berg, Bailey, and others, it was then known that from deposits, chiefly of the siliceous cases of plants of the lowest order, the diatomaceæ, a "cap" of siliceous sand was being formed at the northern, and another at the southern pole. It was also proved that the grand areas of the general sea-bottom of the Atlantic and Pacific oceans were similarly constituted of a girdle of calcareous mud, of indefinite depth, formed by a similar vein of discarded calcareous shells of animals of low organization-the foraminifera. Now this white calcareous matter of the foraminifera shells has been shown by the Challenger researches to be replaced, in certain deep oceanic valleys between Tristan d'Acunha and Kerguelen's Island, and elsewhere, by a very fine red clay. In certain geological deposits, of greater or less antiquity, beds of glauconite, or green siliceous sand, exist, which are composed entirely of the casts of ancient foraminifera formed of a green material, which is a compound of silicate of iron and alumina. The chemist of the Challenger having found that, from the decomposition by weak acids of the calcareous shells dredged up from the 18,000-feet depths, there is a residuum of one or two per cent. of red marl, exactly like that dredged up from the 18,000-feet depths of the valleys referred to, the conclusion is arrived at that the red mud is the accumulation of this small percentage of clayey matter, resulting from the wholesale decomposition of the calcareous polythalamous shells. The novelty of the Challenger discovery consists, therefore, in the fact that clay deposits can also be assigned, like siliceous and calcareous deposits, to the resultant débris of organisms living at the surface of the sea. Supposing, therefore, that the whole globe were immersed under an entire envelope of water, deposits of all the materials of our stratified geological rocks could be going on without the slightest assistance from the degradation and wearing away of any actual land surface at all; and these deposits, subjected in the ordinary natural course of events to ordinary processes and actions, could be modified into gneiss, schists, slate, limestone, and every variety of geologic rocks.-3 A, Jan. 6, 1875, 171.

INFLUENCE OF FORESTS ON CLIMATE.

Claré, in the Revue des Deux Mondes, takes strong ground as to the exercise by forests of a very decided influence upon

climate. In his article he remarks that forests have a chemical, physical, physiological, and mechanical action on the climate of a country. In regard to the physical action, while the foliage of woods allows much less rain-water to reach the ground than in unwooded land, this is more than compensated by the difference of evaporation in the two cases, that of the open fields being nearly five times as great as that of the woods. The melting of snow, too, is retarded by forests, thus causing a more gradual outflow of the water. Again, forests are obstacles to atmospheric movements. An air current meeting a wood is compressed and forced upward, so that it yields part of its moisture in the form of rain. Forests also protect crops against the winds; and it is an established fact that thunder-storms are less frequent and violent in wooded regions than in open countries, as the trees draw from the atmosphere the electricity it contains, which accumulates on regions that are bare. Forests, too, have a decisive action as regards the formation of hail, hailstorms occurring but rarely in a wooded region. A case has lately been noted where a violent hailstorm on approaching and crossing a forest ceased to produce hail, but resumed its formation on passing to the unwooded country beyond.-18 A, June 11, 324.

NOTE ON THE VERTICAL DISTRIBUTION OF TEMPERATURE ON
THE OCEAN.

Mr. Buchanan, chemist on board of the Challenger, writes that the effect of the changing seasons on the temperature of the sea-water seems to him not to have received sufficient attention. During the whole period of the heating of the water it has, from its increasing temperature, been steadily becoming lighter, so that the communication of heat to the water below by convection has been entirely suspended. It has, also, by evaporation, become denser than it was before. at the same temperature. During the approach of winter, the superficial water having cooled, sinks through the warmer water below it, until it reaches the stratum having the same temperature as itself. Nor does it stop there, but continues to sink, owing to its density, carrying its temperature with it to the lower colder layers. The result is that we have during the winter a heating effect going on in the lower re

gions, and during the summer a cooling effect; so that the greater the yearly range of atmospheric temperature, the greater the depth in the ocean to which its effect will be felt. He thus explains the presence of the large body of compara tively warm water in the North Atlantic, the existence of which has been usually ascribed to an assumed back-water of the Gulf Stream. This warm water is, in fact, due to no such extraneous cause, but is the actual effect of the conditions of the climate at the surface, which effects become apparent, because the water is free from the influence of oceanic currents, and exposed to the effect of climate alone.-Proceedings of the Royal Society, 1875, p. 123.

NAUTICAL METEOROLOGY.

