ON THE MOTIONS OF SOUND. 419 



Lighthouse Board Iiave been much more extensive on this suhjoct than 

 those of the Trinity House, and that the latter has established no facts 

 of practical importance wliich liad not been previously observed and 

 used by the former." 



The " Appendix " here referred to is from the pen of the venerable 

 Prof. Joseph Henry, chairman of the Lighthouse Board at Washing- 

 ton. To his credit be it recorded that, at a very early period in the 

 history of fog-signaling, Prof. Henry reported iu favor of Daboll's 

 trumpet, though he was opposed by one of his colleagues on the 

 ground that " fog-signals were of little importance, since the mariner 

 should know his place by the character of his soundings." In the 

 Appendix, he records the various eftbrts made in the United States 

 with a view to the establishment of fog-signals. He describes experi- 

 ments on bells, and on the employment of reflectors to reenforce their 

 sound. These, though effectual close at hand, were found to be of no 

 use at a distance. He corrects current errors regarding steam-whis- 

 tles, which by some inventors were thought to act like ringing bells. 

 He cites the opinion of the Rev. Peter Ferguson, that sound is better 

 heard in fog than in clear aii*. This opinion is founded on observa- 

 tions of the noise of locomotives ; in reference to which it may be said 

 that others have drawn from similar experiments diametrically oppo- 

 site conclusions. On the authority of Cajjtain Keeney he cites an oc- 

 currence, " in the first part of which the captain was led to suppose 

 that fog had a marked influence in deadening sound, though in a sub- 

 sequent part he came to an opposite conclusion." Prof. Henry also 

 describes an experiment made during a fog at Washington, in which 

 he employed " a small bell rung by clock-work, the apparatus being 

 the part of a moderator lamp intended to give warning to the keepers 

 when the supply of oil ceased. The result of the experiment was, he 

 affirms, contrary to the supposition of absorption of the sound by the 

 fog." This conclusion is not founded on comparative experiments, 

 but on observations made in the fog alone ; " for," adds Prof. Henry, 

 " the change in the condition of the atmosphere, as to temperature 

 and the motion of the air, before the experiment could be repeated in 

 clear weather, rendered the residt not entirely satisfactory." 



This, I may say, is the only experiment on fog which I have found 

 recorded in the Appendix. 



In 1867 the steam-siren was mounted at Sandy Hook, and exam- 

 ined by Prof. Henry. He compared its action with that of a Daboll 

 trumpet, employing for this purpose a stretched membrane covered 

 with sand, and placed at the small end of a tapering tube which con- 

 centrated the sonorous motion upon the membrane. The siren proved 

 most powerful. " At a distance of 50, the trumpet produced a decided 

 motion of the sand, while the siren gave a similar result at a distance 

 of 58." Prof. Henry also varied the pitch of the siren, and found that, 

 in association with its trumpet, 400 impulses per second yielded the 



