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CHAPTER XXXVIII.

TIDES, MAGNETISM, ICE.

I. TIDES OF THE PACIFIC.

EXCEPT on the surrounding shores, where they exhibit similar phenomena and magnitude to other parts of the world, the tides of the Pacific are insignificant, and almost unnoticeable to the mariner. In the tables at the commencement of the first part of the present work, we have given the elements of the tides necessary to navigation; that is, the hour of high water, and the rise and fall of the tide, on the coasts of America, Asia, &c. But in the vast space between these two boundaries the tidal wave is scarcely appreciable, except by refined observation, and can form but a small portion of the actuating consideration in navigation. Under these circumstances we deem it unnecessary to enter into the general laws of the tides as founded by the illustrious Newton, or the interesting features elicited by the discussion of the Rev. Professor Whewell and Sir John Lubbock. They can be referred to elsewhere, as in our Atlantic Memoir, 1845, p. 146, et seq. And for those details, as applied to special localities, the reader will find them in the tables referred to, or in the pages of the work to which they appertain. The present consideration, therefore, will be confined to the general view of the Pacific tides, as recently set forth by Dr. Whewell, to whom the main features of the tidal laws, as they are now known, is mainly owing.

THE REV. W. WHEWELL ON THE TIDES OF THE PACIFIC.*

I shall not attempt to determine the general course of the tides in the Pacific, but will remark that the view now given of the distribution of the tides in an ocean explains several of the features of the Pacific tides, which were before very perplexing. If we suppose an ocean tide, from the borders of which proceed tides having their progress marked by cotidal lines, we can easily draw the lines so as to include the following facts and observations:

1. The easterly motion of the tide wave around Cape Horn, which is established by Capt. King's observations, and which is difficult to reconcile with the supposition of a tide revolving from West to East round the South pole. This is explained by its being a tide proceeding from an oceanic tide.

2. The tide being at nearly the same hour along a large portion of the coast of South America, namely, from the Strait of Magalhaens for 20° or 30° northward. This shows that the cotidal line is nearly parallel with the shore.

3. The very small tides, or no tides, at the islands in the centre of the Pacific, Tahiti, and the Sandwich Islands. These belong to a central portion of the ocean, where the rise and fall of the surface nearly vanishes.

Extracts from a paper in the Philosophical Transactions, 1848, pp. 1–28.

There are two sources of inaccuracy in tide observations, namely, the want of a clear understanding as to the thing to be observed, and the irregularity and complexity of the facts themselves. With regard to the former point, I hope that several misapprehensions, formerly prevalent among navigators, are now no longer common; such as confounding the time of high water with the time of the turn of the tide stream. But there is probably still some unnecessary difficulty produced by regarding, as a cardinal point in observation, the "establishment," as vulgarly understood, namely, the hour of high water on the day of new or full moon; for, in fact, the hour of high water on this day is of no more importance than the hour of high water on any other day, except in so far as it gives the means of knowing the hour on other days. And it does not afford the means of doing this any more than the hour of high water for any other given age of the moon does. For just as much inaccuracy as, from whatever cause, there is in deducing the time of high water at all ages of the moon from the time at a given age, just as much inaccuracy is there, from the same causes, in deducing the time of high water for all ages of the moon, from the time for full or new moon. And if the time at which the tide follows the moon on two or three successive occasions be greatly and irregularly different, the observations are equally of little value, whether any of the observed tides fall on the day of the new or full moon, or do not. If the tides are regular, and the observations good, the common "establishment" may be obtained from the observations of any one day; although, to give much value to this deduction, the tides should be observed for a fortnight. And if such observations be made for a number of very distant places, the common "establishment" does not represent a corresponding fact at different places. In some places it means the time of high water one day after the highest tide; in some, the tide two days after the highest tide; in some three days; for the "age of the tide" is different at different places, and the tide which corresponds to the new or full moon comes after the new or full moon by one, two, or three days. Hence, in order that we might compare the tides of distant places by means of a fact which had the same meaning in all of them, I proposed, in a former essay, instead of taking this common establishment, to take what I then called the corrected establishment, namely, the mean of all the lunitidal intervals, that is, of the intervals by which the tide follows the moon's transit. In general the corrected establishment is about thirty minutes less than the common establishment. It has been used by Admiral Lütke, in his discussion of the tides of the Pacific. As the common establishment is still the one familiar to navigators, and as no material error will result from the use of it, I shall make it the basis of my remarks on the tides of the Pacific. It may be useful to bear in mind what I have said, that this establishment may be deduced from observations not made at the new or full moon.* I shall now proceed to give the tide hours for the coasts of the Pacific, according to the best accounts which I find, judging them in the manner I have described. After noticing the course of the tide near Cape Horn, I shall follow it along the whole western coast of America, till, in the North, we

⚫ I have here said that in cases where the tides follow the common laws we may deduce the time of high water on one day from the time on another: I might have said the same thing of the heights.

reach the Aleutian Isles; and then, following this chain of islands, to the shores of Kamtschatka. I shall then consider the islands in the central parts of the Pacific, and proceed from them westward, according to my materials.

