Eicpertments and Inquiries refpeBing Sound and Light. 8g 



tributing to the appearance of the white line in every direftion, in the experiments already 

 mentioned, fo It is pofiible that there may be fpme fecond refleftion at the immediate 

 furface of the body itfelf, and that, by mutual reileftions between thefe two furfaces, 

 fomething like the anguiform motion fufpefted by Newton may really take place ; and 

 then the analogy to the colours of thin plates will be fllU ftronger. A mixture of 

 vibrations, of all pofTible frequencies, may eafily deftroy the peculiar nature of each, and 

 concur in a general effe£t of white light. The greateft difficulty in this fyftem is, to ex- 

 plain the different degree of refradtion of differently coloured light, and the feparation of 

 white light in refraftion : yet, confidering how imperfeil the theory of elaftic fluids ftill 

 remains, it cannot be expe£led that every circumftance fliould at once be clearly elucidated. 

 It may hereafter be confidered how far the excellent experiments of Count Rumford, 

 which tend very greatly to weaken the evidence of the modern do£lrine of heat, may be 

 more or lefs favourable to one or the other fyftem of light and colours. It does not ap- 

 pear that any comparative experiments have been made on the inflexion of light by 

 fubftances poffeffed of different refradlive powers ; undoubtedly fome very interefting con- 

 clufions might be expe£ted from the inquiry. 



XI. Of the Coalefcence of Muftcal Sounds. 



It is furprifing that fo great a mathematician as Dr. Smith could have entertained for 

 a moment, an idea that the vibrations conftituting different founds fhould be able to crofs 

 each other in all diredions, without affeding the fame individual particles of air by their 

 joint forces : undoubtedly they crofs, without difturbing each other's progrefs ; but this 

 can be no otherwife cffedted than by each particle partaking of both motions. If this 

 affertion ftood in need of any proof, it might be amply furniftied by the phsenomena of 

 beats, and of the grave harmonics obferved by Romieu and Tartini ; which M. de la 

 Grange has already confidered in the fame point of view. In the firft place, to Amplify 

 the ftatement, let us fuppofe, what probably never precifcly happens, that the particles of 

 air, in tranfmitting the pulfes, proceed and return with uniform motions ; and, in order 

 to reprefent (heir pofition to the eye, let the uniform progrefs of time be reprefented by 

 the increafe of the abfcifs, and the diftance of the particle from its original pofition, by the 

 ordinate, Fig. 33 — 38. Then, by fuppofing any two or more vibrations In the fame 

 direftion to be combined, the joint motion will be reprefented by the fum or difference of the 

 crdinates. When two founds are of equal ftrength, and nearly of the fame pitch, as in 

 Fig- 36, the joint vibration is alternately very weak and very ftrong, producing the effe£l 

 denominated a beat, Plate VI. Fig. 43, B and C : which is flower and more marked, as 

 the founds approach nearer to each other in frequency of vibrations ; and, of thefe beats 

 there may happen to be feveral orders, according to the periodical approximations of the 

 numbers expreffing the proportions of the vibrations. The ftrength of the joint found is 

 double that of the fimple found only at the middle of the beat, but not throughout its 

 duration ; and it may be inferred, that the ftrength of found in a concert will not be in 



Vol. v.— June i8oj. N cxaft 



