feet in breadth and height, a large part of the light which enters passes through the water. The spectroscope shows that the red is wholly absorbed by the water, and the yellow is so far enfeebled that the D line was scarcely visible. The green, blue and indigo on the other hand shone out brightly, with the F and b lines united in a well-marked absorption line.-Pogg. Ann., clvi, 326.

E. C. P.

11. Action of Magnetism on an Electric Spark.—M. H. BECQUEREL has shown that the action of a powerful electromagnet when its current is broken between its poles, is purely mechanical. The spark in this case is accompanied by an explosion, and takes the form of a small flame, which seems to be projected by the action of the magnet as it would be by a current of air. The same effects may be produced by a bellows or even by the mouth. A little explosion is produced, increasing in strength with the force of the air current. It is generally admitted that the sound accompanying the induction spark is due to a sudden expansion of the air and of the volatilized portions of the electrode, followed by a sudden return of the particles to their former position. According to this view the shorter the time of discharge the louder should be the noise.

The substitution of the purely mechanical action of a current of air seems to show that under the influence of the air current as with the magnet, the effect is due to a sudden rupture of the chain of molecules which transmit the electric current of only short duration, forming the induction current. The time of discharge being thus notably diminished the sound accompanying it assumes a remarkable intensity. The idea of making an air-current act in this way is due to M. de Moncel, who has employed it to separate the spark from the aureole.-Journ. de Phys., iv,

206.

E. C. P.

12. Interference Fringes.-M. NODOT suggests the substitution of rhombs of Iceland spar for Nicol's prisms in forming the interference fringes of Fizeau and Foucault. The sunlight is admitted through a narrow slit, and focussed on the slit of the spectroscope by a lens. Two rhombs of spar are then interposed, and between them is placed a plate of quartz, cut parallel to the axis, and inclined 45 to the planes of polarization of the spars. Three images of the rhomb are thus formed, the central one of double the brilliancy of the others, since it is formed by the superposition of two images. Allowing each image in turn to fall on the slit of the spectroscope, it will be seen that fringes are present in all three, those of the two outer images being complimentary to the fringes of the central image. Evidently the brilliancy of the effect will be double that usually attained, since a Nicol's prism necessarily cuts off at least half the light.

The experiment may be varied by forming the three images one above the other, and allowing all to fall on the slit of the spectroscope, when a spectrum is formed in which the upper and lower parts are fainter than the central portion, and the fringes of the

former are complimentary to those of the latter. In other words the bright portions of one set correspond to the dark parts of the other.-Journ. de Phys., iv, 209.

E. C. P.

13. The Wind Theory of Oceanic Circulation. Objections examined; by JAMES CROLL, of H. M. Geological Survey of Scotland. (From the Philosophical Magazine for October, 1875).*— The fundamental arguments of the advocates of the gravitation theory-1. The gravitation theorists base their argument on two principal assumptions which cannot be conceded. First, they maintain that the existence of polar water in the depths of the ocean is consistent with their theory only; and, secondly, they assume as a necessary condition of the wind theory that the understratum of the ocean should consist of warm water. It is a well recognized fact that the ocean beyond the reach of sun heat is occupied with water of a polar temperature; and they therefore point triumphantly to the fact as at once a proof of their position and a conclusive argument against the wind theory. But, on the other side, it will not be difficult to show that the existence of cold water throughout the ocean depths is as much a necessary result of the wind theory as of the gravitation theory, and that there is no relation whatever between the wind theory and warm water in the depths of the sea.

It is supposed that the return under-currents from the polar regions are by far too insignificant to be able to maintain at a polar temperature the great depths of the ocean.

Let us examine this objection. It is freely admitted, nay even strenuously maintained by the advocates of the gravitation theory themselves, that the heating-power of the sun does not extend to any great depth below the surface of the ocean; consequently there is nothing whatever to heat this mass of water underneath except the heat coming through the earth's crust; but the amount of heat derived from this source is so trifling that an under-current from the Arctic regions of no great magnitude would be sufficient to keep the mass at an ice-cold temperature.

