from among which an animal can choose by little expenditure of energy. By burrowing 

 deep an animal can avoid the extreme heat of day — and by leaving the burrow at night 

 it can even escape the peak temperature below, as at a foot or so beneath the surface 

 there is a lag of about 12 hours in the time of maximum temperature. Almost as big a 

 choice is available in this hilly type of desert by moving into the more or less perma- 

 nent shade beneath the steep south side of the valley. An experiment was made one 

 day in August by artificially shading the sand where the temperatures were being mea- 

 sured. The maximum surface temperature under these conditions was 20°C (36°F) lower 

 than the previous day when the sun had been shining, while at 18 cms. deep the maxi- 

 mum (not reached till about 7 p.m.) was 5°C (9°F) lower than the previous day. 



References 



McKenzie- Taylor, E., & Williams, C.B., 1924. A Comparison of Sand and Soil Temperatures in 

 Egypt. Min. Agr. Egypt. Tech. and Scient. Bull. No. 40. 



Williams, C.B., 1923. A Short Bio -climatic Study in the Egyptian Desert. I.e. Bull. No. 29- 



Williams, C.B., 1924. Bio-climatic Observations in the Egyptian Desert in March 1923. I.e. Bull 



No. 37 



Williams, C.B., 1924. A Third Bio -climatic Study in die Egyptian Desert. I.e. Bull. No. 50. 



27 



