RESTORATION OF THE WATER. 451 



water thus withdrawn — except that which is absorbed by vegeta- 

 tion, that which enters into new inorganic compounds, and tliat 

 which is carried off by evaporation — is finally restored to the 

 original current by superficial fiow or by infiltration. It is gen- 

 erally estimated that from one-third to one-half of the water ap- 

 plied to the fields is absorbed by the earth, and this, with the 

 deductions just given, is returned to the river by du-ect infiltra- 

 tion, or descends through invisible channels to moisten lower 

 grounds, and thence in part escapes again into the bed of the 

 river, by similar conduits, or in the form of springs and rivulets. 

 Interesting observations have lately been made on this subject 

 in France and important practical results arrived at. It was 

 maintained that mountain irrigation is not ultimately injurious to 

 that of the plains below, because lands Hberally watered in the 

 spring, when the supply is abundant, act as reservoirs, storing 

 up by absorption water which afterwards filters down to lower 

 grounds or escapes into the channel of the river and keeps up its 

 current in the dry summer months, so as to compensate for what, 

 during those months, is withdrawn from it for upland irrigation. 

 Careful investigation showed that though this proposition is not 

 universally true, it is so in many cases, and there can be no doubt 

 that the loss in the volume of rivers by the abstraction of water 

 for irrigation is very considerably less than the measure of the 

 quantity withdrawn.* 



* See ViGAN, Etude sur Us Irrigations, Paris, 1867 ; and Scott Monckieff, 

 Irrigation in Southern Europe, pp. 89, 90. 



The brook Ain Musa, which runs through the ruined city of Petra and 

 finally disappears in the sands of Wadi el Araba, is a considerable stream in 

 winter, and the inhabitants of that town were obliged to excavate a tunnel 

 through the rock near the right bank, just above the upper entrance of the 

 narrow Sik, to discharge a part of its swollen current. The sagacity of Dr. 

 Robinson detected the necessity of this measure, though the tunnel, the mouth 

 of which was hidden by brushwood, was not discovered till some time after 

 his visit. I even noticed, near the arch that crosses the Sik, unequivocal 

 remains of a sluice by which the water was diverted to the tunnel. Immense 

 labor was also expended in widening the natural channel at several points be- 

 low the town, to prevent the damming up and setting back of the water — a 

 fact, I believe, not hitherto noticed by travellers. 



The Fellahheen above Petra still employ the waters of Ain Musa for irriga- 

 tion, and in summer the superficial current is wholly diverted from its natural 

 channel for that purpose. At this season the bed of the brook, which is com 



