1878.] Prof. Tyndall. Experiments on Fog-Signals. 245 



at all. This difference may, perhaps, explain why metals conduct so 

 much better than glass, &c. 



III. " Recent Experiments on Fog-Signals." By Dr. Tyndall, 

 F.R.S., Professor of Natural Philosophy in the Royal Insti- 

 tution. Received March 14, 1878. 



Our most intense coast-lights, including the six-wick lamp, the 

 Wigham gas-light, and the electric light, being intended to aid the 

 mariner in heavy weather, may be regarded, in a certain sense, as 

 fog-signals. But fog, when thick, is intractable to light; the sun 

 cannot penetrate it, much less any terrestrial source of illumination. 

 Hence the necessity of employing sound-signals in dense fogs. Bells, 

 gongs, horns, guns, and syrens have been used for this purpose ; but 

 it is mainly, if not wholly, explosive signals that I have now to 

 submit to the notice of the Society. During the long, laborious, 

 and, I venture to think, memorable series of observations conducted 

 under the auspices of the Elder Brethren of the Trinity House at the 

 South Foreland in 1872 and 1873, it was proved that a short 5^-inch 

 howitzer, firing 3 lbs. of powder, yielded a louder report than a long 

 1 8-pound er firing the same charge. Here was a hint to be acted 

 on by the Elder Brethren. The effectiveness of the sound depended 

 on the shape of the gun, and as it could not be assumed that in the 

 howitzer we had hit accidentally upon the best possible shape, arrange- 

 ments were made with the War Office for the construction of a gun 

 specially calculated to produce the loudest sound attainable from the 

 combustion of 3 lbs. of powder. To prevent the unnecessary landward 

 waste of the sound, the gun was furnished with a parabolic muzzle, 

 intended to project the sound over the sea, where it was most needed. 

 The construction of this gun was based on a searching series of ex- 

 periments executed at Woolwich with small models, provided with 

 muzzles of various kinds. The gun was constructed on the principle 

 of the revolver, its various chambers being loaded and brought in 

 rapid succession into the firing position. The performance of the gun 

 proved the correctness of the principles on which its construction was 

 based. 



It had been a widely spread opinion among artillerists, that a bronze 

 gun emits a specially loud report. I doubted from the outset whether 

 this would help us, and in a letter dated 22nd April, 1874, ventured 

 to express myself thus — " The report of a gun, as affecting an observer 

 close at hand, is made up of two factors — the sound due to the shock 

 of the air by the violently expanding gas, and the sound derived from 

 the vibrations of the gun, which, to some extent, rings like a bell. 

 This latter, I apprehend, will disappear at considerable distances." 



