68 CARNEGIE INSTITUTION OF WASHINGTON. 



The influence of slope exposure on the vertical distribution of vegetation 

 has been found to be such that there is a general correspondence between the 

 vegetation of a north slope and a south slope 1,000 feet higher. It has been 

 shown that the ratio of evaporation to soil moisture corresponds on opposed 

 slopes 1,000 vertical feet apart, but a similar comparison of soil-temperatures 

 shows that a south slope has a consistently higher temperature than a north 

 slope 1,000 feet below it. 



The influence of altitude on the temperature of the soil is found to be slightly 

 greater on south slopes than on north slopes. For all of the four comparisons 

 made possible by the data secured in 1922 there is a fall of 6.3° to 6.8° for 

 each rise of 1,000 feet on slopes of the same exposure. The average tempera- 

 ture of the soil at 3 inches on the south slope at 9,000 feet is approximately 20° 

 lower than the soil temperature at the same depth on level ground at the 

 Desert Laboratory, 6,500 feet below. 



Atmometery in South Africa, by W. A. Cannon. 



In connection with studies carried out in 1921 on vegetation and environ- 

 ment in certain portions of South Africa, a comparative investigation of the 

 evaporation power of the air in several unlike stations was initiated and 

 carried out with the assistance of the Botanical Survey of South Africa and 

 various scientists not connected with the Survey. Cylindrical atmometers 

 were employed. In the preliminary studies atmometers were placed and 

 read at the following stations: Messina, Low Veld; Pretoria, High Veld; 

 Pietermaritzburg, Eastern Grass Veld; Grahamstown, Cape Town; Swakop- 

 mund. Southwest Africa; and Beaufort West, Whitebill, and Matjesfon- 

 tein. Great Karroo. Introductory reports from several of the stations are 

 now available and a summary of some of the results can be presented. The 

 period under consideration is from July to December, inclusive, although no 

 one station has records covering the entire time. The seasons, therefore, 

 are those of winter, spring, and early summer. 



A comparison of atmometer readings from the Karroo as contrasted with 

 those of certain other regions will be of interest. At Beaufort West, eastern 

 Karroo, where the average annual rainfall is 9.51 inches, of which over 60 

 per cent occurs in the warm seasons, the atmometer records of August to 

 December show weekly evaporation extremes of from 181 to 615 c. c. These 

 were obtained two successive weeks in September. The total evaporation for 

 September and October was 3,089 c. c. At Matjesfontein, western Karroo, 

 with an annual rainfall of 6.96 inches, of which over 60 per cent occurs during 

 the cool seasons, the total evaporation for September and October was 2,554 c. c. 

 The least weekly evaporation for the two months was 170 c. c. and the greatest 

 was 437 c. c. The former occurred the first part of September and the latter 

 late in October. At Grahamstown, between the Great Karroo and the south 

 coast, with an annual precipitation of 28.79 inches, of which over 60 per cent 

 occurs in the warm seasons, the total evaporation for September and October 

 was 1,577 c. c. The weekly extremes were 119 and 260 c. c. The total evapora- 

 tion at Pietermaritzburg during the same months was more than at Grahams- 

 town but less than at either of the Karroo stations named. The annual 

 precipitation at Pietermaritzburg is 35.97 inches, of which over 80 per cent 

 is in the warm seasons. 



