliott, 
of the United States Army, that our experiments had begun, the Board forwarded to 
the Corporation, for trial, the instrument now mounted at the South Foreland. 
In the steam-syren patented by Mr. Brown, of New York, a fixed disk and a rotating 
disk are employed as in the ordinary syren, radial slits being cut in both disks instead of 
circular apertures. One disk is fixed vertically across the throat of a conical trumpet 
16|f feet long, 5 inches in diameter where the disk crosses it, and gradually opening 
out till at the other extremity it reaches a diameter of 2 feet 3 inches. Behind the 
fixed disk is the rotating one, which is driven by separate mechanism. The trumpet 
is mounted on a boiler. In our experiments steam of 70 lbs. pressure has for the most 
part been employed. J ust as in the ordinary syren, when the radial slits of the two disks 
coincide, and then only, a strong puff of steam escapes. Sound-waves of great intensity 
