were added in confirmation of the general principle. Prof. A. detailed the limits and appropriate regions of various trees of our own country. He remarked that they mostly differ from European species, although analogous in general character. All the walnuts, he remarked, are distinct from European varieties, yet usually resemble them.

DR. DICKESON now read an extended report on "THE SEDIMENT OF THE MISSISSIPPI RIVER," prepared by ANDREW BROWN, Esq. and Dr. M. W. DICKESON, which is here published in full.

To the American Association for the Advancement of Science. GENTLEMEN, The undersigned Committee, charged with the duty of collecting and reporting to this Association, as many of the facts and characteristics respecting the condition and annual conduct of the Mississippi river, as might be practicable, herein respectfully submit a series of facts and observations in connexion therewith, collected by virtue of the most favourable opportunities of daily observation for the last eighteen years, and continued without intermission with a view to this report, for the last two years of that time, beginning the 1st July, 1846, and ending 30th June, 1848; comprising a series of notations and calculations of the quantity of water at the several stages of elevation and depression, during the river's annual oscillations, between its mean high water and low water lines, together with the quantity of detrital or sedimentary matter with which it is charged, &c.

The quantity of water embraced in the respective calculations, being agreeable to the mean of time and velocity for high and low water; and at every intermediate space of twelve inches, between the mean high water and the mean low water line, the several quantities calculated for the respective stages, constituting the aggregate of water passing the point of observation; hence, necessarily, the annual discharge from the valley of the Mississippi in the Gulf of Mexico.

Your Committee have deemed it highly essential to this report, that it should be accompanied by a sectional diagram of the river at the point of observation, accurately delineating the configuration of its bed and banks, the better to enable the Association to perceive the value of the data from which the respective calculations and quantities have been deduced.

In furtherance of such a view, a sectional diagram is herewith submitted, together with a book marked "A," in which is contained the specific quantities of square feet of water at the several stages of ele

vation, as per diagram, which constitutes the basis of calculations for the several times and quantities at the respective stages. The method for arriving at such facts will be better perceived on referring to the book, in which are noted all the important data for obtaining the accurate and aggregate quantity of water, discharged by the river for each twelve months; such being the mean of the above specified two years. Without such a detailed system of observations and calculations, as your Committee have thus adopted, they might have failed to find, with the same accuracy, the required quantities; for they were unable to perceive by what other method the facts could be more surely approximated; and should the tediousness of this report, together with the necessary inspection of the accompanying documents, be a tax on the patience of this Association, we flatter ourselves that a compensation will be found in the unquestionable authenticity of the facts submitted.

It will readily be perceived, that although this report embraces but one point in the whole of this long river, all the descending waters must necessarily pass each point on their way to the ocean ; consequently, the river at this point must be in corresponding depth and velocity, to enable the waters to effect such a passage.

The velocity assumed for the waters at the several stages of elevation, as per calculation in book " A," and which constitutes so important an element in these calculations, is not that of the central current, but is the mean of the lateral quantity obtained by many and repeated experiments and computations, which gave an amount very sensibly less than the central current, and which is variable under varying circumstances, but it may be observed, that while these sensible variations of current exist in the lateral expansion of the river, we have been altogether unable to detect any appreciable difference of current in the vertical quantity, and that, too, after having made many experiments with regard to that particular. What may be regarded as almost, if not altogether, conclusive evidence of that fact, is the circumstance that it is no very unusual thing for tall trees to float down the deepest part of the river, in a perfectly perpendicular attitude, caused by their butt ends being of greater specific gravity than water, while their tops or small ends are so buoyant as often to project many feet above the surface of the water. Such vertical trees are, at times, transported with the same velocity as the surface current where they exhibit themselves; and while such trees are thus floating in a perpendicular attitude, it often occurs that their lower ends are in such close proximity with the bottom, as to come in contact with its pro

tuberances, which throw them down at such angles as often to make their tops disappear below the surface, until they have got over the obstruction; and when such is the case they at once erect themselves as before. We have had many opportunities of witnessing the descent of vertical timber, and frequently when their coming in contact with the projecting parts of the bottom manifested itself; and it is otherwise well ascertained, that the bed of the river projects many protuberances above its general surface, some of which rise to and even project above extreme low water-as, for instance, that represented on the diagram—so that they very much endanger river navigation at the low water stages. They constitute obstructions, with which trees descending in a vertical attitude often come in contact, and by which they are so tossed and agitated, that no particular form of any tree could make its projecting part seem to be perpendicular, while its submerged portion was inclined, comformable with a current which was not uniform in its whole depth; for were the under current more sluggish than that on the top, the lower end of the tree would be correspondingly impeded in its progress, so as to give to the tree an inclined attitude; but the observed circumstance shows that there can be no such inclination, for when agitated by striking against the projecting parts of the bottom, they turn freely round in every direction, and present their several sides to the direction of the current, without any seeming preference as to position.

