278 Length and height of the Wave. [No. 3. 



The half- wave would be in the process of generation until the pent-up 

 waters were exhausted. 



Major Cunningham states in his work on Ladak, that the mass of 

 water which accumulated in 1841 and caused the inundation of the 

 Indus in that year, was estimated at 20,000,000,000 cubic feet. This 

 equals a volume 100 feet deep, 380 feet wide, and 100 miles long ! 

 If the flood of 1858 was only half of this or even much less, the 

 reservoir was large enough to generate a half-wave of enormous 

 length, and to produce a final free whole-wave much longer still. 



As the Indus varies in width and depth, this wave would undergo 

 various modifications as it passed down, especially as we must combine 

 with it the natural downward current of the river — probably as much 

 as from 7 to 10 miles or more at the season when the flood occurred. 

 Thus at Attock where the river is confined at its usual level to a 

 width of less than 800 feet by rocks there is no difficulty in assuming, 

 that the elevation of the water would be greater than in other parts 

 where the stream was wider. 



5. The state of the Indus at Attock in ordinary years is this. 

 The water is lowest in March. By the melting of the snow in May, 

 and by the rains after that, the surface at Attock has risen by August 

 through 50 feet above the lowest or winter level in March. The 

 facts of the phenomenon of 1858, as observed by the late Captain 

 Henderson at Attock (and recorded in the Journal for 1859, p. 199) 

 were these. In August the river was unusually low for that season 

 of the year, being only about 25 feet (instead of the usual 50 feet) 

 above the winter level. On the 10th August at 6 A. M. the water 

 began to rise, and in the first, second, third, and fourth hours rose 

 through 26,12,7,4 feet, and in the next three hours and a half through 

 6 feet, so as at 1^ p. m. to stand at 80 feet above the winter level. 

 After this, it began very slowly to subside and returned to its usual 

 level in about (say) 22J hours, making 30 hours for the whole rise 

 and fall of the water at Attock. The rise occupied one-fourth of 

 this time, and the fall three-fourths. This accords with the form 

 of the wave, the slope of which on the back is much longer than the 

 rise on the front, as explained in para. 2, and represented in fig. 3. 



6. The difficulties in the way of receiving this explanation arise 

 from the possible shallows and rapids and sudden bends in the river. 



