Barometric Pressure. 27 



position : suppose a cylindric vessel of indefinite length were 

 filled with hydrogen gas, and placed perpendicular to the ground 

 plane, having no communication with the atmosphere ; suppose 

 then that a small hole were made in the side of the said vessel at 

 a point where the elasticities of the two gases were equal. A 

 communication being now open an intercourse would immediately 

 commence; but this would not be occasioned by the specifically 

 heavy gas rushing into the light gas, nor the light gas into the 

 heavy gas exclusively. The two gases, having equal elastic 

 forces, would by virtue of those forces be diffused through each 

 other slowly and gradually according to the law which I have 

 pointed out in another Essay, — that of one gas being as a vacuum 

 to another in regard to their mutual diffusion. 



" The application of this principle in accounting for a 

 temporary existence of a warm, vapoury volume of atmosphere 

 supporting itself against heavier, but colder, volumes of 

 atmosphere on the right and left of it is too obvious to be insisted 



upon 



" From these observations it is not difficult to conceive that a 

 vertical column of warm, vapoury air may be projected into a 

 heavier column of cold, dry air, such that their elasticities may 

 be nearly equal for a time, but that the adjustment of their 

 weights may require a slow and gradual operation, sufficient 

 to account for the interval of time observed between the extreme 

 and mean state of the barometer." 



Buchan made the first material advance since the time of Dove 

 in explaining the barometric oscillations, and he followed Dalton in 

 distinguishing between weight and elasticity, without going to the 

 lengths of claiming that the pressure on the cistern of a barometer at 

 any instant is that of the whole weight in the vertical column of air 

 resting upon it. His view I also quote in full from the ninth Edition 

 of the Encyclopcedia Britannica :■ — ■ 



" If the temperature of the whole of the earth's atmosphere 

 were raised, atmospheric pressure would be diminished, for the 

 simple reason that the mass of the atmosphere would thereby be 

 removed to a greater distance from the earth's centre of gravity. 

 Quite different results, however, would follow if the temperature 

 of only a section of the earth's atmosphere were simultaneously 

 raised, such as the section comprised between Long. 20" and 60'' 

 W. The immediate effect would be an increase of barometric 

 pressure, owing to expansion from the higher temperature ; and a 

 subsequent effect would be the setting in of an ascending current 

 more or less powerful, according to the differences between the 

 temperature of the heated section and that of the air on each side. 

 These are essentially the conditions under which the morning 

 maximum and afternoon minimum of atmospheric pressure take 

 place. 



" The earth makes a complete rotation round its axis in 24 

 hours, and in the same brief interval the double-crested and 

 double-troughed atmospheric diurnal tide makes a complete 



