1874] WRITINGS OF JOSEPH HENRY. 391 



several railways in passing over bridges at a distance. Un- 

 fortunately, the question is a difficult one to settle, since in 

 order to arrive at a true result, the effect of the wind must be 

 carefully eliminated. Captain Keeney, who has previously 

 been mentioned, related the following occurrence, in the first 

 part of which he was led to suppose that fog had a very 

 marked influence in deadening sound, though in a subse- 

 quent part he came to an opposite conclusion : He was sailing 

 during a dense fog with a slight wind bearing him toward a 

 light-vessel, the locality of which he expected to find by 

 means of the fog-signal. He kept on his course until he 

 thought himself very near the ship, without hearing the 

 stroke of the bell. He tlien anchored for the night, and 

 found himself next morning within a short distance of the 

 light-vessel, but still heard no sound, although he was 

 assured when he got to it that the bell had been ringing all 

 night. He then passed on in the same direction in which 

 he had previously sailed, leaving the light-vessel behind, 

 and constantly heard the bell for a distance of several miles, 

 the density of the fog not perceptibly diminishing. In this 

 case it is evident that the deadening of the sound was not 

 due to the fog, but (as we shall hereafter see,) in all probabil- 

 ity to the combined action of the upper and the lower cur- 

 rents of air. 



On returning to Washington the writer took advantage 

 of the occurence of a fog to make an experiment as to the 

 penetration of the sound of 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 contrary to the supposition 

 of absorption of the sound by the fog, but 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 result not entirely satisfactory. 



Experiments at Sandy Hook in 1867. 



The next series of experiments was made from October 10 

 to October 18, 1867, under the direction of the writer of this 



