1. The old and popular belief that the treelessness of the prairies was caused by the annual burning of the grasses by the Indians," is now generally admitted to be inadequate.

2. The supposition that trees have been choked out by the tangled roots of cane, which in turn has disappeared under the influence of a burning sun," has no applicability in a region. visited annually by frosts too severe for cane to survive.

3. The supposition that the absence of trees is due to too great dryness of the soil during the summer, is disproved by the fact that trees flourish naturally in drier soils in the same vicinity, while, on being introduced, they flourish equally well in the prairie. The treeless and almost herbless deserts of the far west may have originated in extreme aridity of the atmosphere -as others have from the highly saline character of the soilbut all our discussions have had reference to the prairies of the Mississippi valley.

22

4. A theory often urged is the considerable humidity of the soil of certain prairies," and especially the wetness of the subsoil in contrast with the dryness of the soil during summer." It is singular that such an opinion could be entertained when it is so well known that there is no situation so wet but certain trees will flourish on it-the willow, the cottonwood, the beach, the ash, the alder, the cypress, the tupelo, the water-oak, the tamarack, the American arbor-vitæ or some other tree-some of them standing joyously half the year, if need be, in stagnant water. It is well known that swales are generally devoid of trees; but the reason for this is to be found in the fact that since a soil assumed the place of the ancient lake, the germs of trees have never been introduced; while the introduction of such germs is delayed by the circumstance that neighboring forests are generally such as are adapted to drier situations. Has it been found that a green willow or poplar twig will not root and thrive in a wet prairie? But further than this, large portions of the treeless prairies are not wet. Is there a different cause for treelessness here?

5. Prof. J. D. Whitney " has advanced the opinion that the extreme fineness of the prairie soil is the cause of the absence of trees; and the author of the article on "Plains," in the New American Cyclopedia, seems to have adopted this view. Against this theory we see several weighty objections. Many alluvial soils, as pulverulent as that of the prairies, are densely

20 This Journal, vol. i, p. 331.

21 This Journal, vol. xxiii, p. 40.

22 Does Prof. Dana allude to the prairies of the Mississippi valley when he says, (Manual of Geol., p. 46), " and where the moisture is not sufficient for forests, she [America] has her great prairies and pampas?" See also Cooper, Smithson. Rep., 1858, p. 276; Newberry, Ohio Agric. Rep., 1859; Lambert, Pacific R. R. Rep., vol. i, p. 166.

23 Atwater, this Journal, i, 116; Bourne, Ib., ii, 30; Lesquereux, 2d Ark. Geol. Rep. Western Monthly Magazine, Feb., 1836

24

Engelmann, this Journal, [2], xxxvi, 384.

25 Iowa Geol. Rep., vol. i, p. 24.

wooded, and that in the same latitudes and under the same meteorological conditions. Again, partial or complete destitution of trees is observed on the coarser, sandy borders of the prairies, and on all recent sand dunes, even where no lack of vegetable sustenance exists. But the fatal objection to this theory, and all theories which look to the physical or chemical condition of the soil, or even to climatic peculiarities, for an explanation of the treeless character of the prairies, is discovered in the fact that trees will grow on them when once introduced-not waterloving trees exclusively, but evergreens, deciduous forest trees, and fruit trees-such as flourish in all the arable and habitable portions of our country. Everybody now knows that trees flourish well on the prairies; and the prairie farmers are actively engaged in their introduction." It seems to us that this fact alone militates fatally against the views advanced by Whitney as well as those of Engelmann, Bourne, Atwater and others, who have attributed the distinctive character of the northwestern prairies to an excessive humidity of the soil.

University of Michigan, Aug. 30, 1864.

ART. XXXIV.-On the Nebular Hypothesis; by DAVID
TROWBRIDGE, A.M.

THE following paper on the nebular hypothesis differs from any other with which I am acquainted in several particulars, the principal of which are the arrangement of the parts treated, and the mathematical discussions introduced. It treats the theory of the subject first, and then the phenomena of Nature are compared with the theoretical conclusions.

The only apology which I have to offer for going over so much of the subject is that I attempted to prepare some articles on detached portions of the hypothesis, and found the explanations, which would be necessary to render the subject intelligible to those who had not made the matter a study, so many that the space demanded would be nearly equal to that occupied by the present paper.

That there is a growing interest in the subject, is known to every astronomer that has paid any attention to what has been written. It is a great problem yet to be completely resolved. The complete analytical treatment of the nebular hypothesis presents many difficulties; and without such aid it seems impossible to ascend to the origin of the solar system. I now submit what follows to the candid judgment of my readers.

26 Compare Wells, this Journal, i, 331, where the forest is said to be encroaching on the prairies about St. Louis; Engelmann, Ibid., [2], xxxvi, 389; Edwards, Rep. Dep. of Agric., 1862, p. 495.

THE THEORY.

1. GEOLOGY has revealed the fact that it took immense ages of time to form the earth, and fit it for the habitation of man. The same science also points, somewhat definitely, to a time when the earth was in a highly heated condition. Mathematical science, applied to the problem of the earth's conformation, teaches us that the earth has that form-the asperities of its surface not considered-which it ought to have if it were in a fluid state when it assumed its present form. These facts-to which we might add the condition of Saturn's rings-seem to teach that the earth, and in short the whole Solar System, were once in an aëriform state. An additional argument in favor of this view, is derived from the physical constitution of Comets.

