Theoretical Determination of the Velocity- of Sound. 339 



attended with any thermal change. But whether this be the 

 case or not, the principle itself certainly does not follow as a 

 legitimate consequence, either from the experiments of Dulong, 

 or from those contained in this paper. If, on this hypothesis, 

 we attempt to deduce the heat evolved in the combustion of 

 one litre of the vapour of carbon from the results obtained 

 with marsh gas and defiant gas, we are led in the two cases 



to very different numbers. Thus, 



Dulong. Author. 



One litre marsh gas gives 9588 94'20 



Two litres hydrogen give 6212 6072 



One litre vapour of carbon shonld give . . 3376 SS^S 



One litre olefiant gas gives 15338 15014- 



Two litres hydrogen give 6212 6072 



One litre vapour of carbon should give . . 4563 4'4'71 



The experimental results when interpreted in this way lead 

 therefore to two very different numbers to express the heat 

 due to the conversion of the vapour of carbon into carbonic 



acid. 



[To he continued.] 



XLVI. The Astronomer Royal's Remarks on Professor 

 Challis's Theoretical Determination of the Velocity of Soiind. 



To the Editors of the Philosophical Magazine and Journal. 

 Gentlemen, 



THE publication of an essay in your Magazine is commonly 

 understood to be a challenge to discussion. With this 

 understanding, I beg leave to transmit to you a few remarks 

 on a paper by my friend Professor Challis printed in the last 

 Number of the Philosophical Magazine. 



The conclusions at which Professor Challis arrives are very 

 startling, — that soniferous vibrations may be communicated 

 along a limited cylinder or "filament" of air without affecting 

 the air which surrounds it; and that the theoretical velocity of 

 sound is not that in which all mathematicians, who have hi- 

 therto investigated the subject, have agreed. I need not say 

 that such conclusions cannot be admitted without the most 

 distinct evidence; and I think that I shall be able to show 

 that this eviilence is not to be found in Professor Challis's 

 paper. 



I see no ground to question any important step (except one 

 of interpretation near the bottom of page 280) as far as page 

 281 line 5 from the bottom ; and I will bricffy call your atten- 

 tion to the stale in which the problem is there leit by the 



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