28 REPORT — 1851. 



arc we also of the relation of the central oceanic tides to these. The tides of the 

 Bahamas, the Azores, the Feroe Isles, the Cape Verde Isles, have not been studied, 

 nor connected with each other, nor with the littoral tides. A few notices of what is 

 called "the establishment," tell us little, and it may be, nothing, if we do not know 

 the manner in which these results have been obtained. The same is true of the 

 islands in the central regions of other wide oceans. 



It may appear at first sight that this representation of our ignorance is inconsistent 

 with the results formerly furnished as the amount of our knowledge, exhibited in 

 maps of cotidal lines. But to this, I reply, that the cotidal lines, so far as they are 

 drawn across wide oceans, are fallacious. It appears to be certain that the relation 

 of the tidal movements of a large ocean, such as the Atlantic, the Pacific, or the 

 Indian Ocean, are not justly represented by the notion of waves bounded by such 

 lines. With regard to the shores, the motion of the tides may be rightly represented 

 by such lines ; but the lines formerly drawn on the various shores above mentioned, 

 merely as the best first approximation which could be made, are still liable to great 

 doubt in almost every place, and therefore they require revision and correction, or 

 confirmation. And this is especially the case, now that these systems of cotidal 

 lines are no longer supported by the kind of connection which their continuity through 

 the oceanic spaces formerly gave them. By means of given connected observations 

 (the connesioh in the mode of making the observations is the essential condition of 

 success), the course of the littoral tide-wave on all the extensive shores of the ocean 

 might be determined, and at the islands ; and these results being obtained, the mo- 

 tion of the tidal movements of the ocean on the larger scale would probably be 

 brought into view. 



It may be proper to point out a little more in detail the kind of facts which would 

 have to be noticed. If we suppose a tide-wave, which is nearly straight, moving 

 transversely, to strike a convex shore, there will be some one point which it will 

 first touch, and it will proceed from this point along the shore both ways. This point 

 will be a point of divergence ; the tide will be later and later both ways in proceeding 

 from this point. Such a point is the S.W. point of Ireland, the Land's End in 

 England ; a point on the E. coast of Australia, in S. latitude 25° ; a point on the E. 

 coast of New Zealand, in latitude 37°. On the other hand, if the tide come from 

 two different quarters, as for instance if it come round the two ends of an island to 

 the side furthest from the tide, the two waves will approach and meet at some in- 

 termediate point of the coast, and there will be & point of convergence; and the tides 

 will be later and later in proceeding towards this point. Such a point there is on 

 the coast of England, about the mouth of the Thames, where the tide which comes 

 round by the North of Scotland and by the straits of Dover meet. Such a point 

 there is on the west coast of New Zealand. ^ 



Now in order to determine the general course of the tides, we ought to determine 

 these points of divergence and of convergence on every coast, and especially on every 

 extensive coast. But as yet I am not aware that any others are known besides those 

 •which I have mentioned. On the west coast of America I have placed a point of 

 divergence near Panama, as best representing the succession of the tides along the 

 coast ; but the form of the wave is not satisfactory. On the east coast of South Ame- 

 rica, as I have said, we have very high tides south of the river Plate, which seems as 

 if there were a point of convergence in that region ; but this requires to be more 

 fully proved : and the point of divergence seems to be on the coast of Brazil, but the 

 evidence is very confused and scanty. On the west coast of Africa, I am not able 

 at all to fix upon points of divergence and convergence ; and, in short, we know little 

 or nothing of the course of the tides on that coast. Still more is this the case with 

 regard to the Indian Ocean, although on the shores of the great islands which lie 

 there, there must be many points of divergence and convergence. 



And the connection of the tides of small islands in a larger ocean with those of 

 the shore is so obscure hitherto, that I do not now venture to express it in any definite 

 manner till we have a better collection of facts. In my paper on the tides of the 

 Pacific (Art. 12.), I have suggested a mode in which this connection may perhaps 

 be expressed. 



