560 PEHR OLSSON-SEFFER 



DIFFERENCE OF WIND VELOCITIES PER SECOND IN METER 



In these experiments the anemometers were placed on a board 

 on the ground, thus raised 3 cm. above the surface. The centrum of 

 the instrument was on a level with the top of the low grass turf in the 

 case of the grassy surface. 



We find from these accumulated facts that the mean velocity on 

 the even surface surpassed that on the rough ground by 3.15 m. per 

 second or 34.7 per cent. F. H. King^ gives the velocity over smooth 

 ground as more than 40 per cent, greater than that on a rough sur- 

 face. The difference in my results may have been caused by different 

 conditions under which the experiments were conducted. King does 

 not give any information about the method by which his results were 

 obtained so that actual comparison is impossible. I consider the 

 difference as being of minor importance, as by both these series of 

 experiments it is clearly shown that the velocity of wind over a smooth 

 surface is at least 34.7 per cent, greater than on uneven ground. 

 Again, the velocity on grassy ground is still less than on bare rough 

 surface. This is a fact of some practical importance in connection 

 with planting on sandy soils, and it has a bearing of considerable 

 weight on the vegetation on sand formations. 



Through the investigation of Kihlman,^ Warming,^ Hansen,^ 



1 Destructive Effects of Winds on Sandy Soils and Light Sandy Loams with Methods 

 of Prevention, Eleventh Annual Report of Agricultural Experiment Station of Wis- 

 consin University, 1895, p. 332. 



2 Pflanzenbiologische Studien aus Russisch Lappland, " Acta Soc. F. Fl. F., VI, 

 1890. 



3 Plantesamfund, 1895; "Der Wind als pflanzengeographischer Faktor," Engl, 

 hot. Jahrb., Vol. XXXI, 1902. 



4 Die Vegetation der ostfriesischen Inseln, 1901. 



