270 On the Translation of Waves of Water. [No. 3, 



14. But the speed or force of the river stream varies considerably 



at different times of the year. In the month of March, or the dry 

 season, its rate off Calcutta does not exceed half a mile per hour ; 

 whereas in the month of August, or the height of the freshets it 

 may amount to three miles per hour. Now, if the river stream doe s 

 sensibly retard the passage of the wave itself, it is evident that its 

 effect is far more potent when it is large, and we have thus a direct 

 experiment of variation afforded us to discover if this be the case. 



15. When the time of the lunar transit is Oh. Om. or when the 

 sun and moon are in conjunction ; the crest of the tidal wave passes 

 the floating light vessel, which is 119 miles below Calcutta, at 

 9h. Om. throughout the year. This is not critically correct but suf- 

 ficiently so for the purpose. In the month of March the same wave 

 reaches Calcutta, at 2h. 35m. ; and in the month of August at 

 2h. 10m. by which it would appear that it actually takes less time 

 by twenty-five minutes to travel to Calcutta during the height of the 

 freshets, than it does in the dry season, and this, although the up- 

 ward cm-rent of the particles of which the wave is composed, is 

 entirely neutralised by the increased rush of the river stream. 



16. The fact is, that the speed of the wave depends almost entirely 

 upon two other contingencies, viz. the depth of the water, and the 

 mass of the generated wave. 



17. When the depth of the water is greater than the length of 

 the wave, the rate of translation depends entirely upon the mass of 

 the wave, and is proportional to the square root of its length. 



18. When the depth of the water is small and the wave very 

 great, as in the tidal wave in rivers and those analogous to it, the 

 velocity of translation depends solely upon depth of the stream, and 

 is proportional to the square root of the depth. 



19. From what has been above stated, the cause of the superior 

 velocity of the tidal wave up the Hooghly in August is apparent ; 

 and that, if the river stream exerts any sensible retarding effect what- 

 ever, the increased natural velocity of the wave, through the river 

 being surcharged with water, is sufficient to neutralise it altogether. 



20. The rapidity therefore of a river stream or cm-rent is no crite- 

 rion whatever of the rate of translation of a wave upon it ; and such 

 a supposition may lead to very erroneous conclusions, as the speed of 



