i5a Letter refpeEi'mg Sound and Light. 



2. A current of air, forced by a moderate preffure through a cylindrical pipe, diverges 

 the lefs as its velocity is lefs, SeQ. II. 



3. At a certain point the divergency of fuch a current increafes fuddenly, and the cur- 

 rent mixes with the furrounding air, Se£t. II. 



4. So far is fuch a motion from fpreading equally in al| direflions, that o.n every fide of 

 the current the air is urged mor&towards it than from it, Se6l. II. 



5. Suund, admitted through an aperture, does not by any means diverge equally in all 

 diredlions, and is probably very weak except in directions nearly redilinear. From poli- 

 tion 2 and 4, and from experience, Seft. VI. 



• 6. Sound probably decays in the duplicate ratio of the diftance, Se£l. VII. 



7. A fimilar blaft of air produces nearly a fimilar found, in organ pipes properly cons- 

 njenfurate, Se£t. VIII. 



8. Light is probably the undulation of an elaftic medium, Se£l:. X. 



A. Becaufe its velocity in the fame medium is always equal. 



B. Becaufe all refradlions are attended with a partial refle£tion. 



C. Becaufe there is no reafon to expeft that fuch a vibration fliould diverge equally in 

 all dire£l:ions, and becaufe it is probable that it does diverge in a fmall degree in every 

 diredlion. 



D. Becaufe the difperfion of differently coloured rays is no more incompatible with this 

 fyftem than with the common opinion, which only affigns for it the nominal caufe of dif- 

 ferent eledlive attradtions. 



E. Becaufe refraction and refle£lion in general are equally explicable on both fuppofitions* 



F. Becaufe infledlion is as well, and, it may be added, even much better explauied by 

 this theory. 



G. Becaufe all the phaenomena of the colours of thin plates, which are in reality totally 

 unintelligible on the common hypothefis, admit a very complete and fimple explanation by 

 this fuppofition. The analogy which is here fuperficially indicated will probably foon be 

 made public more in detail; and will alfo be extended to the colours of thick plates, and 

 to the fringes produced by inflection, affording, from Newton's own elaborate experimentSjt 

 a moft convincing argument in favour of this fyftem. 



9. The particles of ajr may be jointly adtuated by two or more founds, and in this cafe> 

 the feveral motions are to be added or fubtradted, in order to find the adlual joint motion^ 

 Sea. XI. 



10. The grave harmonic produced by a major third is accompanied by a very audible- 

 twelfth. This circumftance is explained, and the effe£l of fubordinate notes and fubakera 

 ftops, on the quality of founds, is (hown by figures, Sect. XI. 



1 1. A noife returning every fecond, if audible, would be a C. From Sauveur ; with an 

 experiment, Se£t. XII. 



12. A chord retains always the form of its initial vibration. From experiments, in fa- 

 vour of Euler's theorem, againft the fimple harmonic curve, Sedt. XIIL 



13. The 



