planter. Few rocks are visible on the surface, and gravel is rare; hence the streams have neither falls nor rapids.

.

Two lines, drawn from north to south, crossing the streams one at the distance of 50 miles from the eastern boundary of the State, and the other 60 miles farther west, will divide the upper part of Mississippi into three belts, which may be denominated the calcareous arenaceous and alluvial formations. The first embraces the Tombigbee valley and Prairies, including a small portion of the carboniferous series, in the N.E. corner of the State; the second, the hilly region, where sand-rock occasionally appears, and through which the water-courses, flowing west, suddenly turn south, and enter the third, or the Mississippi bottom.

1st Division.—A low ridge, striking off westward from the carboniferous hills, sweeps around the head of the Tombigbee valley, and turning south, bounds this valley on the west, and is the source of the streams flowing through the other divisions. To the east of the Tombigbee river the soil is red, and no rock is visible; nearer to the river, and partly on its west side, the soil is sandy and gravelly, and may be ascribed to the detritus of the carboniferous rocks in which the branches take their rise.

West of this, and reaching to the ridge just mentioned, is the Prairie region, of slight elevation, but sufficiently undulating for drainage, characterized by its rich, black soil, varying from 2 to 20 feet in depth. Commencing near the north limit of the State, and with an average width of 18 miles, it strikes south, about 200 miles, and then turns eastwardly into Alabama. It is underlaid by Rotten or Prairie limestone, which, near the town of Aberdeen, in N. Mississippi, has a thickness of 1 to 200 feet; but 12 miles further west, and in S. Alabama, attains a thickness of 900 feet or more. It appears to dip slightly towards the south. The limestone is white, soft and amorphous, and crumbles readily on exposure to the air; where it appears on the surface it constitutes a "bald Prairie," destitute of herbage. Springs are rarely found issuing from this rock; and wells dug down to it, merely collect the rain-water, which, by percolation through the black soil, has a strong and disagreeable taste. To obtain pure water, artesian wells are sunk through this rock to the subjacent sand, and the water then rises to within 40 or 50 feet of the surface. In similar wells, in S. Alabama, it rises even to 20 feet above the surface.

To the west of the Prairie region lies the ridge first mentioned, which, with its spurs, has an average width of 10 miles. It is com

posed of a red loam, 25 feet deep, lying upon sand of 30 feet thickness; and this, again, reposing upon a hard, light gray limestone. The rock contains about one quarter sand, burns readily to lime, and seems to be almost wholly composed of gryphæa. It also contains an abundance of echinus and spatangus. The region abounds with springs of pure water, issuing from the sand above the limestone. Spurs, running south-east from the ridge, diminish in height, and occasionally show the rotten limestone near the surface. Their uniformity of slope is remarkable. The streams run south-east, and, in crossing them from south to north the ascent, to the summits of the spurs, is very gradual, but then the descent is very abrupt to the next stream. The springs occur chiefly on this abrupt slope.

2d Division. Following the streams issuing from the red loam ridge, in their west course, the first strip, of some 8 miles width, is a kind of valley between the red ridge and the following sandstone ridges. Its soil is a grayish, sometimes sandy clay, and mostly covered with post-oak, and unproductive, excepting in some rich black bottoms. It seems to be on a level with the hard limestone, is destitute of springs; its streams are sluggish, and water can rarely be obtained by digging.

The next belt, of 10 miles width, consists of broken, intricate, sandy ridges, frequently capped by red sandstone, and covered with pine trees. Fresh water springs abound in the hollows; a feeble chalybeate is sometimes found, and, more rarely, a sulphur spring. Petrified wood and coal (lignite?) occur, the latter in thin strata.

The following belt, of 20 miles width, is arable, and characterized by long, level, dry sandy valleys, by numerous springs and free flowing streams of pure water. The soil is a light vegetable mould. The tops of the ridges, more or less level and fertile, are composed of a light coloured loam, of 10 to 20 feet depth, resting on a quicksand of unknown depth (over 100 feet).

The last belt has an ash-coloured soil, approaching that of the Mississippi bottom, with compact sand and some gravel, and a sandstone rock, showing itself here and there. Springs are scarce, and wells on the ridges are often sunk to the level of the Mississippi river. The ridges of this belt subside by steep descents of 80 or 120 feet, to the Mississippi bottom.

This 3d Division, or the Mississippi bottom, extends from Memphis to Vicksburg, 180 miles in length, and is 60 miles wide in the middle, tapering towards each end. Its features are too well known to require description.

