314 Mr. W. S. Jevons on the Semidiurnal 



minimum of the bai'ometcv at that period of the day, although 

 the low temperature then existing would have an opposite effect ; 

 it is the great increase of vapour which raises the barometer to 

 a maxiuuim at 9 a.m.; but the increasing temperature of the 

 air then becomes predominant in its effects, and the afternoon 

 minimum of pressure is the consequence. The second maximum 

 at 9 P.M. is to be ascribed to the subsequent rapid decrease of 

 the temperature. 



2. This theory is supported by many elaborate and ingenious 

 researches, by Dove and Sabine, into the daily barometrical curve 

 peculiar to various sitnations. In the interior of Russia (and 

 aj)parently in Russia oidy) the curve is of most simple character, 

 the barometer having a single maximum at sunrise and a single 

 minimum in the afternoon : this peciiliarity is supposed to be 

 owing to the comparative absence of aqueous vapour in such far 

 iidand places, the temperature there becoming the entirely pre- 

 dominant agent. 



Colonel Sabine takes the opportunity, when speaking of this 

 theory, to eulogise "the progress which is made in the physical 

 sciences by the aid of nwan numcricol values," and the " new 

 aspect which this beaatilul branch of investigation has assumed 

 by the separation of the "pressures of the aqueous and gaseous por- 

 tions of the atmosjihere.^' Now, while fully admiring the method 

 of investigation by mean restdts, I must object to the almost ex- 

 clusive employment of it which now seems usual in meteorology 

 and some other sciences ; and I proceed to show that in the case 

 of the barometrical oscillation it has led to serious misconcep- 

 tions, which a little close inquiry into the known nature and the 

 simplest mechanical conditions of the atmosphere must have 

 prevented. 



3. The error consists in the whole practice of separating the 

 aqueous and gaseous pressures of the atmosphere, and is in origin 

 perhaps au error of terms. It is said, indeed, and proved, that 

 the pressure of aqueous vapour is independent of the pressure of 

 the air. Thus if water be introduced into a perfect vacuum, a 

 definite quantity will instantly rise from it and exert a definite 

 pressure on the sides of the confining vessel ; for instance, at the 

 temperature of 60° F., the pressure exerted will be nearly equal 

 to that of a column of mercury half an inch deep. But if tlie 

 space into which we introduce the water, instead of being pre- 

 viously vacuous, contain air of any given density and elastic 

 force, the quantity of aqueous vapour emitted will still be the 

 same (or v^ry nearly), and the pressure of this vapour, which is 

 equal to that of half an inch of mercury, will be exerted against 

 the confining vessel, in addition to the previous pressure of 

 the air. 



