20 Prof. J. Le Conte on the Discrepancy between the Computed 



In order to compare the results given by the foregoing formulae 

 with those obtained from experiments on the velocity of sound 

 in the air, it is necessary to reduce the latter to the standard 

 temperature of zero of the Centigrade scale. The more accurate 

 coefficient of expansion for air, given by the experiments of M. 

 Regnault, renders it requisite to make new reductions for tem- 

 perature in all the older observations. Consequently I have in 

 some measure restricted the comparison to such experiments as 

 furnish the data requisite for such new reductions. I have been 

 unable to obtain the data for reducing the results of Benzenberg's 

 experiments. The numbers in the Table relating to the experi- 

 ments of MM. Myrbach and Stampfer, and of MM. Bravais and 

 Martins, are given on the authority of Professor Rankin e, who 

 seems to have employed the corrected coefficient for the dilatation 

 of air*. Most of the observations are deficient in the hygro- 

 metric data for computing the influence of aqueous vapour. This 

 correction is, however, usually very small, and becomes insigni- 

 ficant at very low temperatures — such as existed during the ex- 

 periments of Parry and Foster. But the admirable experiments 

 of MM. Moll, Van Beek, and Kuytenbrouwer (which are cer- 

 tainly in every respect the most reliable) furnish all the data 

 requisite for the most rigorous comparison. The numbers re- 

 lating to these experiments are taken from the Table given by 

 Dr. Simons f, which is restricted to such of the observations in 

 which the guns were fired exactly at the same moment at each 

 station, and in which the time was correctly observed at both 

 extremities of the base. The reductions for temperature and 

 vapour have been computed by means of the well-known 

 formula 



v=v\\f _ 



-0'378.£ . 



, in which 



1 + 0-00367.*' 



v — velocity in dry air at 0° Centigrade. 



t/= velocity given by observation. 



/= tension of vapour, measured by column of mercury at 



0° Cent. 

 b = height of barometer, measured by column of mercury at 



0° Cent. 

 t — temperature on the Centigrade scale. 



* Trans, of Royal Society of Edinburgh, vol. xx. pp. 167 & 168. 



t Phil. Trans, for 1830, p. 211. The mean data furnished by each of 

 the two days' observation have been taken, and the reduction calculated 

 separately. The numerical errors in Dr. Simons's Table, which were pointed 

 out by Professor Miller (Phil. Mag. S. 3. vol. xv. p. 1, 183.9), have been 

 corrected. No correction has been introduced for the influence of the 

 carbonic acid of the air, as its effect must be inappreciable. 



