476 POPULAR LECTURES AND ADDRESSES. 



speed, IO'48 miles an hour, the force required to 

 drag it increases to 268| Ibs. This illustrates that 

 water is not a perfect fluid. It also illustrates the 

 theoretical result in a beautiful and interesting 

 way. If water were a perfect fluid, the forces at 

 the lower speeds would be somewhat less than he 

 has given, perhaps not very much less : at all 

 speeds above 8 miles the force would be nothing ; 

 the boat once started, the motion would go on for 

 ever. On the same canal another boat, weighing 

 12,579 Ibs. (nearly 6 tons), gave these still more 

 remarkable results : at 6*19 miles an hour 250 Ibs. ; 

 at 7'57 miles an hour 500 Ibs. ; at 8^52 miles an 

 hour 400 Ibs. ; and at 9^04 miles an hour only 

 280 Ibs. That is a striking confirmation of the 

 result of the previous observations. Scott Russell 

 says also : " I have seen a vessel in 5 feet water, 

 and drawing only 2 feet, take the ground in the 

 hollow of a wave having a velocity of about 8 

 miles an hour, whereas at 9 miles an hour the keel 

 was not within 4 feet of the bottom." Again he 

 says : " Two or three years ago, it happened that a 

 large canal in England was closed against general 



