POSSIBLE MEASURES OF AMELIORATION. 155 



fixed to leeward of the twigs, as shown in fig. 11, but ordinarily 



Fig. 11, — Diagram of fence intended to stop drift sand. 



lower fences are used and this supporting framework is then omitted. 



The mode of application, size, and distribution of the fences depends 

 on the object to be attained. Where it is merely desired to fix loose 

 moving sand, it has been found best to cover the surface with a net- 

 work of low fences formed of twigs some 20 inches in length, of which 

 about 8 inches are buried and 12 inches stand above ground. Planted 

 in rows at right angles to each other and 4 yards apart, it is said that 

 even strong winds will blow harmlessly over the loose sand. 



This mode of protection is always applied to large areas, and the 

 low fences arranged in straight lines, which are preferably arranged 

 parallel and transverse to the direction of the prevailing wind, and it 

 has been found that, on the coast of Prussia, 4 yards apart is 

 sufficiently close; whether it would be sufficiently close in the case of 

 the long-continued strong winds of Karachi may be doubted, and in any 

 case it is necessary to place the fences closer on the steep sides of the 

 dunes, which can be done, either by running diagonal fences across the 

 squares, or by dividing them into smaller squares by halving the 

 distance between the fences. 



In the case of this protective fencing, when the object is not to cause 

 an accumulation of sand but merely to stop its movement, the ratio of 

 equality between solid twigs and interstices may be exceeded, and the 

 spaces may be twice or even four times as much as the solid part of 

 the fence, as in Plate III, fig. 2, which shows one of the applications 



( 33 ) 



