1874] 



WRITINGS OF JOSEPH HENRY. 413 



one or more cams, depending on the number of blasts to be given in a min- 

 ute ; the length of the bhvst'being regulated by that of the cams. 



The valve for admitting the steam into the whistle is a balance-valve, the 

 diameters of the two disks being respectively 3^ ins. and 2J in-., which differ- 

 ence is sufficient to cause the pressure of steam to close the valve tight, with- 

 out requiring too great a force to open it. The valve is worked by a stem 

 attached to the rocker-shaft at the lower part of the steam-pipe. This shaft 

 passes through a stuffing-box in the steam-pipe, and is provided with a col- 

 lar, which the pressure of the steam forces against the interior boss on the 

 pipe, thus making the joint steam-tight. The exterior arm on this rocker- 

 shaft (as well as that on the engine,) is perforated in such a manner as to 

 allow the throw of the valve to be adjusted. 



In the comments I have made on the report of General 

 Duane, the intention was not in the least to disparage the 

 value of his results, which can scarcely he too highly appre- 

 ciated ; but inasmuch as the true explanation of the phe- 

 nomena he has observed has an important bearing on the 

 location of fog-signals and on their general application 

 as aids to navigation, and is also of great interest to the 

 physicist, who values every addition to theoretical as well 

 as practical knowledge, I have thought not onlj'' that the 

 remarks here offered are necessary, but also that special 

 investigations should be made to ascertain more definitely 

 the conditions under which the abnormal phenomena de- 

 scribed by the General occur, and to assign if possible a 

 more definite and efficient cause than those to which he has 

 attributed them. 



Much thought has therefore been given to the subject, and 

 since the date of General Duane's report I have embraced 

 every opportunity that occurred for making observations 

 in regard to them. The first step we made toward obtaining 

 a clew to the explanation of the phenomena in question re- 

 sulted from observations made at New Haven, namely: 1st, 

 the tendency of sound to spread laterally into its shadow ; 

 2d, the fact that a sound is frequently borne by an upper 

 current in an opposite direction to the wind at the surface ; 

 and 3d, that a sound moving against a wind is heard better 

 at a higher elevation. The first point to consider is in what 

 manner the wind affects sound. That it is in some way 

 connected with the distance to which sound can be heard is 

 incontestably settled by general observation. At first sight, 



