IJ PHENOMENA AT SEA. 



per quantity of line out, when the log is lifted on the top 

 of a wave, marking this time, and measuring the interval 

 from it to the time the ship's stern iS lifted up by the same 

 wave, by a watch with a second hand. The length of the 

 line from the stern, compared with the interval of time, 

 will give the excess of the velocity of the wave over that 

 pf the ship, and these added together will be the velocity 

 of the wave. The velocity of the swell may also be ob- 

 tained when calm, by sending a boat in the direction of 

 the swell, to a moderate distance from the ship, having 

 a line fast to each, to enable their distance to be mea- 

 sured. With a watch, measure the interval to the nearest 

 second, when the boat was lifted up, by a swell, to the 

 ship being lifted by it ; compare the interval of time 

 with the distance measured by the line to obtain the velo- 

 city of the swell. Several observations may be made in 

 either case, and the mean taken as the result. 

 Less in shallow The velocity of the waves seems generally less in shal- 

 water. j^^ water, than in the ocean ; the cause of this may be 



the resistance the particles of water meet with from mud 

 or sand mixed in the water, or from friction against the 

 ground. 

 Various swells I" the ocean it frequently happens that two swells run 

 at same time, in directions opposing each other ; at other tinaes they 

 cross each other obliquely : and sometimes three swells 

 running in different directions, meet and run through each 

 other ; and continue to c]o so for a day or longer time, 

 each retaining its own direction and apparent regular ve- 

 locity. 

 The waves in It frequently happens during a ty-fong in the China 

 the ty-fong. gg^^ that the wavQS run in every direction ; having the 

 appearance of elevated mounts or pyramids^ which in- 

 fringe on each other with great violence. Ships are very 

 liable to lose their rudders, when these pyramids strike 

 against them ; and the masts are endangered by the quick 

 turbulent motion proceeding from such heterogeneous 

 impulse. 

 Causeswhythe You remark that the swell caused by a storm, may be 

 swell of the sea propagated with a greater mean velocity than the storm 

 ^^^v^ dTh t *^^^ causes it, and may therefore arrive on a coast before 

 caused it, it, or come after the stocxn has ceased— this conclusion 



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