t 13 TIDE TABLE, &C. 



Universal tide 2. In the secotid place I was abked for the easiest mode of 

 ^' fiiiding the time of high wqter, with sufficient accuracy for 



common purposes. I have made a table, which, I believe, 

 . is- tolerably correct; but not being so conversant in the sub- 

 ject as I could wish, 1 shoiild be much obliged to any of 

 your correspondents skilled in navigation, if they would 

 compare it with the best observations, and inform me whe- 

 ther they approve of the principle. 

 Waves break 3. Xhe third inquiry related to the cause of the breaking 

 the lip )er part ^^ a wave into surf. Waves seldom break at sea, unless 

 moving fa-'.er the wind is very high : but when they approach the shore, 

 e ower, ^^^^^ always break sooner or later. The general reason of 

 their breaking appears to be the excess of the velocity with 

 which the upper part of the wave advances above that of the 

 lower part: and this may be derived either from the effect of 

 the wind on the upper part, or from the resistance of the 

 bottom to the motion of the lower part, or from a third 

 cause, which is more general, where the magnitude of the 

 wave is at all considerable in comparison with the depth of 

 the fluid; for in this case the upper part of the wave must 

 have a natural tendency to advance more rapidly than the 

 lower, on account of the greater depth which determines its 

 velocity. Beside this, the form of the wave itself, where 

 the water is shallow, may be such as to render it incapable 

 of advancing without a change of the direction of its ante- 

 rior surface into a situation more nearly vertical, 

 Defect in the ^^ *^^ calculations by which the velocity of waves has 

 calculation of been determined, it has been usual to neglect not only the 

 wlfvw!'''''^^ ^"^ difference of the whole depth of the fluid at different parts 

 of the wavers surface, but also the immediate effect of the 

 w horizontal motion of the particles, so far as it is not con- 



cerned in producing an elevation or depression by its varia- 

 . tions. The theory, abstracted from these considerations, is 

 ^ perfectly correct ; and may be combined with their results 



so as to be rendered applicable to some cases, which are not 

 otherwise comprehended by it. Thus if we suppose a wave, 

 terminated by two planes, equally inclined, to be placed in 

 a surface on which it can move without resistance, it may be 

 shown, that the highest point will begin to be flattened with 

 the velocity deducible from the depth at that point, the new 



angular 



