ations of electric excitement under the influence of the sun; after the same manner that the earth's atmosphere is effected by the sun. That an electric influence is directly exercised by the sun upon the upper regions of the earth's atmosphere, or the photosphere of the earth, appears to me to have been established in my later papers on Magnetic Variations, published in former Nos. of this Journal. When repeated electric discharges take place in the higher and rarified regions of the atmosphere of the comet, or of that of the earth, they must have the effect, according to the results of the recent experiments of M. Plücker, to arrange the vaporous matter in columnar masses, coinciding in direction with the lines of magnetic force. We thus have auroral columns in the comet's as in the earth's atmosphere. At the magnetic poles of the nucleus these would have a vertical position; and from these points would gradually decline from this position, until at the equator they would lie parallel to the surface. Now as a comet recedes from the sun its temperature falls, the suspended aqueous vapor begins to condense at certain depths in its atmosphere, the electricity thus set free flows in a series of electric discharges, which follow the course of the auroral columns, as soon as they are established. Condensations extending through a considerable vertical depth in the upper atmosphere, would also be attended with electric discharges from the one elevation to the other. It is these electric discharges along these auroral columns that, as I conceive, disengage the particles of aqueous vapor, or nebulous matter so called; and impel them off with a certain velocity. The same discharges bring the expelled particles into a condition to be repelled by the nucleus. How this result may be produced cannot here be adequately explained. As the temperature of the receding comet continues to fall, the process of condensation, and consequent evolution of aqueous vaper, goes on, and the visible nucleus increases in size. It would seem, from the observations of Mr. Bond, on Donati's comet, that large masses appeared to be disengaged at certain intervals. These phenomena may have arisen from the occasional suspension of the electric discharges taking place in the upper atmosphere. This would produce the appearance of the detachment and expulsion from the surface of the nucleus, of a ring of nebulous matter. Luminous phenomena, precisely similar to those here supposed take place in the upper atmosphere of the earth, to which we have given the name of Aurora Borealis, and Aurora Australis; and probably from the same cause. They are almost uninterrupted at the pole, during the long polar winter, and only at intervals display their coruscations in the skies of the temperate latitudes; where the changes of temperature are less, and the vaporous columns assume a more oblique position. On the other hand while a comet is approach

ing the sun, its temperature rises, and at the same time its atmospheric electricity increases; condensations of aqueous vapor and their attendant electric discharges are now much less frequent. It thus happens that the evolution of vaporous matter, to form the head and tail, is much less copious before than after the perihelion passage; and increases in quantity for a certain interval of time after it. While these auroral phenomena, as they may be styled, are thus subject to great fluctuations, and to sudden interruptions, and are most prevalent in the polar regions* of the nucleus, there would seem also to be an uninterrupted electric discharge, from all points of the nucleus, turned toward the sun, continually detaching particles of aqueous vapor. This should be most abundant at the regions to which the sun is vertical, and where the electric excitement produced by it is the greatest; and may give rise to the hemispherical form of envelope, (see p. 94).

6.

The phenomenon of separate concentric envelopes, or rings, often noticed, shows that the vaporous matter set free, at any time, is not all expelled to the same distance from the nucleus. This would be the case if we were looking down upon the polar regions of a comet whose axis was perpendicular to the plane of its orbit, and the matter was detached in zones from different latitudes. The statement here made will, perhaps, be better understood on glancing at the annexed figure. It would seem also that different intensities of electrical discharge should be attended with different velocities of projection. Upon the theoretical views I have formed these electic variations should also give rise to different intensities of repulsive action, as exerted by the nucleus. Again, if all the particles set free should not be of the same size, the smaller

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ones would experience the greater repulsive acceleration; provided the material repulsion is of the nature of an impulsive action against the surface of the particle.

If the speculative notions just presented be correct, the question arises whether the earth may not be regarded, from our present point of view, as a comet; and if so, why do we not see its luminous train. The proper answer to this inquiry would

It is to be observed that the motion of the nucleus in its orbit, occasions a virtual rotation around an axis perpendicular to the plane of the orbit; so far as exposure to the sun is concerned.

seem to be that the earth is actually, in a certain sense, a comet, and that its luminous train is seen by us in the Zodiacal Light. The nebulous earth-ring contended for by the Rev. Mr. Jones, in explanation of his admirable observations upon the zodiacal light, would seem then, in a modified sense, to have a real existence; instead of being in a condition of statical equilibrium, as sup posed, it is in a dynamical condition of perpetual dispersion and renewal.*

Nor is the expulsion of vaporous matter into the surrounding regions of space confined to the nuclei of comets, and the earth. It occurs at the surface of the sun, and perhaps of all the heavenly bodies. It is beautifully seen, as a solar phenomenon, in a total eclipse of the sun; in the corona, or halo that encircles the sun concealed behind the dark body of the moon; the aigrettes that stream out in various directions, and perhaps also the rose colored flames that here and there project beyond the dim circular disc of the moon.

Note. It would seem that even the visible nucleus of a comet is not in a truly statical condition. It contracts and enlarges with the varying distance from the sun. This may be a mere appearance, arising from the varying luminosity of the photosphere. It is also possible that the inner nucleus, with its atmosphere, may be surrounded by an ethereal atmosphere, which contracts and expands by reason of variations in an impulsive action of the sun, and in the density of the ether of space, in the vicinity of the sun. These remarks may also apply to the entire envelope of Encke's comet, and the complete spherical envelopes sometimes noticed, Spherical envelopes entirely surrounding the nucleus, would also be formed if the cometic matter should be projected from all parts of the nucleus with the same velocity, but with a force insufficient to overcome the gravitating tendency. An apparent spherical continuation of an envelope behind the nucleus, might, perhaps result from the intersections of the orbits of the cometary particles urged past it into space by the repulsive force of the sun.

