The village of Tarifa, which gives its name to the plain at the depression of the central chain of mountains to which we have referred, is seated about 700 feet above the level of the sea; the plain itself is somewhat lower. The hills which bound the plain on the south and southeast, and divide it from the waters flowing south, have but little elevation; and a cut of some fifty or a hundred yards in length would alone be sufficient to enable some of the streams on the plain to run south by the Portillo of Tarifa. One of these hills, the Del Convento, in which the Rio Monetza takes its rise, and thence flows into the Chicapa at San Miguel, stands alone, and on two of its sides, particularly at the north, are clefts so sunken that a cut through them would be a work of extreme

ease.

The direct distance between Tarifa and the river Boccabarra, through which the lakes on the western side of the isthmus reach the Pacific, is about thirty-seven miles. On the southern declivity, the principal streams are the Rio Ostuta, and the Rio Chicapa, both of which empty themselves into the lakes, the latter about eighteen miles from the Bocabarra, and nineteen miles from Tarifa. All the streams on the northern declivity flow into the Huasacualco, and that river, after a very winding course, reaches the Gulf of Mexico, about 105 miles from Tarifa. One of its chief tributaries is the Malatengo, which flows into it on its left bank, about ninety direct miles from its mouth, or 160 miles following its circuitous bed. The Chicapa, gauged above San Miguel Chimalapa, by combining several rivulets flowing from the central chain, rolls a body of water estimated at about twenty-two cubic feet per second. The Ostuta is in volume five or six times larger, and, which is really remarkable, and indicates extreme thirstiness of soil, this volume of water diminishes as it lengthens its distance from its original source. The volume of water in the Huasacualco is great, and at the lower part of its course it would be navigable for ships of any size, were it not for the bar at its mouth. The depth of water upon this bar has been, at different times, ascertained to be, by Dampier, fourteen feet; by Cramer, eighteen feet; by Orbegozo, fourteen feet; by Robinson twenty feet; by Moro, twenty feet. These differences are explainable with great probability, by supposing the bar to be intersected by submarine canals, or clefts, and the soundings not to have been made in the

same one.

At about thirty-three miles from its mouth, near a place called La Horqueta (the Fork), the Huasacualco divides into two branches, the right, and chief of which, the Apotzongo, runs a course of twenty-five miles, and the other or left one, the Mistan, a course of about thirtythree miles-they encircle a large island called Tecamichapa. Below this island, the depth of water always exceeds twenty-two feet, and becomes sometimes thirty-seven or thirty-eight feet. But above the re-union of the two branches, there are shallows, which, it is thought, may be got rid of by damming the Mistan at La Horqueta. Above this latter point, the depth decreases; at some places it is found to be from twenty-five to thirty feet, but in general it is less than twenty-one,

and even sinks to seven or four. Two points only, however, may be regarded as difficult to overcome-the old Mal-paso, and the present Mal-paso; the first being between three and four miles below the confluence of the Melatengo with the Huasacualco, and the second between nine and ten miles below the first, at the confluence of the Rio Sarabria. At these two points, the rocky character of the river's bed would make its deepening difficult.

On the other side of the isthmus, the Boccabarra, which constitutes the pathway between the Pacific Ocean and the lakes, or lagoons, into which flow the Chicapa aud Ostuta, has a depth of about twenty-four feet; but its opening on the lakes is obstructed by a bar, where the depth of water does not exceed nine feet. Moro ascribes the original formation of this bar, in a great degree, to the fact that formerly the river Tehuantepec emptied itself into the lake, and is of opinion that if it were removed by artificial contrivances the result would be permanent. He thinks that this removal might be accomplished by giving a direction to much of the water of the Ostuta above towards feeding the canal, and thus diminishing the force of the current in the lake. After passing the bar the depth increases; but throughout the eighteen miles, or thereabouts, of distance from the bar to the mouth of the Chicapa, not more than nineteen feet of water are to be had, though modes of augmenting its depth could be readily found,

