1S76.] Inequality oftlie tivo Semidiurnal Oscillations. 325 



And t= V P -—tc (2) 



Wherein p is the density of air at the standard pressure P and absolute 

 temperature To and c its specific heat at constant volume, compared with 

 water as unit y. 



If now the sanie quantity of heat t be employed in evaporating water 

 at temperature T, (the whole being consumed as latent heat) and filling the 

 volume of ah J^with vapour at presstirep', the total pressure will become 



, -, „»' To , 



P + p and t = Vs — — — A 



when s is the hypothetical density of water vapour at P and To, and A its 

 latent heat at temperature T. Substituting for s its approximate equivalent 

 #P 



r=Vi P f^A (3) 



and equating (2) and (3) and eliminating common factors, 



From (1) and (4). 



P = — (4) 



P=P sTc (5) 



which gives the ratio of the increase of pressure produced by the same quan- 

 tity of heat, employed in the one case simply in heating dry air, and in 

 the other in charging it with vapour. At a temperature of 80° Fahr. = T 

 = 541, 



p = 7-36 p' 

 that is to say, when a given quantity of heat is employed in heating dry 

 ah at the temperature of 80° it raises its pressure more than seven times as 

 much as when it simply charges it with vapour without altering the tem- 

 perature. With lower values of ^Tthe difference will be still greater.* 



This great difference is no doubt much reduced in nature by the effects 

 of radiation ; and while some evaporation is effected on the land surface, there 

 is some increase of temperature over the sea : but it may be expected tbat 

 some part of tbis difference will manifest itself in tbe greater intensity of 

 the forenoon pressure in the lower strata of the atmosphere on the land as 

 compared with the sea, and in fine clear weather as compared with cloudy 

 weather, when banks of clouds present an evaporating surface. With 



* Substituting for A the general value determined by Regnault 109T7 — '695 

 (2 1 — 493"2) tbe general expression for tbe ratio becomes 



/5400-S7 „„,\ , 



# = (— ^ 2-6i) y 



