184 



NA TURE 



[December 25, 1902 



region, and from ninety-five measurements of these negatives he 

 has obtained the following values for the relative yearly motion 

 of the Nova in regard to the comparison stars here given : — 



Comparison Srar. 



« (B.D. +43730) 

 #(B.D. +43732) 



c (B.D. +43748) 

 a? (B.D. +43751) 



Herr Bergstrand has obtained +o" - 033 as the final value for 

 the absolute parallax of the Nova. 



Star with Probable Large Proper Motion. — In 

 making observations of Comet 1902 b, M. J. Pidoux, of 

 Geneva, has found the position of the star B.D. 

 - I°'3359, relative to the position of the star B.D. - i"'336o, 

 to be Ao= -os. '03 and AS = 10' 33"'6. In the catalogue for 

 1S55, these values are given as - 2S/4 and - I2'-I respectively, 

 thus showing — if the observations of M. Pidoux are confirmed 

 — that the star has a large proper motion (Astronomische 

 Nachrichten, No. 3834). 



Report ofthe Government Astronomer for Natal, 

 1901. — This report deals with all the meteorological data col- 

 lected during 1901 at the Government Observatory at Durban 

 and at the thirty subsidiary meteorological stations which are 

 scattered throughout the colony. 



The equipment of the observatory has undergone no change 

 during the year. 



The table giving the yearly rainfall shows that the amount of 

 rain which fell at Durban during 1901 was considerably above 

 the average, being more than double the quantity recorded 

 during 1900. 



Total Eclipse of the Moon, April 22, 1902. — Several 

 series of observations of this eclipse are recorded in this month's 

 Bulletin de la Socii'ti Astronomiqut tie France, and an excellent 

 coloured plate, showing the appearance of the moon at various 

 phases of the eclipse as seen by Dr. W. van der Gracht, of 

 Graz (Styria), accompanies the observations made by him. 



THE CREA T IRRIGA TION DAM A T 

 ASSUAN. 



""THE country of Egypt consisted principally in its natural state 

 of level, arid plains with a few patches of vegetation on 

 the higher parts. Its agricultural prosperity depends entirely 

 on the irrigation derived from the River Nile. It is many 

 thousands of years ago that the first attempt was made to 

 regulate this river and make it serviceable to mankind. In the 

 time of Menes, the west side of the river was embanked, and 

 the water led by a system of canals and embankments to the 

 land lying between the river and the Libyan mountains. When 

 the river was in flood, openings were cut in the banks and the 

 country converted into a series of lakes, the land being enriched 

 and rendered fertile by the warp brought down in suspension 

 by the turbid water of the river. When the floods subsided, 

 the water drained off and the openings made in the banks were 

 again filled up. 



This system remained in existence until after the English 

 occupation, when regulating sluices took the place of the more 

 primitive method of cutting and making good the banks. A great 

 depression on the Libyan side of the river was also, in the time of 

 the Pharaohs, converted into a vast regulating basin known as 

 Lake Maris which was reckoned one of the wonders of the 

 world. Afterwards the right side of the river was also 

 embanked, and the channel enlarged and regulated. 



To Joseph of scripture fame belongs the merit of having made 

 one of the principal canals used for irrigating the land, and after 

 the lapse of 4000 years the Bahr Usuf, or Joseph's waterway, 

 is still doing useful work. 



For records of further works of importance, it is necessary to 

 skip over a very long period to the time of Mehemet Ali, about 

 the year 1833, who, under the advice of French engineers, 

 caused to be constructed the great barrage above Cairo across 

 the Rosetta and Damietta branches of the Nile, and, by thus 

 holding up the water when plentiful, a very large area of land is 



NO. I73O, VOL. 67] 



irrigated and rendered highly fertile during the dry period. 

