164 EARL DE GREY'S ADDRESS. [May 28, 1860. 



Not that these rules are without occasional apparent exceptions 

 — apparent rather than real — caused by a second, perhaps even a 

 third cyclonic (or oval) eddy impinging on the first circulation, 

 either horizontally or angularly (with reference to the horizon). 



The first movement may be likewise more or less inclined to 

 the horizontal plane, if not occasionally almost veiiical, as in a 

 *' descending squall." 



Such phenomena are readily explicable, after due consideration 

 of Dove's theory of polar and equatorial currents (translated and 

 published by the Board of Trade), and they are so marked by 

 " weather-glasses " that it is now inexcusable to navigate without 

 them or to undervalue their warnings. 



Why the barometer rises and falls, how it and its indispensable 

 companion the thermometer are affecte^ by a coming change, are 

 questions often asked by the inexperienced in their use, and may 

 be answered here in a few sentences (from the Meteorological 

 Department) for the benefit of such young travellers or voyagers as 

 have yet the world and its marvels before them. 



" Cold, dry air, coming from a polar direction, is heavier in 

 specific gravity than warm, moist air (containing gas or aqueous 

 vapour) flowing from tropical or equatorial regions. 



" The normal condition of our atmosphere is a continual rising 

 and westward movement of inter-tropical, or rather sub-solar, atmos- 

 phere, consequent on its expansion, and being lightened by the 

 sun's action while the earth is rotating on its axis. 



" This rise and westward impulse is accompanied by general 

 movement, from polar directions, to fill the space that would other- 

 wise become comparatively vacant. Air, like water, seeks equi- 

 librium, but, unlike water, it is very elastic and excessively mobile. 



" Yet air, however rarefied, cannot rise beyond a certain dis- 

 tance. Cold and gravity check its elevation. It must, however, 

 move onwards somewhere. Having momentum, and being pressed 

 behind by ever-rising air, it overflows (as it were) the polar under- 

 currents and moves towards those regions which the polar currents 

 have quitted and are continually quitting. But those regions are 

 vastly smaller in area than the equatorial, and opposition, if not a 

 conflict, occurs soon between the main streams or currents, so un- 

 equal in breadths and characters. 



" Portions of the overflowing quantities from the sub-solar 

 regions combine, between the tropical limits and near thirty degrees 

 of latitude, with the normal and general movement (called trade- 



