1823.] for Barometrical Measurements, 35*7 



force, so that its presence in the different strata renders them 

 proportionally capable of sustaining with a less density an equal 

 column of mercury. 



Lastly, the decrease of gravity as we recede further from the 

 centre of the earth is another cause of the change ; for by this 

 decrease a column of mercury whose length is hy weighs so much 

 the less, as we recede from the centre : if it weigh less, it com- 

 presses less the strata of air into which it is carried : thus the 

 ratio of their density to the length of the column of mercury, or 



-, is no longer the same for these strata as for those which are 



below. If all other circumstances are alike, the densities of the 

 strata of air which these columns compress will be likewise pro- 



portional to them. The ratio -, or C, therefore, ought to vary 



from one stratuni to another proportionally to the force g, 



(3.) The amount of each of these corrections may be calcu- 

 lated on the following principles : — 



First, the action of temperature. From the influence of this 

 cause, a mass of air whose volume is 1 at zero (centig.) becomes 

 at t degrees, 1 4-^ . 0'00375, the barometrical pressure remaining 

 the same. Under a constant pressure, the densities of this mass 

 are reciprocally as the volumes, and, therefore, if the density at 



zero be 1, the density at t degrees will be ^ ^ 0.00375 i^^^^r ^ 



ft 

 constant pressure ; the ratio -, or C, must, therefore, vary pro- 

 portionally to this quantity. 



Secondly, the influence of aqueous vapour. According to 

 the experiments of De Saussure and Watt, the weight of this 

 vapour is to that of air as 10 to 14, while their elastic forces and 

 temperatures are the same ; that is to say, while the air and the 

 vapour being at the same temperature, sustain equal columns of 

 mercury. The substitution, therefore, of this vapour in the 

 strata of the air, renders them specifically lighter without dimi- 

 nishing their elastic force. To obtain the value of this effect, 

 let h be the barometrical pressure which supports a certain, 

 stratum of air : let us call F the elastic force of the aqueous vapour 

 contained in it ; that is to say, the part of the barometrical 

 pressure which the vapour sustains. The whole weight of the 

 stratum may be considered as composed of two parts, viz* of a 

 certain quantity of vapour whose elastic force is F, and of a 

 certain quantity of atmospheric air perfectly dry, whose elastic 

 force is ^ — F. Let p be the whole weight of the stratum, if it 

 were composed entirely of dry air under the pressure h. The 

 weight of the same volume of dry air under the pressure ^ — F 



will be p ^~ . The weight of the same volume under the 

 pressure F will be —, Lastly, if this volume remaining always 