 When the difficulty and cost of obtaining the stone necessary for 

 this great work was pointed out to the Egyptian ruler, it is said 

 he at once gave orders for the destruction of so many of the 

 pyramids as would provide the necessary material, and these 

 monuments were only preserved by the engineers assuring the 

 Khedive that the cost of this would be greater than transporting 

 the stone from other places. Until the English occupation, this 

 barrage was more or less a failure, as, owing to defective 

 foundations, the water could not be held up sufficiently high to 

 make the irrigation effective as it otherwise would be. When 

 the English Irrigation Department obtained control over the 

 works, this defect was with great skill and difficulty remedied. 



It has long been recognised by the English irrigation 

 engineers that the present system of irrigation very imperfectly 

 makes use of the fertile properties of the Nile floods. The 

 most perfectly irrigated lands command a rent equal to 5/. an 

 acre ; imperfectly irrigated land is not worth more than I/, an 

 acre, while one-third of Egypt, or about two million acres, is yet 

 undeveloped. It is estimated that the rental value of Egypt may 

 be increased six millions a year by an effective system of irri- 

 gation. The great bulk of the land is dealt with by the original 

 plan of basin irrigation, where the water is carried on to the 

 land during the Nile floods and after remaining there for about 

 six weeks is drained off. The more effective and profitable plan 

 is where perennial irrigation is carried on, that is, where water 

 can be supplied, not only in times of flood, but in summer and 

 dry seasons. To effect this it becomes necessary to store up the 

 water in floods in impounding reservoirs and let it out as required 

 in the dry season. 



The great dam at Assuan, which was opened with much cere- 

 mony in the presence of the Duke and Duchess of Connaught 

 and the Khedive at the beginning of December, has been con- 

 structed for this purpose. This dam, erected across the Nile, will 

 hold up the water for a distance of 147 miles. 



For several years, the staff of the English Irrigation Depart- 

 ment was engaged in surveying the country in order to arrive at 

 the best site for the intended reservoir, and finally it was decided 

 that the first cataract at Assuan offered the most eligible 

 conditions for this purpose. A scheme designed by Mr. 

 Wilcocks, the chief of the Engineering Department, was 

 approved. This scheme was opposed because the Temple of 

 Philie would be submerged, and ultimately, in deference to the 

 objections of archaeologists and the foreign members of the Inter- 

 national Commission who had to be consulted, a compromise 

 was arrived at and the height of the dam was reduced, allowing 

 the temple to stand out above the level of the water. The dam 

 has, however, been so designed that at any future time the 

 additional height can be added to it so as to take full advantage 

 of the natural reservoir capacity. When this is done, ten 

 millions of rental will be added to the resources of the country 

 at a cost of about a quarter of a million a money. 



The great dam ,is a Cyclopiean work. It is a mile and a 

 quarter long, constructed of solid granite and cement, and is 

 founded on the natural bed of granite over which the river runs. 

 The height is S2 feet, and when full it will have a head of 

 65 feet of water against it. The base is 80 feet and the top 

 24 feet wide. It is pierced by 140 lower openings 23J feet high 

 by 6 feet wide, and 40 upper openings. These openings are pro- 

 vided by doors so hung and balanced that they can be lifted and 

 lowered with very little labour. Through these openings, the 

 Nile water will flow in floods and the scour will carry with it 

 the sediment that may have settled when the water is still. 

 As the flood waters decrease, the doors will be closed and the 

 water impounded and only allowed to escape in such quantities 

 as will be required for irrigation during the summer months. 

 For the navigation, a canal a mile long has been cut through 

 the rocks with a width of 50 feet, and a lock constructed having 

 a descent of 69 feet in four drops. 



For the further regulation of the water, another dam has been 

 constructed across the Nile 330 miles lower down, above the 

 entrance to thelbrahimeh Canal at Assiout, to control the irriga- 

 tion below this point. Here also a lock has been made of 

 sufficient size to allow the largest steamers using the Nile to 

 pass through. 



When this scheme of irrigation was ripe for commencement, 

 a question arose as to how the large sum of money required 

 for its execution were to be raised. The International Com- 

 mission charged with the finances of Egypt refused to allow a 

 charge to be made on the public debt, and without this per- 



