and the Observed Velocity of Sound in Air and Gases. 5 



Beek, in their account of the experiments made in Holland, seem 

 to have overlooked this fact, as they evidently ascribe an influ- 

 ence to the variation of gravity with latitude*. On the other 

 hand, Dr. Simons, in a paper "On the Theoretic Velocity of 

 Sound," written in 1829, clearly shows that the velocity is inde- 

 pendent of the latitude, and consequently of the intensity of 

 gravity f. Mr. Herapath likewise manifests a distinct apprecia- 

 tion of the grounds of this physical conclusion J. 



Under the same head must be included a more serious diffi- 

 culty which has from time to time arisen in the minds of many 

 excellent philosophers, in relation to the influence of the heat and 

 cold developed by waves of condensation and rarefaction on the 

 velocity of sound as deduced from the theory of Laplace. Pass- 

 ing by the singular idea of Winter, who thought that, if Laplace's 

 view was correct, a sonorous body kept in a state of continual 

 vibration in the air must finally render it sensibly hot§, I pro- 

 ceed to a brief consideration of the more serious objections which 

 have been urged on this score. Prof. William Ritchie, in a paper 

 presented to the Royal Society in January 1837, regards the 

 hypothesis of Laplace as untenable, " from the fact that a rarefied 

 wave advances through the air with the same velocity as a con- 

 densed wave, which would not be the case," he asserts, u if in 

 either instance their progress were influenced by the heat 

 evolved" || . More recently this difficulty has been revived by 

 Prof. Challis, who contends that, if a sudden condensation, by 

 developing heat, produces a greater elastic force, a sudden rare- 

 faction, by lowering the temperature, must produce a less elastic 

 force If . This view has been endorsed by Prof. Potter, who insists 

 that, by considering the cold produced by rarefaction, "Laplace's 

 reasoning would have been equally available if the velocity of 

 sound in air had proved one-sixth less than the theoretic velocity, 

 instead of one-sixth more" **. 



b 

 the value of the factor -, remains constant amid the fluctuations of b. Again, 



since any variation in g must equally influence the weight of the mercury in 

 b, and the weight of the column of air which balances it, it follows that, 

 cceteris paribus, variations in g are without influence on the value of b. But, 

 by Mariotte's law, d varies directly as the compressing force, or y ; hence 



9 

 the value of the factor -, remains constant amid the variations of g with 



. gbD . . . 



latitude and height. Consequently the value of ~r ^gh is invariable, 



* Phil. Trans, for 1824, p. 446. f Ibid, for 1830, p. 212, 



X Mathematical Phvsics, vol. ii. p. 65. London, 1847- 



§ Phil. Mag. S. 1. vol. xliii. p. 206 (1814). 



|| Ibid. S. 3. vol. x. p. 220(1837). 



IF Ibid. S. 3. vol. xxxii. p. 283 (1848). 



** Ibid. S. 4. vol. i. p. 101 (1851). 