894

On a former occasion I showed that, taking the rate at which internal heat passes through the earth's surface to be that assigned by Sir William Thomson, the total amount received per annum by the North Atlantic, between the equator and tropic of Cancer, including the Caribbean Sea, is equal to only 4 of that conveyed by the Gulf-stream, on the supposition that each pound of water carries 19,300 foot-pounds of heat, and that consequently an under-current from the polar regions of not more than the volume of the Gulf-stream would suffice to keep the entire mass of water of that area within 1° of what it would be were no heat derived from the crust of the earth, and an under-current of less than that of the Gulf-stream coming from the polar regions would keep the entire North Atlantic from the equator to the arctic circle filled with ice-cold water. A polar under-current half

*Received for this Journal from the author.

+ Philosophical Magazine, June, 1874; "Nature," vol. x, p. 52.

the size of the Gulf-stream would be sufficient to keep the entire water of the globe (below the stratum heated by the sun's rays) at an ice-cold temperature. Internal heat would not be sufficient under such circumstances to maintain the mass 1° Fahr. above the temperature it possessed when it left the polar regions.

In short, whatever theory we adopt regarding oceanic circulation, it follows equally as a necessary consequence that the entire mass of the ocean below the stratum heated by the sun's rays must consist of cold water. For if cold water be continually coming from the polar regions, either in the form of under-currents, or in the form of a general underflow as Dr. Carpenter supposes, the entire under portion of the ocean must ultimately become occupied by cold water; for there is no source from which this influx of water can derive heat, save from the earth's crust, which amount is so trifling as to produce no sensible effect.

It is therefore evident that the great mass of cold water occupying the depths of the ocean cannot be urged as an objection to the wind theory.

2. But it is asserted that the impulse of the wind on the surface of the ocean cannot produce and maintain deep under-currents. This is an objection which has been urged by some eminent physicists; but it is based upon a misapprehension of the manner in which, according to the wind theory, under-currents are produced.

It is true, as the objectors maintain, that a wind simply impelling the water forward will not necessarily produce an under-current, since compensation will more readily take place by return surface-currents, as in this case the path of least resistance will generally be at the surface. But when the general surface of one half of an ocean basin is being constantly impelled forward by prevailing winds in a contrary direction to that in which it is being impelled in the other half, compensation cannot possibly take place by means of return surface-currents. For a full discussion of this point I must refer the reader to my work, "Climate and Time," Chap. XIII.

It is, however, needless to advance arguments à priori against the possibiliy of such under-currents; for we have actually several well-known examples of such currents, the particulars of which will also be found in the work to which I refer.

3. But supposing it could be shown that the winds cannot directly produce under-currents, it can nevertheless be demonstrated that they can do so indirectly. A vertical circulation filling the deep recesses of the ocean under the equator with polarcold water, follows as readily and truly from the wind theory as it does from the gravitation theory. It has been shown that the general tendency of the system of the winds is to impel the surface-water of the equatorial regions into the temperate and polar regions as rapidly as it is heated. But such a transference of surface-water must tend to destroy static equilibrium by making the equatorial too light and the temperate and polar columns too heavy, as truly as though the transference had taken place by

means of difference of temperature. The effect must be to produce a constant ascent of the equatorial column and an inflow of cold water below equal to the outflow above. In short, the wind must produce a system of circulation precisely the same as that supposed to take place by difference of temperature.

By both theories the cause of the vertical motion is the transference of water from the top of the one column to the top of the other. This vertical motion is therefore as much a necessary consequence of the wind theory as it is of the gravitation theory.

