DEW : FACTS AND FALLACIES 45 



Regularly spaced heaps of stones cover several hillsides in the Negev 

 region of Israel. It is generally agreed that they were built by Nabatean 

 communities about the first and second centuries a.d., and that their pur- 

 pose was agricultural; their Arabic name is teleilat el einab, 'the hillocks for 

 grapevines' (Glueck, 1959). Tadmor and others (1957) discounted a sugges- 

 tion that the teleilat were built to collect dew, on the negative evidence 

 that no plant material has yet been found within them. Assuming from 

 present dimensions that the teleilat were originally conical with a diameter 

 of 3 m and a height of about 1-2 m, then p= 1-15. The maximum yield 

 from a saturated atmosphere at a temperature of 20°C would be about 

 0-5 htres per hour and, allowing for unsaturation in the early evening, 

 condensation may have reached 2-3 htres per night. If dewy nights were 

 frequent this would be sufficient to maintain several small plants with their 

 roots beneath the stones. 



Table 4 



Theoretical and measured values of p for single plants of geranium 

 and bean (after Hofmann) 



Gigantic stone piles on the hills above the port of Theodosia in the 

 Crimea had roughly the same shape as the teleilat, but their height was 

 10 m and their diameter 24 to 30 m (Hitier, 1925). Maximum yield, 

 calculated from potential condensation and p= 1-15, is about 300 litres per 

 night. From the size of the aqueducts leading from the cairns to the famous 

 fountains of Theodosia, Zibold estimated the yield from each to be 55.400 

 Htres per day: dismissing dew as totally inadequate, the only alternative 

 mechanism for moisture collection is the interception of fog and cloud 

 droplets. On Table Mountain, the annual mean fog precipitation is 3-75 

 mm/hr (Nagel, 1956) and, assuming the same figure for Theodosia, each 

 cairn could have produced about 12,000 Htres per day in favourable 

 weather. 



At MontpeUier in the south of France, L. Chaptal attempted to estimate 

 the yield of the Theodosia cairns by building a miniature version in the 



