AND TlIK ACTINIC CONSTITUTION OF THE ATMOSPHERE. 23 



Bothnia. When that vast mass of air, in relative repose, whicli I have named The 

 Isle of Calms, rests over our part of the world and gives us fine weather, the equa- 

 torial current, which tui-ns it northward, is over Sweden and Norwaj', to which it 

 brings overcast skies and rains; when, on the other hand, the Isle of Calms rests 

 over the north of Europe, we are in Fi'ance subject to stormy disturbances which 

 come to us through the Mediterranean or the Gulf of Gascony, or we are subject again 

 to the return curreut, which, after having lounded the " Island," comes back to us in 

 the shape of cold east and northeast winds. To find favorable coincidences in this 

 grand atmospheric dance, we should need months of continued observations, which 

 neither Mr. Elfving nor myself were in a condition to undertake. 



In this difficulty we availed ourselves of the meaning of the word "fine day," 

 as I have shown above (page 13), which is so uncertain as to its actinometric defini- 

 tion that every effort to make it absolute as to perfect equality of experimental con- 

 ditions becomes rather illusory. We could be content, more modestly, with a first 

 approximation ; it was enough to compare the actinometric combustion of the finest 

 days in the Gulf of Finland and in Fi-ance, at the same time of the year. 



Nor is this all. The length of the day is greater at the North than at the 

 South duiing the period of vegetation, and the length of the insolation has, we all 

 know, a direct influence on the I'elatlve (piantum of combustion. Hence I requested 

 Ml'. Elfving to make every day two sets of experiments, one with vessels exposed 

 to the sun from 8 a.m. to 4 p.m., like those which I was using in France, and the 

 other with vessels left out from 8 a.m. till the setting of the sun. 



Ml'. Elfving made at Helsingfors between August 27th and September -ith, 1887, 

 five series of experiments, which I cannot compare with those which I was making at 

 the same time at the foot of the Puy-de-Dome, and the records of which have been 

 lost. But I am fortunately able to compare them with those which I had begged 

 M. Ch. Mascart to make at the same time near the seashore in the Channel. These 

 may perhaps be better fitted for comparison with those made by Mr. Elfving, as 

 both were made at maritime stations. All that I noted when I received them was 

 that they gave much higher figures than those which I obtained at the same time 

 on the bare table-land which carries the Puy-de-Dome. 



In the first place, here is a table of the observations made in France ; it is 

 formed in the same way as those which have already been given in this Memoir. 



