164 LECTURES ON 



and in consequence the streams dried up, and the productiveness of 

 the country was seriously diminished. 



3. The angle at which the sun's rays strike a soil is of great influence 

 on its temperature. The more this approaches a right angle the 

 greater the heating effect. In the latitude of England the sun's heat 

 acts most powerfully on surfaces having a southern exposure, and 

 which are inclined at an angle of *&° and 30°. The best vineyards. 

 of the Rhine and Neckar are also on hill-sides, so situated. In Lap- 

 land and Spitsbergen the southern side of hills are often seen covered 

 with vegetation, while lasting or even perpetual snow lies on their 

 northern inclinations. 



4. The influence of a icall or other reflecting surface upon the warmth 

 of a soil lying to the south of. it, was observed in the case of garden 

 soil by Malaguti and Durocher. The highest temperature indicated 

 by a thermometer placed in this soil at a distance of six inches from 

 the wall, during a series of observations lasting seven days, (April, 

 1852,) was 32° Fahrenheit higher at the surface, and 18° higher at a 

 depth of four inches than in the same soil on the north side of the 

 wall. The average temperature of the former during this time was 

 8° higher than that of the latter. 



In the Rhine district grape vines are kept low and as near the soil 

 as possible, so that the heat of the sun be reflected back upon them 

 from the ground, and the ripening is then carried through the nights 

 by the heat radiated from the earth. — {Journal Highland and Agricul- 

 tural Society, July 1858, p. 347.) 



5. Malaguti and Durocher also studied the effect of a sod on the 

 temperature of the soil. They observed that it hindered the warm- 

 ing of the soil, and indeed to about the same extent as a layer of earth 

 of three inches depth. Thus a thermometer four inches deep in green- 

 sward acquires the same temperature as one seven inches deep in the 

 same soil not grassed. 



It is to be remembered that the soils that warm most quickly, also 

 cool correspondingly fast, and thus are subjected to the most exten- 

 sive and rapid changes of temperature. The greensward which 

 warms slowly, retains its warmth most tenaciously, and the sands that 

 become hottest at noon-day, are coldest at midnight. 



Of no little practical importance is the shrinking of soils on drying. — 

 This shrinking is of course offset by an increase of bulk when the 

 soil becomes wet. In variable weather we have therefore constant 

 changes of volume occurring. Soils rich in humus experience these 

 changes to the greatest degree. The surfaces of moors often rise and 

 fall with the wet or dry season, through a space of several inches. 

 In ordinary light soils containing but little humus no change of bulk 

 is evident. Otherwise, it is in clay soils that shrinking is most per- 

 ceptible; since these soils only dry superficially they do not appear 

 to settle much, but become full of cracks and rifts. Heavy clays may 

 lose one-tenth or more of their volume on drying, and since at the 

 same time they harden about the rootlets which are imbedded in them, 

 it is plain that these indispensable organs of the plant must thereby 

 e ruptured during the protracted dry weather. Sand, on the other 



