SUPPLEMENT. 



OBSERVATIONS MADE IN 1894 IN FRANCE AND ALGIERS. 



Siuce sending luy Memoir on Atniospbeiic Actiuometry, I have been enabled 

 by the courtesy of M. Gessard, Chief Pharmacist of the Military Hospital at Setif 

 (Algeria), to make a number of combined observations in a temperate region and in 

 a hot climate. It was interesting to discover whether we would meet here with 

 the same differences as between the observations made in France and in Finland, 

 that is to say, if for equal lengths of insolation the chemical activity of the solar 

 rays would continue to diminish in proportion as we api)roacli the equator, and 

 as theii- calorific power incieases. 



For such a comparison the choice of the stations was of some importance. 

 Setif is situated about eleven hundred metres above the level of the sea, on a buttress 

 of the southern slope of the high mountains of the sea-coast, the chain of theBabei-s 

 or the Bibans. Towards the south it oveilooks from a height of two or thiee hun- 

 dred metres an immense plain, which in its turn is bordered at a distance of 35 or 40 

 kilometres (22 to 25 miles) by a chain of not very high mountains, which cuts it 

 off from another more extensive plain, the basin of the Hodna. Beyond this, sepa- 

 rated a"ain by an insignificant mountainous elevation, lies the Sahara and the desert 

 climate, which makes its influence felt as far as the plain of Setif. This vast heat- 

 ing-centre south of the city frequently procures for the latter, towards evening, a 

 fresh cui'reut of air from the north, and in ordinary times it stands on the boundary 

 line where two contraiy influences enter into direct conflict, the wind blowing from 

 the coast and the high summits, and tlie liuriiing wind fi'om the desert. Tims 

 Setif enjoys a relative freshness on certain .lays when the }.lain at its feet is given 

 up to the full ardor of the sirocco, and when it even may happen that the clniid of 

 dust, propelled Ijy this wind, stopping at a distance of 15 or 20 kilos, from the 

 town, screens the neighboring mountains at the very time Avhen the atmosphere 

 remains quite clear al;out Setif and the immediate surroundings. 



The station which I have chosen in France for my comparative observations 

 is also situated on the side of a slope, overlooking the plain of Vic-sur-Cfere (Caiital), 

 and 750 metres alx.ve the level of the sea. I might have gone highei-, but I have 



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