L. MECHANICS AND ENGINEERING. 509 



which have valves on their upper and lower sides : the up- 

 per valves being for the admission of air, and connected with 

 a valve house; the lower valves for the admission of water. 

 On the two tubes which are placed on the outside are flanges, 

 which are placed on the upper side so that vertical tubes 

 can be bolted to them. On these vertical tubes is placed a 

 tramway, together with the houses containing the air pumps 

 and air valves. The operation of the whole is as follows : 

 The dock being sufficiently submerged, the ship is floated 

 over it. The water valves are then opened, and the air 

 valves closed ; the air pumps are started and air forced into 

 the tubes, thus displacing the water and lifting the vessel. 

 The water valves are then closed, and the dock floats with- 

 out any reference to the air valves. In order to submerge 

 the dock, the lower or water valves are opened, the air es- 

 caping by the upper air valves at the valve house until the 

 dock is sufficiently lowered. The advantages of this dock 

 are the cheapness with which it can be built, the great 

 strength attained by the use of cylindrical tubes, and the 

 great facility with which it can be moved from one place 

 to another, as occasion may require. 



THE PROPOSED FLOODING OF THE SAHARA DESERT. 



A report lias recently been made by Captain Roudaire, 

 charged by the French government with the investigation 

 of the possibility of converting the interior of Algeria into a 

 sea, to which enterprise frequent reference has been already 

 made in our previous Records. An appropriation of $2500 

 was made by the National Assembly of France in 1874 for 

 the special inquiry, and a commission was selected, of which 

 Roudaire was placed in charge. The special object was to 

 determine carefully, by leveling, the area that it was thought 

 could be flooded by cutting away the barriers and introduc- 

 1112: the water of the Mediterranean. 



CD 



The party left Biskra on the 22d of December, and in a 

 few days reached the station of Chegga, the altitude of which 

 had already been determined in 1873. The work was prose- 

 cuted with great care, and continued until April, 1875, at 

 which time the party returned to its starting - point, after 

 traversing a distance of 650 kilometers. 



The general result of the investigation went to show that 



