and the Observed Velocity of Sound in Air and Gases. 11 



some experiments on sound, it was observed by persons stationed 

 at a distance, tbat the report of the cannon reached them before 

 they heard the command to fire ; thus showing, as he thinks, 

 that the sound of the gun's report outstripped the sound of the 

 officer's voice. Subsequently, from the observations of Professor 

 C. Montigny of Antwerp, in a memoir printed in the Bulletins 

 de VAcademie Royale de Belgique, Mr. Earnshaw has been satis- 

 fied that the thunder-clap comes fairly under his exponential type 

 of sound-waves, and that in such cases sound is transmitted 

 with a velocity far greater than he had imagined. In one of 

 the instances recorded by M. Montigny, " the report of a thun- 

 der-clap reached him after an interval of two seconds, which 

 according to theory should have occupied more than fifteen 

 seconds;" that is, its velocity was about seven times as great as 

 that of the reports of the cannon used by experimentalists for 

 determining the velocity of sound. Other cases of thunder- claps, 

 less striking, but pointing towards the same conclusion, are 

 recorded in the memoir*. 



It is proper to remark that inductions drawn from observa- 

 tions such as those last mentioned must be received with extreme 

 caution. Strokes of lightning are sometimes three, four, or even 

 five English miles in length^. The velocity of electricity being 

 incomparably greater than that of sound, the thunder may be 

 regarded as originating at one and the same instant of time along 

 the whole of this extended and, frequently, irregular line of 

 disturbance. The observer estimates one element for determining 

 the velocity, viz. the distance, under the assumption that the 

 sound is generated at the point struck by the electricity ; while the 

 probability is that the sound reached him by a much shorter 

 route, from some nearer portion of the line of genesis. In fact 

 this consideration is of itself sufficient to vitiate all inferences 

 deduced from observations on thunder-claps. 



In reference to the evidence derived from the anomalous 

 observations of Capt. Parry's men, it is scarcely necessary to 

 remark that, being entirely unsupported by confirmatory obser- 

 vations in other quarters, they can hardly be considered as in- 

 validating the records of the most trustworthy experimentalists, 

 which concur in establishing the general fact that all sounds travel 

 at the same rate. As will be shown in the sequel, the experi- 



* Phil. Mag. S. 4. vol. xx. pp. 37, 38 (1860). 



t M. Gay-Lussac, from observations in mountainous districts, ascer- 

 tained that the striking distance of lightning frequently exceeds three miles 

 (Quart. Journ. of Science for Oct. 1825, pp. 1/2,1/3; from Ann. de 

 Chim. vol xxix. p. 105). Sturgeon concluded, from observations made 

 at Woolwich, that he saw flashes nearly six miles in length (Phil. Mag. 

 S. 3. vol. v. p. 421, 1834). In several instances I have estimated the 

 length of strokes of lightning to be at least four miles. 



