VOL. XC.] PHILOSOPHICAL TRANSACTIONS. 6l3 



g. On the Vibrations of different Elastic Fluids. — All the methods of finding the 

 velocity of sound agree in determining it to be, in fluids of a given elasticity, 

 reciprocally in the subduplicate ratio of the density: hence, in pure hydrogen gas 

 it should be s/ 13 = 3.6 times as great as in common air; and the pitch of a pipe 

 should be a minor 14th higher in this fluid than in the common air. It is there- 

 fore probable that the hydrogen gas used in Professor Chladni's late experiments 

 was not quite pure. It must be observed, that in an accurate experiment of this 

 nature, the pressure causing the blast ought to be carefully ascertained. There 

 can be no doubt but that, in the observations of the French academicians on the 

 velocity of sound, which appear to have been conducted with all possible attention, 

 the dampness and coldness of the night air must have considerably increased its 

 density: hence the velocity was found to be only 1109 feet in a second; while 

 Derham's experiments, which have an equal appearance of accuracy, make it 

 amount to 1 142. Perhaps the average may, as has been already mentioned, be 

 safely estimated at 1130. It may here be remarked, that the well-known eleva- 

 tion of the pitch of wind instruments, in the course of playing, sometimes 

 amounting to half a note, is not, as is commonly supposed, owing to any expan- 

 sion of the instrument, for this should produce a contrary effect, but to the in- 

 creased warmth of the air in the tube. Dr. Smith has made a similar observation, 

 on the pitch of an organ in summer and winter, which he found to differ more 

 than twice as much as the English and French experiments on the velocity of 

 sound. Bianconi found the velocity of sound, at Bologna, to differ at different 

 times, in the ratio of 152 to 157. 



10. Of the Analogy between Light and Sound. — Ever since the publication of 

 Sir Isaac Newton's incomparable writings, his doctrines of the emanation of par- 

 ticles of light from lucid substances, and of the formal pre-existence of coloured 

 rays in white light, have been almost universally admitted in this country, and but 

 little opposed in others. Leonard Euler indeed, in several of his works, has ad- 

 vanced some strong objections against them, but not sufficiently powerful to justify 

 the dogmatical reprobation with which he treats them ; and he has left that system 

 of an ethereal vibration, which after Huygens and some others he adopted, equally 

 liable to be attacked on many weak sides. Without pretending to decide positively 

 on the controversy, it is conceived that some considerations may be brought for- 

 wards, which may tend to diminish the weight of objections to a theory similar to 

 the Huygenian. There are also one or two difficulties in the Newtonian system, 

 which have been little observed. The first is, the uniform velocity with which 

 light is supposed to be projected from all luminous bodies, in consequence of heat, 

 or otherwise. How happens it that, whether the projecting force is the slightest 

 transmission of electricity, the friction of 2 pebbles, the lowest degree of visible 

 ignition, the white heat of a wind furnace, or the intense heat of the sun itself, 

 these wonderful corpuscles are always propelled with one uniform velocity? For, 

 if they differed in velocity, that difference ought to produce a different refraction. 



