460 RESEARCHES IN SOUND. 



but is confined entirely to the atmosphere above the river, while the 

 other class of fog exists over the laud as well. 



Fog-Signals. 



The importance of fog-signals as aids to navigation, especially on the 

 northeastern portion of our coast, of which the shore is exceedingly bold 

 and to the approach of which the sounding-line gives no sure indicatiou, 

 has been from the first an object of special attention. 



At the beginning of the operations of the Light-House Board such 

 instruments were emjiloyed for i^roducing sound as had been used in 

 other countries ; these consisted of gongs, bells, guns, horns, &c. The 

 bells were actuated by clock-machinery, which was wound up from time 

 to time and struck at intervals of regular sequence by which their x)Osi- 

 tion might be identified. The machinery, however, by which these bells 

 were struck was of a rude character and exceedingly wasteful of power, 

 the weight continuing to descend during the whole period of operation, 

 including the successive intervals of silence. This defect was remedied 

 by the invention of Mr. Stevens, who introduced an escapement arrange- 

 ment, similar to that of a clock, which is kept in motion by a small 

 weight, a larger one being brought into operation only during the 

 instant of striking. 



Bell-buoys were also introduced at various points. These consisted 

 of a bell supported on a water-tight vessel and rung by the oscillation 

 of the waves, but all contrivances of this kind have been found to be 

 untrustworthy, the sound which they emit is comparatively of feeble 

 character, can be heard at but a small distance, and is frequently inefii- 

 cient during a fog which occurs in calm weather. Besides this, auto- 

 matic fog-signals are liable to be interfered with by ice in northern 

 positions, and in all sections to derangement at times when no substi- 

 tute can be i)ut in their place, as can be in the cases of the bells rung 

 by machinery under the immediate control of keepers. A signal which 

 is liable to be interrupted in its warnings is worse than no signal, since 

 its absence may give confidence of safety in the midst of danger, and 

 thus prevent the necessary caution which would otherwise be emi)loyed. 



Guns have been emi)loyed on the United States coast, first under the 

 direction of General Bates, engineer of the twelfth district, at Point 

 Bonita, San Francisco Bay, California, The gun at this station con- 

 sisted of a 24-pounder, furnished by the War Department. The neces- 

 sary arrangements being made, by the construction of a powder-house, 

 and laying of a platform, and employment of a gunner, notice to mari- 

 ners was given that after the 8th of August, 1850, a signal-gnn would 

 be fired every hour and half-hour, night and day, during foggy or thick 

 weather. The first year, with the exception of eighty-eight foggy days, 

 omitted for want of i>owder, 1,390 rounds were fired. These consumed 

 5,560 pounds of powder, at a cost of $1,487, pay of gunner and incident- 

 als excluded. The following year the discharges were 1,582, or about 



