272 On the Translation of Waves of Water. ). 3. 



stream to those spots by the simple pressure of gravitation ; long- 

 before the particles x, b, <fcc. can reach them — (always conditionally 

 that D E, F (x, &c. are sufficiently long to float the wave, which con- 

 tingency will be treated of hereafter.) In other words the particles 

 x, b, move down the river with a speed proportional to the sine of 

 the angle of the inclination of its bed ; minus friction, and plus the 

 wave motion communicated to them ; but the wave crest is translated 

 directly according to the known ratio of gravitation, minus only the 

 retard of friction. 



2ndly. That we have evidence that the phenomenon presented all 

 the characteristics of a wave. We are told that in the morning the river 

 at Attockwas " unusually low." This was the preceding hollow, — then 

 that " the river at first came swelling up quite quietly but very 

 rapidly, not less for a little time, than a foot per minute." This was 

 the rise on the anterior slope, — then " this of course did not last very 

 long, for as the width, the depth and the velocity increased, so did the 

 discharge, &c." This was the onward motion of the particles at the 

 passage of the crest. — Then we hear of a more gradual fall on the 

 posterior slope of the wave, which seems to have been much flatter 

 then the anterior slope, as is always the case with large waves in 

 shallow water. These facts are apparently in accordance with the 

 wave laws, and inconsistent with any other theory. And it may be 

 replied, 



3rdly. That those who doubt may make the matter a direct subject 

 of experiment upon a small scale. 



23. From the nature of the phenomenon of the flooding of the 

 Indus ; and for the reasons above detailed, I have no hesitation in 

 considering it, when it passed Attock, as an immense wave, the mass 

 of which was very considerable in proportion to the depth of the 

 stream. I consider it therefore analogous to the tidal wave, and it 

 is, on that account, that I have treated so fully of waves of that class. 

 I have little hesitation in ascribing to the Indus wave a length of 

 one hundred miles when it passed Attock, but if it were only five 

 miles or as much as five hundred, the following results would not be 

 materially affected. 



24. Where the depth of the stream above Attock was not more 

 than twenty-five feet — the rate of translation of the wave should 



