RESEARCH AND DEVELOPMENT -51 
modification of clouds and precipitation. On the contrary, they indicate 
that further effort is necessary. 
It is therefore recommended that: 
(a) new scientifically designed and rigorously checked experiments be 
undertaken in all regions where there is a possibility of success; 
(2) precise methods of evaluating the amount of precipitation result- 
ing from such experiments be developed; 
(c) in all research experiments and applied operations, the collabora- 
tion of the meteorological authorities be sought, in order to ensure the 
greatest reliability in conducting the experiments and assessing the 
results; 
(d) information on all projects already carried out or those in opera- 
tion now be released and made available to all scientific workers, putting 
development of science before commercial and other interests. 
It is well known that advanced civilizations formerly occupied 
what are today arid regions. Excavations show the remains of 
reservoirs, canals, fortifications, and human habitations, pre- 
suming an intensive and extensive cultivation by a large popula- 
tion. Such is the case in India, Pakistan, North Africa, the Near 
East, and even in the desolate region of Lob Nor in Central Asia 
there are remains of oasis towns and irrigation works. The dis- 
appearance of these civilizations may have been brought about 
by severe climatic changes or by the doings of man not for the 
benefit of mankind. 
Tixeront, our colleague from Tunisia, in asking whether it is 
possible to forecast weather over long periods, points out that 
we have not yet sufficient, reliable records, but he states that in 
Tunisia they use one of nature’s records in the growth rings of 
trees and one of man’s in archaeological studies. The meteorological 
service of Tunisia studied the climate of Ain Draham from 1736 
to 1955 by examining the growth rings of an oak tree. The infer- 
ence drawn was that from 1736 to 1790 there was significantly 
more rain than later, and this seems to be confirmed from histor1- 
cal documents which refer to abundant crops in the eighteenth 
century. (See pp. 91-92.) 
Tixeront considers that a study of the ruins of Arab and Roman 
irrigation works, of historical texts, of the continuity of cultiva- 
tion methods, and of the cultivated plant species indicates that 
the climate is stable and has not become decidedly drier. Simi- 