I have, in my first essay, shown that round Cape Horn the tide wave has an easterly motion. Thus, as I have there said, according to Capt. King, * at Cape Pillar it is high water on the day of full and change; at York Minster, 5° lon. to the East, it is at 3; at Cape Horn, 3° farther East, it is at 31"; in Good Success Bay, in Strait Le Maire, the hour is 4; on the East side of Strait Le Maire it is 5h. It appears also from Capt. King's observations (p. 17), that the tide travels in the same direction along the coast, that is, to the northward, on the eastern shore of Patagonia. This direction appears, by Capt. FitzRoy's Tables, † to continue as far northward as lat. 40°, the wave employing about twelve hours in travelling from lat. 50° to 40° S. Along this coast the tides are very large; at Gallegos River, in lat. 52°, they rise 46 feet. This circumstance might lead us to imagine that they are the result of accumulated waves converging from the North as well as the South; and this is probably the case. Yet it is remarkable, especially when considered in connexion with this view, that in the great estuary of the Plata there is no perceptible tide.‡

I shall not, however, dwell at present upon the tides of the Atlantic, and shall proceed to those of the western coast of America.

From this point the coast turns westward, and the stations are arranged according to longitude, without regard to their latitude.

9 52

......

6

10 52

......

10 33

6

42

11 41

7 52

1 0

9 16

:6:00

8

1 30

...

12 30

10 40

8228

Looking at the general assemblage of the numbers which occur in the column marked "Greenwich time," it is evident that the tide wave of the hour 8, which is at Cocos Island and the Galapagos about eight o'clock, comes to the continent at Nicoya and Realejo, about 10° and 12° N. lat., at about three-quarters of an

hour later; while the tide is at hours later than this, both to the northward and the southward. Proceeding first southward, we find the line of XI. not far from Callao, that of II. near Coquimbo or Valparaiso; and that of III near Valdivia; and farther South we have the line of V. at Chiloe, and of VI. at Cape Pillar; whence the wave moves to the eastward, round Cape Horn, as already stated. Considering these points as fixed, it is easy to interpolate the other cotidal lines along this coast. The observed hour at Guayaquil is later than its position would give, a result which we should expect, since the tide will occupy some time in travelling up the gulf in which Guayaquil is situated.

Again, proceeding from Nicoya and Realejo, to the northward, we find a like progression of tide hours. The line X. is not far from Acapulco, according to the data here collected. But the tide at Acapulco is small, and hence the accuracy of the result is doubtful. Perhaps the smallness of the tide is an indication that the point of divergence of the tide wave, which occurs on this part of the American coast, is not far from Acapulco. It appears that the line of III. passes near San Blas, and also near the Bay of S. Magdalena, on the coast of California. At Mazatlan, somewhat within the Gulf of California, the time is an hour or two later, as we should expect. When we reach Monterey and San Francisco, the hour is about 6, according to Capt. Beechey's observations. The more recent ones are too anomalous to proceed upon. At Port Bodega, in lat. 38°, we have the VIII. tide line; and at Nootka Sound, Cook's observatory, which give 12h 30' (whence Greenwich IX. nearly), are confirmed by Capt. Kellett's observations in the Straits of De Fuca, South of Vancouver's Island. The next point is the Russian settlement, New Archangel, in the Island of Sitka, where the tides exhibit very curious features, as I have already stated from the observations of Admiral Lütke,* and, as I find, further confirmed by the observations of Sir Edward Belcher. The line belonging to Sitka appears to be IX.

From this point we depend upon Russian observations, which are given by Admiral Lütke in his "Notice." These enable us to see that the cotidal lines bend, as usual, deep into the head of the bay in which is Cook's River (Inlet), in lat. 60°. The coast here trends to the West, and the wave follows it, and pursues its course along the chain of the Aleutian Islands, where it is traced by Admiral Lütke and the navigators of the Russo-American Company. It appears that the lines of XI., XII., I., II., fall near this chain, and that the line of V. is near the coast of Kamtschatka. It is not difficult to arrange the cotidal lines so as to conform to these data.

Admiral Lütke has observed the tides at other places on the Asiatic coast, as far North as 65°, but I shall not attempt to arrange them.

Our next attempt must be to arrange the tides of the oceanic isles of the Pacific, taking, in the first place, those South of the Equator.

See Philosophical Transactions, 1840, part i. p. 107.