On the Physical Properties of Soil 



205 



right angle, or 90 degrees, with the surface. If the actual in- 

 crease of temperature produced by the rays of the sun be- 

 yond the temperature in the shade be between 45° and 63°, 

 as is often the case on clear summer days, this increase would 

 be only half as great if the same light spread itself in a more 

 slanting direction, over a surface twice as large. Hence it is 

 sufficiently explained why even in our own climate the heat 

 so frequently increases on the slopes of mountains and rocks, 

 which have an inclination towards the south. When the sun is at 

 an elevation of 60 degrees above the horizon, as is more or less 

 the case towards noon in the middle of summer, the sun's rays 

 fall on the slopes of mountains which are raised to an inclination 

 of 30 degrees to the horizon, at a right angle ; but even in the 

 later months of summer, the sun's rays frequently fall on them 

 under a right angle, in cases where the slopes are yet steeper. 

 Such declivities, particularly in our own geographical latitude (of 

 Germany), are therefore peculiarly suited for the cultivation of 

 plants which require a high temperature, such for instance as the 

 vine. 



If we compare accurately the power of the sun's rays to warm 

 the soil with reference to the different seasons, we shall per- 

 ceive more distinctly the influence of the different inclination of 

 the ground towards the sun. I made some careful observations at 

 Tubingen some years ago on this subject, the results of which I 

 have arranged in the following table, in comparison with other 

 observations which I had made previously at Geneva. Those 

 observations, which are marked as having been made in fine 

 weather, exhibit the mean highest temperature of an ordinary 

 blackish-grey garden-mould, the temperature of which was ob- 

 served on the south side of my house, in perfectly fine weather, 

 between noon and one o'clock, whenever the weather happened 

 to be perfectly fine at that part of the day. They are founded 

 on the average of two years' observations : the bulb of the 

 thermometer was covered only the twelfth of an inch high with 

 earth, and its scale being of clear glass could contribute nothing 

 to the elevation of temperature. Those figures in the table which 

 refer to variable weather rest on observations made in the Botanic 

 Garden at Geneva, in the year 1796: they contain the mean of 

 the observations made every day, and not merely of those taken 

 in fine weather. The elevation of temperature by the rays of the 

 sun was therefore considerably less according to the average re- 

 sults of these observations, because the temperature of the upper 

 surface of the earth on cloudy and rainy days often accords exactly 

 with that of the air ; but on the other hand, they give us more 

 accurately the mean temperature of the ground at some depth. 



