374 WRITINGS OF JOSEPH HENRY [1874 



northern regions, is colder than the aii- in the vicinity. The 

 air over the river being thus cooled below the temperature 

 of a gentle wind from the south, the moisture of the latter is 

 precipitated. This fog, which occurs in the last of winter, 

 during the spring, and beginning of summer, is very dense, 

 but is confined entirely to the atmosphere above the river, 

 while the other class of fog exists over the land as well. 



Fog-signals. — The importance of fog-signals as aids to nav- 

 igation, especially on the north-eastern portion of our coast, 

 the shore of which is exceedingly bold and to the approach of 

 which the sounding-line gives no sure indication, has been 

 from the first an object of special attention. 



At the beginning of the operations of the Light-house 

 Board, such instruments were employed for producing 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 posi- 

 tion might be identified. The machinery however by 

 which these bells was 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 

 arrangement, 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 of comparatively feeble character, can be heard 

 at but a small distance, and is frequently inefficient during 

 a fog which occurs in calm weather. Besides this, automatic 

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

 positions, and in all sections — to derangement at times when 

 no substitute can be put in their place, as can be in the cases 

 of the bells rung by machinery under the immediate con- 