Our observations induce in us the conviction, that in a descending aqueous current, there is no appreciable difference of velocity throughout the vertical quantity; and unquestionably for the reason, that the superincumbent pressure urges forward the under stratum to the point of least resistance, with the same velocity that it may itself have acquired.

Governed by the foregoing considerations, and estimating quantities by the method adopted in book "A," your Committee have found the aggregate annual quantity of water discharged by the Mississippi river, to be 14,883,360,636,880 cubic feet, or 551,235,579,143 cubic yards. 101.1 cubic miles=101.

Now the fact being notorious, that the Mississippi river is the only visible outlet for all the surplus waters of that vast valley, through which it passes on its way to the ocean, there are involved considerations of no little importance; for the Mississippi valley has been found to contain an area very little, if any, short of fourteen hundred thousand square miles.

What then is the relative difference between the quantity of rain

water falling annually in this valley, and that discharged by the river?

We find by examination of the meteorological register of the late Dr. H. Tooley, of Natchez, kindly furnished by his family, that the mean quantity of rain water, which falls annually at Natchez, is between fifty-five and fifty-six inches; but, as such is the quantity for the southern extremity of the valley, it may be regarded as overestimating, if taken for an average of the whole area, we will therefore assume the mean quantity to be 52 inches, and then we will have for the whole valley, 169,128,960,000,000 cubic feet, which is about 11g or 11.3636 times the quantity which is discharged by the river.

8

There can be but two ways by which this immense quantity of water can make its escape from the valley; one is by the course of the river, and the other by evaporation. Hence, we perceive that there is but one relative portion of this quantity passing off by the river, for every 103 parts which are exhaled into the atmosphere, or parts are carried off by the river, and parts by evaporation. Thus we arrive at a fact of the most momentous importance to the planting interests of Louisiana and Mississippi; for it will be at once perceived, that the more exhalations are promoted, the less liable will the low or bottom lands of these two states be to the periodical inundations by the river.

91

If it be asked, by what process it is expected that evaporation can be promoted, over such an extensive area as the Mississippi valley, so as visibly and permanently to affect the planting interests of the above named States, the answer will be found in the fact, that the process has been, and is now, in the most rapid and successful progress, and of that kind which is the best calculated to produce so desirable a result, viz: the clearing of such large portions of the valley of its forests for the promotion of agriculture, and the consequent exposure of the lands to the action of the sun and winds, the very best promoters of the evaporating process, particularly on a large scale.

It will not be difficult to perceive the great difference there must necessarily be in the evaporation from a surface of country exposed to such action, and that which is covered by the primitive forests and their almost impervious undergrowth, through which neither sun nor wind can penetrate but with difficulty.

So rapid is the progress of this increased exposure and its consequent evaporating tendency, and so visible has been its effects on the Mississippi river, that we may hazard the assertion with safety,

that there is not now, by twenty or twenty-five per cent., as much water passes down the Mississippi as there was twenty-five years ago; for at and prior to that time there were annual inundations of many feet, and long periods of submergence of almost all the bottom lands, from the bluffs or highlands on one side of the river bottom to those on the other side. Such lands were at that period in a great measure accounted valueless : and to such a degree, that but little or no hopes were entertained of the practicability of their redemption by any artificial means, that is, on any general scale; but such has been the diminution in the annual quantity of water discharged from the valley, that those lands have been progressively and rapidly redeemed from overflow, until very great portions of them are now in the very highest state of cultivation, and with but comparatively slight assistance from art in the way of embankments, and these such as could not have been at all available against the overwhelming effects of floods, and the length of time of their continuance; for then there were annual inundations, both deep and expansive, of the waters, over almost all the bottom lands; but now the river seldom rises to the same elevation as formerly, and when it does, it is of much shorter duration, and the waters are almost exclusively confined to the channel of the river, in place of being spread over almost all the bottom lands the whole spring and early part of the summer.

These changes in the quantity of water discharged are so progressive, that they fail to excite general notice; so that the lands which are at one time considered to be of little or no value, are subsequently taken up, occupied and improved with success, without any consciousness that such an important change is in progress, the opinion being prevalent, that in nature there can be but little change, and that the annual quantity of water descending the river at one period must approximate very nearly to the quantity that descends at every other, there being no visible cause for suspecting it otherwise. All the advantages are progressively but rapidly extending themselves while the cause remains unsuspected or overlooked, but none the less sure. As a further evidence of the altered condition of this river, we may mention the circumstance, that in former times, and in the spring of the year particularly, steamboats ascending or descending the river were detained about half their time by dense fogs, while now hardly any such obstructions prevail; so that steampackets succeed in making their trips to an hour, with no fear of such retardation.

Assuming that the diminution of the waters will continue in some