These minute details, translated and transferred from the French publication already mentioned, the accuracy of which I perceive no reason to doubt, are introduced here in order to show that the subject has not been lightly considered. They prove that the two principal obstacles to the creation of a ship channel across the isthmus are the bars at the respective mouths of the Huasacualco and Boccabarra. Now, the bar at

the mouth of the first of these rivers ceases to be an impediment as soon as its main submarine canal, over which twenty feet of water have been found, is fixed with accuracy and marked by lines of buoys; and the bar at the mouth of the other river, within the lake, is removable, either in the manner suggested by Moro, or upon the plan heretofore successfully pursued, under the direction of our corps of topographical engineers, with bars at the entrances of our rivers from the northern lakes. Then, by surmounting the inferior obstacles,-namely, the occasional shallowness of the Huasacualco, and of the lake, either by sinking the bottoms or raising the water, and the short dividing swell of land between the source of the Rio Monetza, running south into the Chicapa, and the source of any one of the streams running north into the Huasacualcoan uninterrupted water highway is effected. There does not, indeed, appear on the whole line of this route any difficulty which the present resources of our science and mechanical art may not vanquish.

We should all, perhaps, prefer a canal of large dimensions, fit to accommodate vessels of every size; and the expenditure for such a one, having reference to the route specially described, is estimated differently at from fifteen to twenty-five millions of dollars. These estimates are liberal, and are probably exaggerated by an imperfect knowledge of

the local aids in the presence and cheapness of material and labour which would be furnished. Still, the highest valuation is, in contemplation of the purpose and its incalculable results, far from being appalling. But it may be, that a canal much smaller, and, of course, less costly, would be attended by every substantial advantage; and, indeed, it is even easy to imagine that consequences very salutary and important might flow from leaving inducements to the construction of two free cities, one on the bank of the Huascualco, and the other on the banks of the Boccabarra, or the Pacific, as termini, to a railway.

It will appear, from a careful consideration of the commerce which may be expected to direct its course through this passage, that the number of vessels of heavy tonnage and deep draught would bear a small proportion only to those of light tonnage and shallow draught; and, in all probability, the great additional expense incident as well to the construction, as to the keeping up, of a canal on the larger scale, would not be repaid by corresponding advantages. The average burden of vessels from the United States to the ports on the Pacific, including the Sandwich Islands, to China, and on whaling voyages during the years 1845-46, was short of 400 tons; and I should doubt whether, if this junction were effected, the burden of nineteen-twentieths of the craft passing through it would exceed that engaged in our coasting navigation. A canal adequate to the perfect accommodation of the great influx from our own country might, therefore, exact an outlay of but ten millions; and trans-shipments of large cargoes, though always more or less inconvenient, could be indemnified by the rapidity of transit by steam power. It is hardly necessary to say that the American people, in the event of a speedy restoration of peace, will find themselves in circumstances of extraordinary prosperity, which will enable them to afford, out of the national treasury, to appropriate for five years five millions of dollars for the superior, or two millions for the inferior, order of canal.

Having given you this sufficiently precise description of the contemplated work, showing it to be practicable, and nothing repulsive in its probable cost, let me indulge in a few remarks of a more general character, or rather hints for you to meditate upon.

The chief objects to be attained are, a speedy communication between this country and the western coasts of North and South America, especially with our Territories of Oregon and California, an easy and quick access to China, the groups of the South Sea Archipelago, the Sandwich Islands, Russian settlements, and even before long I hope, the tempting and untouched treasures of magnificent Japan; and, finally, the facilitating and enlarging of that great source of wealth, as well as nursery of able seamen, the whale fishery.

Now, I cannot resist the impression that this junction of the two oceans at the Isthmus of Tehuantepec would Americanize this vast and augmenting portion of the commerce of the world. It would give to the people of the United States the overwhelming advantage of an abridgment, by fully one half, of geographical distances. Against the

There

merchants of Europe it would give ours two voyages to one. is scarcely a region in the limitless South Sea, with which a trade would be lucrative, that could not be reached by them in half the time that would be consumed by English, French, Spanish, Dutch, or Swedish navigators. "If," says Mr. Scarlett, "this scheme were realized, it has been calculated that the navigation from Philadelphia to Nootka Sound and the mouth of the Columbia river, which, by Cape Horn, is now 5000 leagues, would be reduced to 3000 only!" In fact, the reduction would be greater. But at this rate what would the reduction necessarily be as regards the navigation in that direction from New Orleans, Mobile, St. Augustine, Savannah, and our entire southern seaboard? The interchanges of commodities between our great and teeming valley of the West, and the rich and rising regions of the Pacific, would be accomplished almost at the mouth of the Mississippi.

