a stream of air driven through a pipe with a lateral aperture, like a 

 French flute, where at the part where the stream issues out of the 

 orifice, vibrations are manifestly perceived, which are rendered still 

 more evident if the current be impregnated with smoke. 



4. Of the Velocity of Sound. -The velocity of any impression trans- 

 mitted by the common air, being corrected by the experiments of 

 various observers, is at an average here estimated at 1130 feet in 

 a second of time. 



5. Of Sonorous Cavities. What is here said relates chiefly to the 

 reflection of sounds in rooms or galleries. This we find takes place 

 as often in a second as double the breadth of the room or passage is 

 contained in 1130 feet, that breadth determining the pitch of the 

 musical note thereby produced. 



6 Of the Divergence of Sound. Various observations are here re- 

 lated which militate against the received opinion that sound diverges 

 equally in all directions, and that there is no substance impervious 

 to sound. On this head, however, the author admits that a more 

 ample investigation will be required than has hitherto been insti- 

 tuted ; and he intimates that he shall engage in it as soon as his 

 leisure will permit. 



7. OftheDecay of Sound. The two hypotheses, 1st, that sound de- 

 cays nearly in the simple ratio of the distances, and 2nd, that this di- 

 minution is in the subduplicate ratio, are here stated, and some falla- 

 cies are mentioned, which will likewise render a further inquiry ne- 

 cessary. 



8. Of the Harmonic Sounds of Pipes. The object of this section 

 appears from a table exhibiting the results of a set of experiments, 

 made with a view to ascertain the velocity with which organ-pipes 

 of different lengths require to be supplied with air, according to the 

 various appropriate sounds which they produce. 



These were made on pipes of the same bore, and of different 

 lengths, both stopped and open. The general result was, that a si- 

 milar blast produced as nearly the same sound as the length of the 

 pipe would permit, or at least that the exceptions, though very nu- 

 merous, lie equally on each side of this conclusion. 



9. Of the Vibrations of different Elastic Fluids. The difference of 

 these vibrations has been received as being reciprocally in the sub- 

 duplicate ratio of the density of the fluid. Hence in pure hydrogen 

 gas they must be 3' 6 times greater than in common air. And 

 this will explain why an instrument will often appear out of tune, 

 when, in fact, the fault lies in the change of temperature of the at- 

 mosphere ; and why the pitch of an organ will be found to differ ma- 

 terially in summer and winter. 



10. Of the Analogy between Light and Sound. While the author 

 vindicates the Newtonian theory of light against the criticisms of 

 Euler, he freely admits that it is liable to some objections, among 

 which he chiefly insists upon the uniformity of the motion of light in 

 the same medium, and the partial reflection from every refracting 

 surface. Having reasoned largely upon this subject, he admits the 



