ON THE MOTIONS OF SOUND. 425 



the phases of thought then passed through, one of the solutions then 

 weighed in the balance and found wanting, being identical with that 

 which Prof. Henry now offers for acceptation. 



But though it thus deflected me from the proper track, shall I say- 

 that authority in science is injurious ? Not without some qualifica- 

 tion. It is not only injurious, but deadly, when it cows the intellect 

 into fear of questioning it. But the authority which so merits our 

 respect as to compel us to test and overthrow all its supports, before 

 accepting a conclusion opposed to it, is not wholly noxious. On the 

 contrary, the disciplines it imj^oses may be in the highest dco-vee salu- 

 tary, though they may end, as in the present case, in the ruin of au- 

 thority. The truth thus established is rendered firmer by our strug- 

 gles to reach it. I groped day after day, carrying this problem of 

 aerial echoes in my mind ; to the weariness, I fear, of some of my col- 

 leagues who did not know my object. The ships and boats afloat, the 

 " slopes and crests of the waves," the visible clouds, the clifis, the ad- 

 jacent lighthouses, the objects landward, were all in turn taken into 

 account, and all in turn rejected. 



With regard to the particular notion which now finds favor with 

 Prof. Henry, it suggests the thought that his observations, notwith- 

 standing their apparent variety and extent, were really limited as re- 

 gards the weather. For did they, like ours, embrace weather of all 

 kinds, it is not likely that he would have ascribed to the sea-waves an 

 action which often reaches its maximum intensity when waves are en- 

 tirely absent. I will not multipl}"- instances, but confine myself to the 

 definite statement, that the echoes have often manifested an astonishing 

 strength, when the sea was of glassy smoothness. On days when the 

 echoes were powerful, I have seen the southern cumuli mirrored in the 

 waveless ocean, in forms almost as definite as the clouds themselves. 

 By no possible application of the law of incidence and reflexion could 

 the echoes from such a sea retiirn to the shore ; and, if we accept, for 

 a moment, a statement which Prof. Henry seems to indorse, that 

 sound-waves of great intensity, when they impinge upon a solid or 

 liquid surface, do not obey the law of incidence and reflexion, but 

 " roll along the surface like a cloud of smoke," it only increases the 

 difficulty. Such a " cloud," instead of returning to the coast of Eng- 

 land, would, in our case, have rolled toward the coast of France. 

 Nothing that I could say in addition could strengthen the case here 

 presented. I will only add one further remark. When the sun shines 

 uniformly, on a smooth sea, thus producing a practically uniform dis- 

 tribution of the aerial currents to which the echoes are due, the direc- 

 tion in which the trumi^et-echoes reach the shore is always that in 

 which the axis of the instrument is pointed. At Duugeness this was 

 proved to be the case throughout an arc of 210 an impossible re- 

 sult, if the direction of reflexion were determined by that of the ocean- 

 waves. 



