1860.] and its transmission down the Indus. 277 



them to move at 27 miles an hour.* Waves may be made, as is well 

 known, to move niuch slower than this, if the pressure producing 

 them is less. The rapidity of the communication of pressure, and 

 therefore the velocity of translation of the wave, depends upon the 

 intensity of the cause producing the pressure. 



4. In order to apply these results to the phenomenon in question, 

 I suppose, instead of the plug pressing down the surface, a large 

 body of water to have fallen upon the surface of the Indus by the 

 bursting of the barrier, as represented in fig. 2. According to the 

 forcé with which this descending mass struck the river, would be 

 the velocity with which the front of the generated half-wave would 

 begin to move down the river. As the cataract poured down from 

 the broken barrier, its successive portions, after causing the pressure 

 by their impact and weight and so aiding in the generation of the 

 half-wave, would become themselves in turn part of the river, and so 

 part of the médium through which the pressure of the next falling 

 portion was transmitted, to continué the generation of the wave. 



* During a voyage from England by the Cape in 1838 I made the following 

 experiment with the assistance of the first and second oíficers. A day was 

 chosen when the swell was moving from ahead aft, and the ship was making only 

 about three knots. At one end of the log line a large bung was fastened, and 

 40 fathoms further up another large bung was tied on, the intermedíate forty 

 fathoms of line having a number of smaller corks attached to it to make it float. 

 The line thus furnished was thrown into the water astern, and more line allowed 

 to run off the reel till the bungs were well clear of the ship. The second ofRcer, 

 who held the reel, then checked the line from running out further : and the 

 40 fathoms of line between the two bungs were drawn out straight by the way 

 the ship made. As the wave which was to be observed approached the vessel 

 from ahead, at the word " let go" the line was allowed to run off the reel, and 

 the bungs, with the line between them stretched straight, instantly remained 

 stationary in the sea. The moment the wave lifted the first bung to its highest 

 point was marked by my giving a " now," and the moment the second bung was 

 raised to its highest point by the same wave a second " now." The first officer, 

 who had a chronometer in bis hand, marked the interval ; it was found to be 

 about 6 seconds. That is, the wave moved over 40 fathoms in 6 seconds, or 1 

 mile in 132 seconds, or about 27 miles in one hour. Each of us took the several 

 places in turn of reel-holder, time-keeper, and observer ; and the results were the 

 same. 



