RECENT EXPERIMENTS ON FOG-SIGNALS. 275 



ing men how they should die for their creed, there must also be those 

 who were ready for science' sake to lead a life of self-denial, or even, if 

 need were, to die for it. In inspiring man with the ardent longing for 

 absolute knowledge, Christianity made atonement for the wrongs its 

 asceticism had for so long done to science. 







RECENT EXPERIMENTS ON FOG-SIGNALS. 



By Professor JOHN TYNDALL, F. E. S. 



OUR most intense coast-lights, including the six-wick lamp, the Wig- 

 ham gaslight, 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 sirens, have been used for this purpose ; but it is main- 

 ly, 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 

 pounds of powder, yielded a louder report than a long 18-pounder 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, arrangements 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 pounds of powder. To 

 prevent the unnecessary landward waste of the sound, the gun was fur- 

 nished 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 experiments executed at Woolwich with small 

 models, provided with muzzles of various kinds. The gun was construct- 

 ed on the principle of the revolver, its various chambers being loaded 

 and brought in rapid succession into the firing position. The perform- 

 ance 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 April 22, 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 



