VI. A LE'n'ER TO MR. NICHOLSON. 



FROM THOMAS YOUNG, M.D. F.R.S. 



PROFESSOR OP NATURAL PHILOSOPHY IN THE ROYAL INSTITUTION, 



RESPECTING SOUND AND LIGHT, 



AND IN REPLY TO SOME OBSERVATIONS OF PROFESSOR ROBISON. 



From Nicholson's Journal, for August 1801. 



SIR, 



J.N the supplement of the Encyclopaedia 

 Biitannica, are inserted several excellent ar- 

 ticles by Professor Robison, of Edinburgh : 

 one of them appears to require some public 

 notice on mj' part, and I consider your valu- 

 able Journal as the most eligible channel for 

 such a communication, especially as you 

 have lately done me the honour of re- 

 printing the paper which gave rise to the 

 Professor's animadversions. But in the first 

 place, I shall beg leave to recall the atten- 

 tion of your readers, by a summary enume- 

 ration, to the principal positions which I 

 have in that paper endeavoured to establish. 

 1. Sound, as transmitted through the at- 

 mosphere, consists in an undulatory motion 

 of the particles of the air, Sect. III. This is 

 generally admitted ; but as the contrary has 

 even very lately been asserted, it is not su- 

 perfluous to have decisive evidence of the 

 fact. Professor Robison's experiment with 

 a stopcock, furnishes *an argument nearly 

 •imiliir. 



2. A current of air, forced by a moderate 

 j)ressure through a cylindrical pipe, diverges 

 the less as its velocity is less. Sect. II. 



3. At a certain point the divergency of 

 such a current increases suddenly, and the 

 current mixes with the surrounding an". 

 Sect. II. 



4. So far is such a motion from spreading 

 equally in all directions, that on every side 

 of the current the air is urged more towards 

 it than from it, Sect. 11. 



5. Sound, admitted through an aperture, 

 does not by any means diverge equally in all 

 directions, and is probably very weak except; 

 in directions nearly rectilinear. From posi- 

 tion 1 and 4, and from experience. Sect. VI. 



6. Sound probably decays in the duplicate 

 ratio of the distance. Sect. VII. 



7. A similar blast of air produces nearly a 

 similar sound, in organ pipes properly com- 

 mensurate, Sect. VIII. 



8. Light is probably the undulation of an 

 elastic medium, Sect. X. 



A. Because its velocity in the same me- 

 dium is always equal. 