2. With these preliminary facts and considerations, perhaps we may be justified, in treating of a hypothesis-in assuming that the Solar System was once but a single body, and in a gaseous condition; and that this single solar body was but a detached portion of a still larger and more extensive mass, that being in a still more primitive state. If we ask what is the cause that made the materials from which worlds have since been made assume that primitive gaseous condition, we must look for our answer to the operations of caloric. Philosophers, in their investigations, have arrived at this general conclusion respecting the operation of heat, namely, that mechanical action develops it, and the greater the action the greater the heat; and that as soon as heat becomes sensible, it tends to change the condition of bodies. This, then, reduces the cause of the primitive gaseous state of the stellar and planetary worlds to mechanical action. As the mechanical action becomes less and less, the operations of heat become less and less potent.

I. Of Stellar Systems.

3. Let us assume that there once existed an immense and widely extended nebulous mass, from which solar and planetary bodies were afterward developed. It is extremely doubtful that the astronomer will ever obtain, with the telescope, any information respecting the existence of any such nebulous mass, even if such a body now exists within the range of telescopic vision, as I have attempted to show in a former paper in this Journal.' 4. We may now ask what was the probable nature of this supposed nebulous body? Was it homogeneous in structure, or was it heterogeneous? Let us try to determine this point. But first, what was the form of this great nebulous mass? Here 1 Chemical action should not be left out of the consideration, but at present we cannot define its operations so well. * Vol. xxxvii, [2], p. 210. AM. JOUR. SCI.-SECOND SERIES, VOL. XXXVIII, No. 114.-Nov., 1864.

AM.

again we are reduced to conjecture. We cannot determine, with any approximation to truth, what the original form was. We may suppose it to have had no definite form, according to our conceptions, but rather to have been an amorphous mass, like the clouds of vapor which float in our atmosphere. If we could suppose this great nebulous mass to have been symmetrical in form and homogeneous in structure, in the process of cooling and condensation there would be little probability of its generating a rotatory motion on an axis; and under such conditions the matter would condense into a sphere. But we have not the remotest evidence-nay, all our evidence is to the contrarythat the original nebulous mass was homogeneous. It is a very difficult thing to find even a small amount of matter perfectly homogeneous. We must, therefore, conclude that the probability against the homogeneity of the original gaseous mass, is many millions to one in favor of it. Even if it were originally perfectly homogeneous, but of irregular shape, the attraction of gravitation, and the other forces of nature which acted upon it, would soon' cause it to become heterogeneous in its constitution.

5. Again, if we suppose the original fluid mass to have been symmetrical in form (at least one of some particular forms), and arranged in strata that were homogeneous in themselves, the process of cooling and condensing would not generate a rotatory motion, and the mass would ultimately become a sphere. But the chances against such an arrangement of the nebulous mass would be equally as great as against the condition previously described. These are the only cases in which it seems possible for the mass to condense without generating a rotatory motion.

4

6. The science of mechanics teaches us that if a fluid mass be not so constituted as to be arranged in strata, homogeneous in themselves the strata being of different densities or otherwiseit will not be in equilibrium; and in this case the attraction of gravitation, radiation of heat, and whatever other active and modifying causes might operate upon the nebulous body, would cause the fluid mass to change its parts so as to seek a condition of equilibrium; and such a change would generate a motion of rotation about one or more axes. It is highly probable— caused by the form of the body, and the heterogeneous nature

In saying that such a result would be soon reached, it must not be supposed that we mean 64 soon according to our ideas of time. A million of years might be "soon" for such a change to take place. In speaking of such changes as are described in the text, we must extend our ideas of periods of time.

Courtenay's Mechanics, pp. 355-6; and Airy's Mathematical Tracts, pp. 163-6; and other works on the subject.

It must not be supposed that because the conditions of equilibrium are not satisfied that there will be a "breaking up" and a separation into distinct masses, but only a change in the distribution of the material composing the original fluid mass, so that it may ultimately attain the conditions of equilibrium. This change will generate a motion of rotation.

of the materials composing it, and perhaps other things-that the fluid would cool unequally in different parts, and in such places it would condense unequally, and thus, if they did not already exist, different centres of attraction would be produced. Around these different centres, matter would accumulate and condense, and these nuclei, so formed, would revolve around their common centre of gravity. As soon as a rotatory motion had commenced, centrifugal forces would begin to act; and as the process of cooling continued, the attraction of gravitation would have a greater control, (for the tendency of heat is to expand all bodies, and thus to operate against the attraction of cohesion, and also of gravitation in the case which we are considering,) and thus the mass would be condensed, and the rotatory motion thereby increased. But an increase of rotatory velocity would also increase the action of the centrifugal force, and this process would continue until in some parts the centrifugal force might equal the force of gravity, and a separation would take place; and in this way each nucleus might ultimately be separated from all the others, when each would pursue its own course around the common centre of gravity of the whole.

7. Each nucleus would itself be in a condition very similar to that which at first existed in the original great fluid mass. The same laws and forces acting on each nucleal mass, would generate a motion of rotation, if the previous separation had not already given it an initial rotatory velocity, and this motion would generate a centrifugal force, as before, and each of these masses would separate into parts. This process of separation would continue until such a result could no longer ensue. During all this time, it must be recollected that the original nucleal parts would continue their revolutions around their common centre of gravity, while the parts into which each original nucleus was divided would continue their revolutions around their common centre of gravity. In this way each division, or system, would continue its own independent revolution, and at the same time it would be carried around the common centre of gravity of the greater system of which it formed a part; just as the satellites revolve around their primaries, while the primaries are at the same time carried around the sun, and the sun around some more distant centre. It is in this way that we would ac count for the existence of clusters of stars.

8. Notwithstanding the probability that the detached parts would have a motion of rotation impressed upon them, yet we cannot so easily infer that this motion would be in the same direction

It is difficult to see how one of these supposed nucleal masses could be detached from the parent mass through the influence of a rotatory motion, without impressing on the detached part, at the time of separation, a motion of rotation. From the fact that the whole system is yet supposed to be fluid, such a result of the separation would seem to be still more probable than if it were solid.