1875] WRITINGS OF JOSEPH HENRY. 471 



disturbing the general flow of wind both at the surface of 

 the sea and at an elevation above. But although the local- 

 ity was unfavorable for obtaining results tending to exhibit 

 the effects of broad currents of wind flowing in one direction 

 it had the advantage of offering more varied phenomena 

 than could otherwise have been exhibited. Before com- 

 mencing the experiments, instructions were given to attach 

 a rotating iron neck to the trumpet of one of the sirens, in 

 order that it might be directed to the zenith, while the 

 other siren remained with its axis in a horizontal direction. 

 The observers in these investigations consisted of the chair- 

 man of the Board; General Woodruff, engineer of the third 

 district; Mr. Porter Barnard, assistant superintendent of con- 

 struction ; Captain Keeney, and other officers of the Mistletoe; 

 with an assistant who acted as one of the observers and re- 

 cording clerk. The Mistletoe was daily employed, though 

 on two occasions the Cactus — another of the light-house 

 steamers, rendered assistance. 



Observations on the Echo. — The first observations to be men- 

 tioned are those relating to the echo ; the results however in 

 regard to this are not very satisfactory. The sirens were of the 

 second order, and therefore the echoes produced were not so 

 distinct as those from the larger instrument at Block Island. 

 The echo from the horizontal trumpet was distinct, and in the 

 prolongation of its axis ; the interval however between the 

 blast of the siren trumpet and the commencement of the echo 

 was very brief; so short indeed that the ending of the one and 

 the beginning of the other were generally difficult to distin- 

 guish. A slight break in the apparatus of the siren produced 

 a continuous hum, which interfered somewhat with the dis- 

 tinct appreciation of the sound of the echo. The keeper 

 thought the weather was not favorable for the production of 

 echoes. He thinks they are heard most distinctly during a 

 perfect calm, which did not occur during the course of these 

 investigations. 



The axis of the siren with the movable trumpet being 

 directed to the zenith, strict attention was given by all the 

 observers to any echo which taight be produced from it ; 

 but in this case, as in that at Block Island, the slight echo 



