98 Mr. J. H. Alexander on the Tension of Vapour of Water, 



tween the two consecutive surfaces either of no condensation 

 or of no velocity, is exactly the same as that which would take 

 place along the axis of the ray between two consecutive posi- 

 tions of no condensation or of no velocity, if two series of 

 waves for which X is the same were propagated along that 

 axis in opposite directions. The time in which each particle 

 executes a vibration is the same in the two cases. We must 

 consequently have 



X 



H 



ence 



^=2- 



xVe 1 ^ e>? 4 



= 1, and — 2" ^ -g. 



2 71" 



Recurring now to the expression for the velocity (a') o^ 

 propagation of a ray, obtained in the Phil. Mag. vol. xxxiii. 

 p. 363, and neglecting the term involving 7»\ it appears that 



Hence if we take for a the value in art. 66 of Sir John Her- 

 schel's Treatise on Sound in the Encyclopcedia Metropolitana, 

 we shall obtain 



ft. / r 



a' = 916,322Y/ 1+ -^ 



= 1086,25 feet. 



The value of a' obtained by experiment is 1089,42 feet, as 

 given in the same work. The slight excess may be owing to 

 the neglect of the term involving m"^. 



I have thus obtained a value of the velocity of sound, closely 

 agreeing with experiment, on purely hydrodynamical principles. 

 As this result is not in accordance with received ideas on this 

 subject, I shall at a future opportunity give a careful resume 

 of the course of reasoning by which it has been arrived at. 



J. Challis. 



XII. On a neiv Empii-ical Formula for ascertaining the Tension 

 of Vapour of Water at any Temperature. By J. H. 

 Alexander, Esq.^ 



[Concluded from p. 15.] 

 N the last number of this Journal, I gave the formula itself, 

 the principles from which it was deduced, and a compa- 

 rison of results by it, with those by experiment at numerous 

 identical temperatures. Want of room excluded then what 

 * From Silliman's Journal for Nov. 1848. 



1 



