THE TIDES. 



217 



tion with respect to the sun, and the declination of the two bodies, are in many 

 cases altogether obliterated by the effects of the disturbing influences, or can 

 only be detected by the calculation and comparison of long series of observa- 

 tions. 



By reason of these disturbing causes, it becomes a matter of great difficulty to 

 trace the propagation of the tide wave, and the connexion of the tides in different 

 parts of the world. In the Philosophical Transactions for 1832, Sir John Lub- 

 bock published a map of the world, in which he inserted the times of high 

 water at new and full moon at a great number of places on the globe, collected 

 from various sources, as works on navigation, voyages, sailing directions, &c., 

 and in order that the march of the tide wave might be 'traced more readily, the 

 times were expressed in Greenwich time, as well as the time of the place. In 

 the same Transactions for 1833, Mr. Whewell prosecuted this subject at 

 greater length, and availing himself of a-priori considerations, as well as of a 

 mass of information collected in the hydrographer's office at the admiralty, in- 

 serted in the map a series of cotidal lines, or lines along which high water 

 takes place at the same instant of time. But these cotidal lines, as Sir John 

 Lubbock remarks, are entirely hypothetical ; for we have few opportunities of 

 determining the time of high water at a distance from the coast, though this is 

 sometimes possible by means of a solitary island, such as St. Helena. Lub- 

 bocft's Elementary Treatise on the Tides, 1839. 



According to Mr. Whewell's deduction, the general progress of the great 

 tide wave may be thus described ; it is only in the Southern ocean, between 

 the latitudes of 30 and 70, that a zone of water exists of sufficient extent to 

 allow of the tide-wave being formed. Suppose, then, a line of contemporary 

 tides, or cotidal line, to be formed in the Indian ocean, as the theory supposes, 

 that is to say, in the direction of the meridian, and at a certain distance to the 

 eastward of the meridian in which the moon is. As this tide-wave passes the 

 Cape of Good Hope, it sends off a derivative undulation, which advances 

 northward up the Atlantic ocean, preserving always a certain proportion of its 

 original magnitude and velocity. In travelling along this ocean the wave assumes 

 a curved form, the convex part keeping near the middle of the ocean, and ahead 

 of the branches, which, owing to the shallower waters, lag behind on the Amer- 

 ican and African coasts, so that the cotidal lines have always a tendency to make 

 very oblique angles with the shore, and, in fact, run parallel to it for great dis- 

 tances. The main tide, Mr. Whewell conceives, after reaching the Orkneys, 

 will move forward in the sea bounded by the shores of Norway and Sibe- 

 ria on one side and those of Greenland and America on the other, will pass 

 the pole of the earth and finally end its course on the shores in the neighbor- 

 hood of Behring's straits. It may even propagate its influence through the 

 straits, and modify the tides of the North Pacific. But a branch tide is sent 

 off from this main tide into the German ocean ; and this, entering between 

 the Orkneys and the coast of Norway, brings the tide to the east coast of Eng- 

 land and to the coasts of Holland, Denmark, and Germany. Continuing its 

 course, part of it passes through the strait of Dover and meets in the British 

 channel the tide from the Atlantic, which arrives on the coast of Europe 

 twelve hours later ; but in passing along the English coast, another part of it 

 is reflected from the projecting land of Norfolk upon the north coast of Ger- 

 many, and again meets the tide ..wave on the shores of Denmark. Owing to 

 this interference of different tide-waves, the tides are almost entirely oblitera- 

 ted on the coast of Jutland, where their place is supplied by continual high 

 water. 



In the Pacific ocean the tides are very small; but there are not sufficient, 

 observations to determine the forms and progress of the cotidal lines. Off Cape 



