POSSIBLE MEASURES OF AMELIORATION. 153 



fix, to prevent, and where desirable to cause, the formation of sand- 

 hills. The results of this experience have lately been summarised in 

 an exhaustive treatise. 1 



To a large extent they are inapplicable to the climatic conditions 

 of Karachi, or of any other part of India, as the final aim in all cases 

 is the establishment of a continuous cover of vegetation of some kind 

 or other, and the details both of plants used, and method of planting 

 are only adapted to the special climatic conditions of Europe. In 

 part, however — that part which deals with the preliminary fixing of 

 the loose sands — the principles are of world-wide application, and 

 may be as profitably employed in India as in Europe. 



The principle underlying the operations is in all cases the same, 

 whatever may be the variation in details of application ; it is the 

 setting up of obstacles which will break the force of the wind, and 

 prevent scouring of the loose sand, while causing a deposit of that 

 which is blown along, or near, the surface, by the wind. In regard to 

 this, the effect of an obstacle varies very largely with its nature. A 

 solid obstacle, such as a wall or plank fence, causes a certain check 

 in the wind currents and sand is deposited in front of the wall, but 

 not in its immediate vicinity. Here a strong eddy is formed and not 

 only is no sand deposited, but there may even be scour, as indicated 



WZZ&F 



Fio. 8. — Diagram to illustrate the effect of a solid impermeable obstacle. 



in fig. 8, and it is only when the heap of sand in front of the wall 

 gradually rises to a greater height than it that this hollow fills up, 

 and the wall is completely buried in sand. 



1 Handbuch des deutschen Diinenbaues, by Paul Gerhardt, Johannes Abromeit, 

 Paul Bock, and Alfred Jentzsch : Published by order of the Prussian Ministry of 

 Public Works : Berlin, 1900. 



( 21 ) 



