32 



Lieut.-Col. Sabine on some Points hi 



sure into its constituent pressures of vapour and air; since the phy- 

 sical causes of the diurnal variation of the component pressures have 

 been respectively traced to the variations of temperature produced 

 in the twenty-four hours by the earth's revolution on its axis, and 

 to the different properties possessed by the material bodies at the sur- 

 face of the globe in respect to the reception, conveyance, and radia- 

 tion of heat. 



Annual variation. — We now proceed to the annual variations, 

 which are shown in the subjoined table. 



Table TIL 



We here perceive that the leading features of the phenomena are 

 clearly analogous to those which have been seen to present them- 

 selves at Toronto, Prague and Greenwich ; viz. a correspondence of 

 the maximum of vapour pressure and minimum of gaseous pres- 

 sure with the maximum of temperature, — and of the minimum of 

 vapour pressure and maximum of gaseous pressure with the mini- 

 mum of temperature ; and a progressive march of the three varia- 

 tions from the minimum to the maximum, and back to the minimum 

 again. The epochs, or turning-points of the respective phenomena, 

 are not in every case strictly identical ; but their connexion, which 

 is the subject immediately before us, is most obvious. 



We have thus a further illustration of the universality of the prin- 

 ciple of the dependence of the regular periodical variations, annual 

 as well as diurnal, of the pressures of the dry air and of the vapour, 

 on those of the temperature *. 



* In the tropics and in the temperate zone the heat of summer produces and 

 accompanies a low gaseous pressure, and the cold of winter a high gaseous pres- 

 sure. When we consider how large a portion of the northern hemisphere is occu- 

 pied by land, which becoming greatly heated in summer rarefies the superincum- 

 bent atmosphere, causing it to overtop the adjacent less heated masses, and to 

 overflow them, we should be led to expect that in parts of the Arctic Circle situ- 



