AND THE ACTINIC CONSTITUTION OF THE ATMOSPHERE. 43 



11. Northern countries add to tins cause of superiority, which they owe to 

 the constitution of their atraosphei-e, another, ^vhic•ll is due to their geograpliical 

 position, namely : that the actinic effect of the sun increases more rapidly than the 

 duration of its presence above the horizon. The very long days of the north 

 during the period of vegetation, are, therefore, in their actinic effect, more active 

 than an equal number of days in our temperate regions, and we can thus exi)lain 

 tlie j.articularly intense rate of the progress which vegetation makes in the vicinity 

 of the polar circle. 



12. Tliis increase of sensitiveness which oxalic acid experiences in the sun, 

 does not cease when tlie light begins to fade, and may continue several days. 

 Hence follows a conclusion which may also be applied to our temptrate reo-ioiss: 

 this is, that the actinic effect of a number of fine days in succession increases more 

 I'apidly than its duration, and also, that the effect of a fine morning is not lost by a 

 dark and cloudy evening. 



13. We must, therefore, give up the hope of finding, in the duration of a day 

 oi- of solar action, a measure of its effects, and meteorological instruments, which 

 accept such a pi'oportionality, are to be rejected. 



14. The importance of these actinic phenomena in the general economy of 

 the world is great enough to make it necessaiy that we should ap2:)roach the 

 investigation by appropriate means. 



