240 SIXTH REPORT — 183G. 



rature, and ii be the tension of the vapour which the moist air 

 contains, the density of the air in the mixture will be D I I — A 



that of the vapour D — * , and consequently the density of the 



oil 



compound D' is equal to D (l — ^)- Thus the actual density 



is given in terms of the density of dry air under the same pres- 

 sure. In any instance to which this expression is applied, the 

 quantity n will have to be determined by observation of the hy- 

 grometer. 



The manner in which the theoretical formula for determining 

 the velocity of sound is brought to the greatest possible degree of 

 perfection having been now exhibited, it will be interesting to 

 compare the result it gives with the most accurate experiments. 

 Those which claim the greatest confidence in this respect are the 

 experiments undertaken by Professor Moll, of Utrecht, and Dr. 

 Van Beek in June 1823, a detail of which is contained in the 

 Philosophical Transactions of the Royal Society for 1824 (p. 

 424.) Measures were taken to secure that the firing of the guns 

 at two stations should take place as nearly as possible at the 

 same instant, which was effected with much greater precision 

 in these experiments than in those of the French Academicians 

 in 1822. By this precaution the cause of error arising from the 

 wind is removed, the velocity of propagation in still air being 

 assumed to be the arithmetic mean between the velocities in- 

 ferred from the observations at the two stations, as in fact it 

 might be shown to be theoretically. The difference between 

 two determinations on different days, when this precaution was 

 attended to, was only 2*166 feet, whereas two other determina- 

 tions on different days, when the shots were not reciprocal, 

 differed by 20*84 feet. The mean velocity which the experiments 

 gave for dry air atO° of temperature was 332"-05 ( = 1089*744 

 English feet). The mean excess of the experimental determina- 

 tion over the theoretical, supposing the ratio of the specific 

 heats to be 1*3748*, was 4™*58 ( = 15*032 feet). 



In Dr. Gregorys experimentsf, made in 1823, an anemometer 

 was employed whose indications were found to agree with the 

 velocity of the wind, as inferred from the difference of the velo- 



* It is shown by Dr. Simons (Phil. Trans., 1830, p. 213.) that the mean value 

 of this ratio as derived from the experiments of Drs. Moll and Van Beek is 

 1-4152. 



t Cambridge Philosophical Transactions, vol. ii. p. 119. 



