188 ANNUAL RECORD OF SCIENCE AND INDUSTRY. 



is sufficient distance, pass over the observer's head. This 

 explanation was propounded by Professor Stokes in 1857, but 

 was discovered independently by the author. 



The paper then contains the description of experiments 

 made with a view to establish this explanation, and from 

 which it appears that: 



1. The velocity of wind over grass differs by one half at ele- 

 vations of one and eight feet, and by somewhat less over snow. 



2. When there is no wind, sound proceeding over a rough 

 surface is destroyed at the surface, and is thus less intense 

 below than above. 



3. That sounds proceeding against the wind are lifted up 

 off the ground, and hence the range is diminished at low ele- 

 vations ; but that the sound is not destroyed, and may be 

 heard from positions sufficiently elevated with even greater 

 distinctness than at the same distances with the wind. 



4. That sounds proceeding with the wind are brought down 

 to the ground in such a manner as to counterbalance the ef- 

 fect of the rough surface (2) ; and hence, contrary to the ex- 

 periments of Delaroche, the range at the ground is greater 

 with the wind than at right angles to its direction, or where 

 there is no wind. 



On one occasion it was found that the sound could be 

 heard three hundred and sixty yards with the wind at all ele- 

 vations, whereas it could be heard only two hundred yards 

 at right angles to the wind standing up; and, against the 

 wind, it was lost at thirty yards at the ground, seventy yards 

 standing up, and at one hundred and sixty yards at an ele- 

 vation of thirty feet, although it could be heard distinctly at 

 this latter point a few feet higher. 



As might be expected, the effect of raising the bell was to 

 extend its range to windward, to even a greater extent than 

 was obtained by an equal elevation of the observer. 



These results agree so well with what might be expected 

 from the theory, as to place its truth and completeness be- 

 yond question. 



It is thus argued that, since the wind raises the sound so 

 that it can not be heard at the ground, by causing it to move 

 faster below than above, any other cause which produces 

 such a difference in velocity will lift the sound in the same 

 way; and therefore that an upward diminution in the tern- 