Nautical meteorology has for its object the study of both atmosphere and ocean in their relations to navigation, and the utilization of our knowledge of the winds and currents, the laws of tempests, etc., in order to accomplish the shortest possible voyages between given points. The foundation of this application of scientific study to the wants of navigation was laid by Maury, and a recent work by Ploix and Caspari seems to embody many of the recent improvements that have been made. This work is intended as a guide to the mariner in the use that he can make of the charts published by meteorologists. After giving a general descriptive account of interesting phenomena, instruments, methods of observation, and the general climatological features, the volume gives a résumé of the oceanic routes recommended for the different months of the year throughout the navigated oceans. For instance, it insists especially upon the importance of attending to the point where the ship crosses the equator in passing from one hemisphere into the other. Thus, in order to go from Europe to either the Cape of Good Hope or to Cape Horn, we should cross the equator at the same point; but, varying with the seasons, we should pass either near the African or near the Brazilian coasts, but never in the intermediate region where the navigator is exposed to persistent calms. Another interesting point is the navigation in high southern latitudes, where we meet almost constantly west winds. Thus, in order to go to Australia, we sail from Europe to the south of the Cape of Good Hope, but in order to

return it is preferable to sail from Australia eastward past Cape Horn.-Bulletin Hebdomadaire, XVI., 28.

ON THE THEORY OF TORNADOES AND WATERSPOUTS.

In a general investigation into the phenomena of the cyclonic movements of the atmosphere, Cousté states that, starting from the general principle that there exists at the centre of the cyclone a column of ascending gyrating air, he deduces logically the following conclusions: First, the whole columu must rotate about its geometrical axis, in the opposite direction to the gyration. Secondly, there must be a vertical oscillating movement by which the column alternately rises above and descends to the ground, carrying devastation before it. Thirdly, there must be a movement of translation, which is accomplished, as shown by observation, with a rapidity varying between twenty and seventy miles per hour. These three movements are derived from the centripetal forces developed by the gyration which give rise to lateral streams of air, which he calls radiating filaments, in opposition to those interior filaments which gyrate within the helix, and which he calls helicoidal filaments. These radiating filaments of air form a nappe which incloses the whole convex surface of the tornado, and they constitute the wall of the column, which wall, for a given state of dynamic equilibrium, is as solid as if it were a solid matter of sheet iron, for example, yet is permeable and indefinitely extensible according to the conditions of its dynamical equilibrium. These filaments are directed from below upward, following the tangent to the helix farthest from the axis, producing reactions similar to those in turbine wheels. Cousté has also determined the character of the movement along the surface of the earth; this is, in general, of a spiral nature, at least for waterspouts properly so called, which appear as truncated columns, suspended from a cloud. But for those tornadoes whose trajectory is nearly rectilinear, and for the cyclones and hurricanes whose birth takes place upon the ocean the trajectory takes the form of a parabola, whose summit is always near the side of some large continent. These remarkable peculiarities he explains by the following principles, which he has deduced from his theory: First, if the angular velocity of gyration increases or diminishes-that is to say,

if the energy of the meteor increases or diminishes-the curvature of the trajectory will rapidly increase or diminish. Second, the energy of the meteor is greater in proportion as the air which it draws in is dryer, or has a low relative humidity.

In reference to the rigidity of the column of the waterspout, Cousté says that the equilibrium between the reacting forces due to the radiating filaments exists even when the various diameters of the waterspout are made unequal by the pressure of the wind. The normal components of the movement of the wind have the effect substantially of pressing the helicoidal filaments together normally to the surface, thus concurring to maintain the rigidity of the column, to which the gyratory movement contributes. This we can easily comprehend if we compare the column to the gyroscope of Foucault; for the column of the waterspout can be likened to a series of gyroscopes having a common vertical axis, the revolving disks being formed by parallel horizontal sections of the column. In the case of the tornado the disks are gaseous, it is true, but the gas is kept in its place by the normal components of the forces; and if they have but little mass, they are, on the other hand, actuated by very rapid velocity of rotation.

Theoretically, a waterspout is a collection of parallel whirling tubular masses; a complete illustration of this is, however, very rare in nature, though such have been figured by Mouchez and others.-Nouv. Meteorologiques, 1875, p. 61,

TREATISE ON METEOROLOGY BY MOHN.

81.

An important treatise on meteorology has lately been. published by Mohn, the distinguished chief of the Norwegian Weather Bureau. It constitutes an original German edition, with many improvements, of the work published two years ago in Norwegian, under the auspices of the Society for the Dissemination of Useful Knowledge. As was to be expected, Mohn has especially developed in this work the ideas that he has for some years defended with reference to the influence of moisture in the air upon the movements of storm areas. His whole work, in fact, corresponds to the present condition of meteorology, except, possibly, that the attempt to provide a purely popular explanation of the mechanical