22 ATMOSPHERIC ACTINOMETRY 



the statements just made, I foiiiul myself face to face, as Piofessor of Meteorology, 

 with the following question : 



It is an acknowledged fact, that the activity of the vegetative j)rocess in the 

 northern parts of Europe is very great. The interval between sowing and harvest- 

 ing, for si)ring wheat, lasts on an average 14.') days in Alsace. According to M. 

 Tisserand it amounts to only l.">."5 days at Ilalsno, in 50° 30' N. latitude; .iml it is 

 oidv 114 days at Skibbotten, in (i9° 30' N. latitude. It decreases therefore as we 

 approach the pole; notwithstanding that the average temperature of the peritxl of 

 vegetation diminishes likewise with the increase of latitude. 



This decrease in the number of days needed for vegetation, as we draw neaier 

 the north pole, seems to be a general law. According to Arnell, barley recpiii'es 

 117 days to grow in Southern Sweden, 9'2 in Middle Sweden, and 89 in Lajiland. 

 It is true that these variations depend in part at least on the power which the plant 

 has to a<lapt itself to external conditions, for if sown in our country the Norwegian 

 grain grows more ra})idly tlian ouis, while our own native grain, carried to Norway, 

 lags behind the acclimated variety. l>ut this is not sufficient to explain ;dl, ;uh1 we 

 must in the end always return, as a final analysis, to the influence of dinuite. 



We may go even a little farther in our induction. According to Griesbach 

 the increased ra[)idity in the development of plants cultivated at the extreme noith 

 does not affect the whole evolution of the plant, but onl}' the period between ger- 

 mination and blooming. It a[)plies, therefore, only to the green organs of the plant, 

 and thus starts once more the question of light, which actually appears to be of 

 greater importance than that of temperature. In fine, to return to the subject of 

 our Memoir, the actinic influence of the solar rays seems to increase with the 

 latitude. 



To what is this increase due? This (pu'stion is still open and to it I have 

 tried to find an answei'. The first point to determine was whether the solution of 

 oxalic acid also showed such an increase of actinic effect? 



It was on this account that I asked Mr. Elfving to assist me, whose interesting 

 experiment I have mentioned above (page 17). 1 sent to him at Ilelsingfors an 

 oxalic solution, and ten vessels exactly alike, such as I had used myself in France, 

 in Older to make sure that at least, and as fai' as possible, those experimental condi- 

 tions which we could conti'ol should be as identical as they could be luade. 



Unfortunately there were other conditions which were entirely beyond our 

 control. The ideal would have been attained with a series of days equally fine, 

 occurring simultaneously in France and in Finland, and permitting us to make our 

 observations under precisely the same circumstances. But there are obvious reasons 

 why the weather could haidly ever be the same in France and in the Gulf of 