K

The divisions and even the subdivisions, given above, are not only characterized by their topography, geology and soil; but, as might be inferred by the peculiar kind of forest covering them, hickory, pine, and various kinds of oak. We are struck with the complete and regular system of drainage; the size of the streams and their valleys being always proportional to the extent of country drained. It is evident, from this fact, conjoined with the character of the formations, that the whole has been quietly and simultaneously elevated, and has not materially suffered by subsequent subterranean action. It appears further, that the Prairie belt, east of the red loam ridge, and the clay belt west of the same, were two marine valleys, and that the remainder of the country constituted a great level, probably with a very gentle southern. slope, which, as the land rose, was scooped into hill and valley, with the peculiar features which are its present characteristics. The Section then adjourned till Monday.

R. W. GIBBES, Secretary.

September 23.

SECTION OF GENERAL PHYSICS, &c.
Third Meeting.

On motion, Lieut. M. F. MAURY, U. S. N., was called to the chair. A paper was read "ON CHEMICAL PHILOSOPHY." By Mr. T. S. HUNT.

This paper has been published in the American Journal of Science and Arts for May, 1849.

A paper was read by Prof. JAMES H. COFFIN "ON THE MEAN VALUES OF DIFFERENT POWERS OF THE RADII VECTORES OF THE ELLIPSE," in which he showed

1st. That the mean value of their first powers is equal to the semi-transverse axis.

2d. That the mean value of their squares is equal to the square of the semi-transverse axis, plus half the square of the eccentricity.

3d. That the mean value of their cubes is equal to the cube of the semi-transverse axis, plus the square of the eccentricity, multiplied by three-fourths of the transverse axis.

Prof. M. H. Boyè's paper, "ON THE COMPOSITION OF BITTERN," was now read. It has been published in the American Journal of Science and Arts for January, 1849.

The next communication was by Mr. SEARS C. WALKER, through Prof. ALEXANDER

ON THE OPPOSITION OF NEPTUNE OF 1848.

Mr. SEARS C. WALKER Communicated to the Section the comparison of his Ephemeris of Neptune, for the opposition of 1848, with the observations received from Hamburg and Cambridge, England. After applying to the Ephemeris a correction published by Mr. W. in the American Journal of Science, the differences between the computed and observed places of Neptune, are as follows:

This comparison shows that no correction is needed as yet, either to Mr. Walker's elements of Neptune's orbit, or to the perturbations of the planet, as computed by Prof. PEIRCE.

The following paper was read :—

ON THE TOPOGRAPHY OF THE STATES OF PENNSYLVANIA AND OHIO, &c. BY SOLOMON W. ROBERTS, Esq.

The speaker commenced by saying, that the maps which he exhibited showed the results of numerous and extensive surveys which had recently been made. A rail-road is in use from Philadelphia to Harrisburg, on the Susquehanna, a distance of 107 miles, passing over a rolling and highly cultivated country, but not crossing any high ridges. The Pennsylvania Rail-road, now in course of construction from Harrisburg to Pittsburg, at the head of the Ohio river, will be 251 miles in length, making the whole distance from Philadelphia to Pittsburg, 358 miles. This line crosses the Alleghany mountain at Sugar Run Gap; and from Harrisburg to the base of the mountain, a distance of 133 miles, the line follows the valley of the

Juniata river, and has no grade greater than 21 feet per mile. The curvatures are easy, and the road adapted to high velocities. The mountain is ascended on the eastern side by 123 miles of a grade of 80 feet per mile, similar to that on the Western Rail-road of Massachusetts. The summit of the mountain is then passed by a tunnel 700 yards long, and the line from the summit to Pittsburg is 106 miles long, with a maximum grade of 52 feet per mile.

The rail-road distance from Pittsburg to Cincinnati will be 330 miles, by the way of Massillon, Wooster, and Columbus, while the distance by the Ohio river is 495 miles, or one-half longer than the rail-road; and the rail-road may be traversed in about onefourth of the time required by steamboats on the river. The railroad in Ohio for the greater part of its length will traverse the elevated table lands of that state, which are very favourable for rail-road construction.

The speaker described the principal topographical features of the States of Pennsylvania and Ohio, and exhibited a profile of the crest line of the Alleghany mountain for a distance of 44 miles.

In conclusion, he explained that the best and shortest rail-road route from Cincinnati to New York and Boston, passed through Pennsylvania and Philadelphia; and that the same was true of a road from St. Louis. And also that from Cleveland on Lake Erie to New York, the distance by the rail-road through Pittsburg and Philadelphia, will be 80 miles shorter than by the way of Dunkirk and Piermont.

A communication was read on-

DEFINITIONS AND DISCRIMINATIONS RESPECTING MATTER, VOID SPACE AND NIHILITY. BY PROF. ROBERT HARE.

In what respect does a void space resemble nihility?
Either implies a negation of matter.

In what do they differ?

Space can hold either matter or spirit, nihility neither. Nothing can hold nothing.

Space implies the capacity to hold something, whether corporeal or spiritual, and differs from nihility as a mathematical solid differs from a mathematical point. It is unity in imagination, multiplied three times by any number representing imaginary length, breadth, and thickness, whence results an imaginary solid.