passage of the same through the atmosphere, and thus produce an aberration which, in so far as the earth's motion was concerned, would be the opposite to that which actually exists. As to aberration being, both in mode and measure, what it ought to be, if the earth had no atmosphere, Prof. A. suggested that the explanation of this was to be found in the enormous porosity of the atmosphere; by far the greater portion of the rays, so passing through as to escape the mechanical action of the molecules. When, however, the quantity of atmosphere to be traversed was so great that light must be nearly absorbed, some sensible portion of it might be subject to the influence in question. Prof. A. then referred to the phenomenon of a blue band, seen by himself and others, bordering that edge of the earth's shadow into which the moon entered at the time of the last lunar eclipse, but which was less distinct on the side at which the moon emerged; and showed that these phenomena were consistent with the supposition of such an impulse, accompanied by the dragging of the shadow.

Lastly, Prof. A. suggested that these considerations might have a bearing upon the question of a systematic aberration of the double stars; for if the nature of the envelope of a star were such that its mechanical impulse could be communicated to the light of the star, the theory of emission, with reference to such a star, would be quasi true, and the aberration admissible.

The Section then adjourned to meet at 10, A. M., to-morrow.
B. SILLIMAN, JR., Sec'ry.

September 22.

SECTION OF NATURAL HISTORY, GEOLOGY, &c. Second Meeting.

The Section met and was organized-Dr. SAMUEL GEO. MORTON in the chair.

ON THE FORCES IN NATURE WHICH RUPTURE, CONTORT, UPHEAVE, AND DEPRESS THE SUPERFICIAL STRATA OF THE EARTH. BY PROF. L. J. GERMAIN.

Mr. G. remarked, that it was familiar to every one, that the planes of the terrestrial equator and the plane of the ecliptic, form an angle

of about 23° 28'. There are two motions of the earth, aside from its perturbations, namely, its diurnal revolution around its own axis, and its annual revolution around the sun-each of which revolutions tends to flatten the earth in the direction of their resultant force, in a plane whose direction is intermediate between the direction of the two, operating on the crust of the earth most effectively within the limits of the torrid zone. It was this resultant centrifugal force to which Mr. G. called particular attention, as the substance of his theory of flattening, upheavals, and depressions.

Prof. AGASSIZ favoured the Section with his Observations

ON THE PHONETIC APPARATUS OF THE CRICKET.

The Professor gave an account of the structure of the Phonetic Apparatus of insects of the order Orthoptera, particularly the crick

ets.

He observed that he had been led to these investigations by some interesting remarks upon the wings of the grasshopper, in the Transactions of the Academy of Natural Sciences of this city. The wings of the grasshopper have elastic springs, by which they are folded, when in a state of repose. Prof. A. proceeded to demonstrate the structure of these wings on the black-board. It had occurred to him that the same structure might be found in other insects also, and upon investigation, he had discovered this to be the case.

The same general formation is common to all insects of the order of Orthoptera, and is a distinguishing feature of all the varieties of that large and interesting group. It is (remarked Prof. A.) by the action and contact of these springs, that these insects produce their music.

Prof. HALDEMAN exhibited a series of specimens of Phacops Hausmanii. (Calymene Micrurus. Green).

The head and tail of this species are the only portions hitherto discovered; but one of the specimens, now presented, contains the body which has eleven segments.

Prof. HALL added some remarks on the same subject.

Prof. AGASSIZ now gave his views―

ON THE COMPARISON OF ALPINE AND NORTHERN VEGETATION.

His remarks referred principally to the general observations of Humboldt; but many facts observed on the North American continent

F

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 is 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, conformable 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