Nor can securing, in this way, for our own country, the just benefits of her relative position be made the subject of complaint by other nations. We do not propose at least I hope not-to monopolize the uses of the canal; on the contrary, it would be thrown open on terms, if not of equality and freedom, of the utmost liberality. Although an artificial structure, reared by the money and policy of our people, I would give it the unchangeable character of a public highway of nations.

G. M. DALLAS.

REMARKS ON SURFS, ON ACCUMULATION ON WAVES, AND ON THE REVERBERATING ACTION OF OCEANIC WATER.

As your valuable Magazine combines such a host of knowledge, and useful information one can never be tired in reperusing articles that have come under their notice before. I for one am ever anxious to improve, I am again threading my way through your volumes, and must be pardoned for going so far back as 1837, in which will be found 441 an interesting paper on the surf at Nice.

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As the above are subjects of such vast importance and deep interest, and embrace points which have hitherto been little considered, they admit of some brief observations in reference to the note added to the article I have mentioned, supplying Mr. Webster's original theory of the rollers.

It is a point scarcely disputable that the equatorial current in its oblique course across the South Atlantic towards Cape St. Roque, the north-east angle of South America, receives the drift waters pressed thither by the south-east trade wind, and that the accession, although comparatively trifling, may serve to augment the volume, though it should add nought to the velocity of that great stream.

This oceanic river which

passes round the elbow of the continent, in conjunction with the alternating inshore currents preserves the level of the ocean there from an

undue elevation, by carrying off the superfluous supplies as fast as these are pressed onward by existing causes, distributing and returning the amount by circuitous routes.

This view is taken not alone from the disposition of the land, the ocean, the winds and the currents; but also from the consideration that, the latter generally are designed to fulfil certain purposes in the economy of nature-the effecting a constant interchange of waters, and the preventing accumulation and preserving a general level, and that, the operation is perfect.

Admitting, the principle of the theory, which is extremely reasonable, the re-action seems to be a consequence of the constant pressure upon the fluid, occasioned by the maximum strength of the trade wind during its greatest height and regularity, declining in the season when its minimum force succeeds to that, and its constancy becomes interrupted by calms and contrary winds.

That the waters occasionally flow back in a wide extended stream there is no disputing, but this action does not appear to be the effect of accumulated waters upon the coast from the trade wind drift re-acting, as will be shown, therefore the immediate cause of the periodic return of the rollers on the islands alluded to in the note, is, the reverberatory motion if the incumbent waters en masse, irresistible truly, but easy and rapid, and as an action probably unseen or unnoticed in the open ocear. unless meeting with sudden resistance;-in other words the return merely of the impressions created by the trade wind without necessarily any flowing or streaming; and if this and the perfection of currents be admitted what need have we to insist upon an accumulation of water on the western coast?

The spherical form of the moleculæ of the water, and the active principle by which these are constantly striving to acquire an equilibrium, whenever there may be undue force, would ensure the re-action as soon as the fluid became relieved from the unequal pressure; this re-action is inevitable, but it does not necessarily follow that there should be an accumulation of water for its fulfilment, upon a long line of coast open to the free ocean, whatever cause may press the surface waters towards that line, could not, I imagine, possibly create an undue accumulation of water there; the nature of the fluid and the activity incessantly exerted in relieving itself of the action of unequal pressure, would prevent it where there is no obstacle to impede its energetic impulse. Besides, in such an extent of ocean, as we are speaking of, the lateral pressure required to raise the mass of waters, independent of the moon's attraction, upon the coast, (if there were no intervening cause to prevent it,) of the western land, even only a few feet, would be tremendous, and far exceed ing any effect the wind is capable of producing in that quarter.

A great mass of water, such as the South Atlantic, acted upont by the constancy of a brisk wind for a given time, would swing back, as it

*It is not improbable that this takes place, annually in the wet season. † We do not here allude to accumulation, but to the undulatory impression which on its subsidence would impart a reverberatory action to the mass.