II. GEOLOGY AND MINERALOGY.

1. On the Gravel and Cobble-stone deposits of Virginia and the Middle States; by Wm. B. ROGERS. (Proc. Boston Soc. Nat. Hist., vol. xviii, 1875, 101.-The deposits here described as occurring in the great river valleys and on the adjoining slopes, at Richmond, Va., Washington, D. C., and elsewhere, consist chiefly of layers of water-worn gravel and stones, with ferruginous sands and clays. In most localities the larger pebbles are found in the upper part of the deposit; but in others, as at Alexandria and Richmond, the cobble-stone deposit is overlaid by bedded sands and gravel. Casts of Scolithus occur in some of the pebbles collected at Washington and Richmond. The deposit at Washington covers the entire plain on which the city is built; it averages 75 feet in height above mean tide, but rises on the north to about 100 feet. Thence it spreads over the slopes, covering the grounds of Columbian College, and the higher hill of the Soldiers' Home, over 200 feet above sea level. In the vicinity of the Capitol the stones are often a foot in diameter, and near Georgetown in a recent excavation some are much larger. The facts point to transportation along the valleys, but by streams of much greater width than those now there. The distance transported may be learned from the fact that the nearest Potsdam or Scolithus sandstone to Richmond is 80 miles, and along James River 160 miles; and that from Washington to the western side of the Blue Ridge is 40 miles, and along the Potomac 50 to 60 miles. Prof. Rogers remarks on the origin of these deposits as follows:

Speculating on the causes by which these deposits have been formed, it may, on the one hand, be imagined that during the Glacial period the icy covering of the north and west prolonged itself in the valleys of the great rivers, as far south as the James, and even the Roanoke River, bringing down to the belt of land now marking the limit of tide water debris from the Appalachian rocks, mingled with materials derived from the intervening region, and that the grinding and sorting action of the waters subsequently obliterated glacial marking, and gave to the whole deposit the distribution and stratification which it now presents; or, on the other hand, it may be conceived that the transporting force of the rivers themselves, swollen and rapid as they must have been in the closing ages of the Glacial period, brought about the same results. But even in this case, it is

highly probable that glacial action had much to do with the original accumulation of the rocky debris on the flanks of the Blue Ridge, and in the Appalachian valleys beyond."

Prof. Rogers further observes that there is a siliceous and argillaceous formation in Virginia, easily confounded with the finer drift beds, which underlies the Tertiary, and is placed by him at the base of the Cretaceous formation or top of the Jurassic; that sections are exposed in a deep railroad cut between Washington and Baltimore, and on the way to Wilmington; that it is seen beneath the Cretaceous green-sand in Maryland, Delaware, and New Jersey, and near Baltimore was found by Prof. Tyson to contain stumps of Cycads. When the Cretaceous and Tertiary are absent these beds are easily confounded with the stratified drift. Its contact with the superficial deposits was well presented in April, 1875, near Washington, at a cut at the extremity of 16th street, at the base of the Columbian College Hill, and on 14th street where it ascends the same bill.

2. Report of the Geological Survey of North Carolina, Volume I. Physical Geography, Resumé, Economical Geology; by W. C. KERR. By authority of the General Assembly. 326, 120 pp. 8vo. Raleigh. 1875.-The Geological survey of North Carolina under Prof. Kerr was commenced in 1866. The report now issued was presented to the Legislature in 1870, but not then ordered for publication; and now it appears in 1875 "under a permissive resolution, out of the working fund of the Survey." A second volume, the preface states, will go to press during the year.

The report commences with an introductory chapter on the Physical Geography of the State. In the course of it the fact is brought out that on the south side of the rivers in eastern North Carolina there are usually bluffs and high banks, and on the north, swamps and low flats; and that this is a feature also of eastern South Carolina. Further, the Miocene shell-beds were found only on the south side of these large rivers. "The cause," according to the author, "is doubtless the rotation of the earth co-acting with the river current ;" and he cites the law of motion, wrought out with mathematical demonstrations by Prof. Ferrel, that, "in whatever direction a body moves on the surface of the earth, there is a force arising from the earth's rotation which deflects it to the right in the Northern hemisphere, but to the left in the Southern." The chapter treats of the topographical features and climate of the State and contains a long table of altitudes.

On its Geology the author gives only the general outlines. The series of rocks includes those of the Archæan, which are made to cover (1) a large area running northeastward across the State immediately west of its center and extending westward to include the Blue Ridge; also (2) an area in central North Carolina running north-northeast from Raleigh, and some small areas in that vicinity, one of them to the south on the Cape Fear River; and (3) an area west of the Blue Ridge. The first consists largely of syenyte and related hornblendic rocks, which rarely con