88 



and moisture from the rain have dried up more than on the south and south- 

 cast; it has previously been warmed to some extent and the same amount of 

 warmth falHng on a drier soil correspondingly warms it up more. 



The monthly mean temperature, however, and in any case the maxi- 

 mum warmth in the different seasons, is more important for plants than is 

 the annual average. In this connection Kerner's thermometer observations 

 show that only in winter (from November to April) is the maximum soil tem- 

 perature found on the southzvest side and that conversely, from May until 

 August, the southeast side shows the greatest ivarmth; in September and 

 October the south side is the warmest. This shifting of the maximum may 

 undoubtedly be explained by the dry east and southeast winds of midsum- 

 mer which, a similar physical composition of the soil being assumed, dry 

 the soil more quickly and thereby make it more capable of being Avarmed up. 



While Kerner's investigations were made on a natural hill, consisting of 

 alluvial sand and provided v/ith pretty steep, grass slopes near Innsbruck, 

 Wollny experimented with an artificial hill made of sifted calcareous sandy 

 humus whose surface formed an angle of 15°. Here, therefore, the con- 

 ditions were adapted to a land whicli could be used agriculturally. 



Wollny's observations confirm first of all those of Kcrner, that the max- 

 imum of warmth shifts from southeast in summer to southwest in winter. 

 Further, in general, the southern slopes (S.JV., S., S.E.) are exposed to great- 

 er fluctuations in temperature /ban the northerly slopes which respond to the 

 smallest fluctuations. In another series of experiments ascertaining the 

 temperature of the slopes of beds set at different angles to the compass, com- 

 pared during the warmer season with the temperature on a level field sur- 

 face depressed 15 cm., gave the follov/ing results. The south side is the 

 warmest, then follows, as the medium, the level worked surface ; then in the 

 third place the east and west sides, while the northern exposure of the bed 

 seems to be the coldest. If now the bed is placed east and west, one long 

 surface lying to the south, the other to the north, these two surfaces show 

 the greatest difference in temperature when vegetation can still be found. 

 Therefore, if the field is to be laid out in plots, it is better to have them run 

 north and south. Cultivation on level surfaces with a lower temperature 

 than on the slope inclined to the south but exceeding that of other exposures 

 is the most advantageous on account of the even, and, on an average, higher 

 warming of the soil. 



Later experiments', however, show the advantages of a position 

 inclined to the south, but these are only evident when the moisture is suffici- 

 ent and constant. In dry weather or irregular precipitation the harvest is 

 smaller. Indeed, in extremely dry weather, the greatest yield is from the 

 northerly side, which- otherwise gives the smallest. In fact the yield becomes 

 less as the angle of inclination increases. Then follow the west and east ex- 

 posures. The smallest yield was usually on the south side. 



1 Wollny, E., Untersuchumgen liber die physikal. Eigenschaften des Bodens auf 

 das Produktionsvermogen der Nutzgewachse. Forsch Geb. d. Agrikluturphysik XX, 

 Part 3, 1899, p. 291. 