A more accurate determination of the orbits of the cometic particles will be giv en in a succeeding No. of the Journal.

ART. XV.-Review of Hall and Whitney's Report on the Geology of Iowa.t

THE first volume of the Report on the Geology of Iowa has just been published, and in a style highly honorable to the State. The State embraces much that is of scientific and practical interest, and Professors Hall and Whitney who have had the survey in hand, have accomplished great results considering the time afforded for exploration. The Carboniferous formation is a prominent feature in the geology, and has afforded a splendid collection of

The vaporous matter which is incessantly streaming off from the sun into remote space, should enhance the brightness of the zodiacal light.

Report on the Geological Survey of the State of Iowa: embracing the results of investigations made during portions of the years 1855, 56 and 57. By JAMES HALL, State Geologist, and J. D. WHITNEY, Chemist and Mineralogist. Published by authority of the Legislature of Iowa. Vol. I. pp. 724. 1858.

Crinoids for description and illustration. Even the Permian has some representative beds, though discovered too recently-the Introduction states, after the Permian fossils of Kansas had been made known-to be described in detail in this volume. Prof. J. D. Whitney contributes chapters on the Physical Geography, Geology, and Chemical and Economical Geology including Mines,-Prof. Hall, on the General Geology and Paleontology, and A. H. Worthen, (Assistant,) on the Geology of the Des Moines valley and some other parts of the State.

The geological investigations have thus far been confined to the eastern half of the State, the western portion of Iowa, which is equal in area to New York, Massachusetts, Connecticut and Rhode Island put together, not having yet been examined. The work is divided into eight chapters, or divisions, of which the first is devoted to the Physical Geography, the second to some general remarks on the Geology of the Northwest, the third to a review of the Geological formations occurring in Iowa, as exhibited in a section on the Mississippi river, the fourth, fifth and sixth to the detailed Geology by counties, the seventh to the Economical Geology, and the eighth to the Paleontology, the whole work being comprised in 724 pages, with 29 steel and lithographic plates of fossils, a geological map and a diagram of the leadbearing crevices near Dubuque.

In Chapter I, we have a concise view of the topography of Iowa. The State is a vast plain, gradually rising as we proceed from the east towards the west, and from the south, northwards :-the absolute elevations are given from the railroad surveys, when such could be obtained. In this plain the streams have cut deep and narrow valleys, which are bordered by precipitous "bluffs," as they are termed, the river-bottom, bluff and prairie being the three conspicuous features of the topography of the State. The height and steepness of the bluffs decrease gradually from the north towards the south. The course of the tributaries of the two great rivers forming the eastern and western boundaries of the State, the Mississippi and Missouri, is adduced as evidence of the direction of the drainage having been determined by two sets of low and narrow flexures of the strata, one set running in a northwest and southeast direction and the other nearly at right angles to this. The Wapsepinicon is mentioned as a remarkable instance illustrative of this; this river having a length of about 250 miles and draining a valley which is only from eight to twelve miles in width. The "mounds" which break the monotony of the landscape in the lead region, are described as outliers of Niagara limestone, overlying strata of the Hudson river group, and rising in isolated, flat-topped hills, presenting evidence of extensive denundation. These mounds are from 400 to 600 feet above the Mississippi river, and about 200 above the general evel of the prairie at their bases.

The prairies, those wonderful treeless and fertile plains, the most marked feature of the Northwest, are described and the subject of their origin, a much controverted question, is briefly discussed. The theories which have, at different times been brought forward to account for the absence of trees in the prairie region are discussed and pronounced to be inadequate. It is attempted in the report to show that the extreme fineness of the particles of which the soil is made up is the predominating cause of this peculiar condition of the vegetation, and some facts are stated which go to confirm this theory. Reasoning from analogy of the smaller prairies to the thickly wooded region of the Upper Peninsula of Michigan, it is inferred; "that the whole region now occupied by the prairies of the Northwest was once an immense lake, in whose basin sediment of an almost impalpable fineness gradually accumulated, under conditions, the discussion of which is postponed to another volume in which the drift phenomena of the Northwest will be taken up; that this basin was drained by the elevation of the whole region, but, at first, so slowly, that the finer particles of the superficial deposits were not washed away, but allowed to remain where they were originally deposited. After the more elevated portion of the former prairies had been laid bare, the drainage becoming concentrated in narrower channels, the current thus produced, aided perhaps by a more rapid rise of the region, acquired suf ficent velocity to wear down through the finer material on the surface, wash away a portion of it altogether, and mix the rest so effectually with the underlying drift materials, or with abraded fragments of the rock in place, as to give rise to a different character of soil in the valleys from that of the elevated land. This valley soil, being much less homogeneous in its composition, and containing a larger proportion of coarse materials than that of the uplands, seems to have been adapted to the growth of forest vegetation; and in consequence of this, we find such localities covered with an abundant growth of timber."

"Wherever there has been a variation from the usual conditions of soil, on the prairie or in the river-bottom, there is a corresponding change in the character of the vegetation. Thus, on the prairies we sometimes meet with ridges of coarse material, apparently deposits of drift, on which from some local cause, there has never been an accumulation of fine sediment: in such localities we invariably find a growth of timber. This is the origin of the groves scattered over the prairies, for whose isolated position and peculiar circumstances of growth we are unable to account in any other way."

At the close of the chapter on Physical Geography, a review is given of the meteorological observations which have been made in different parts of the State by different observers. From

SECOND SERIES, VOL. XXVII, No. 79.~JAN., 1859.