1858.] Meteorological Observations on Parisnath Hill. 37 



of accounting for the absence of any visible effect of the great and 

 irregular variations in the tension of vapour upon the daily barome- 

 trical curve might be obviated by applying the view which Bessel has 

 developed in his paper on barometrical measurement of heights* 

 with regard to the action of watery vapour. According to it, the 

 watery vapour, so long as it is not saturated for the existing tem- 

 perature, would in no way differ in its physical behaviour from a gas. 



To explain more distinctly in what way the application of this 

 view would obviate the difficulties I mentioned, I will make use of 

 the following illustration. 



Suppose Fig. 5, an upright hollow Cylinder, a, b, c, closed at the 

 bottom, to be filled with a gas g, which I will suppose to be not 

 acted upon by gravity. This gas shall be subject to a pressure, 

 which is represented in the figure by the moveable weight p, resting 

 on the gas, and closely fitting the cylinder like the piston of an air 

 pump. In a state of equilibrium between the elastic force of the gas, 

 and the weight it has to support, the gas fills the volume 2 V and 

 exerts an elastic pressure, equal to the weight p. If now, through 

 a stopcock 13 a quantity of another gas, g', of a different density be 

 introduced, which, under the pressure p would occupy the volume 

 Y, the weight p will be raised from b to c, and the space filled by 

 the two gases will now be 3 V. Each of the two gases will be dis- 

 tributed equally throughout the space 3 Y, and both together will 

 exert the pressure p. 



By the increase of volume, to which each of the gases has been 

 subjected, their densities must have been diminished in proportion, 

 that of the first gas g, being now one-third, and that of the second 

 gas, g', two-thirds less than what it was before. The pressure 

 or elasticity of a gas, all other conditions remaining unchanged, 

 will vary in the same proportion as its density, and consequently 

 the elastic pressure of the first gas g, which was equal to p, will now 

 only be | p and that of the second gas, g', only i p, the sum of both 

 being equal to p, the weight they have to support. 



If we now deduct the elastic pressure of the gas g' from the sum 

 of the pressures of both gases, we shall not obtain the pressure 



* Astronomische Nachrickten Nov. 356, 357. Taylor's Scientific Memoirs, 

 Vol II. 1841. 



