and the polar water. Whence it follows that these two differences nearly compensate each other, and that to produce equilibrium, the sea ought to be, under the equator, a little lower than under the poles, and that if this difference of level establishes a current, it should take place at the surface, from the poles to the equator, and in the interior in a contrary direction. Let us now see the effect of a thaw of the polar ice.

We have two kinds of polar ice, that which is formed upon the continent, the continent being land covered with snow and ice, or simply ice and that which is formed by the sea itself. The first are evidently only glaciers, like those which are formed in the alpine regions of every climate. They contain absolutely no salt, this is confirmed by the observations of MM. Egédé, Sabie and Wrangle. The second are the waters of the sea frozen, and M. Wrangle informs us that between the 70° and 71° of N. Latitude this congelation does not exceed the depth of nine or ten feet. Thus these enormous masses of floating ice, which rise even four hundred and five hundred feet above the sea, and have at least eight to nine times more thickness under the sea, are glaciers of the former kind, formed upon a base of frozen sea water, which cannot be twenty feet thick ; and this base itself contains so little salt, that it was believed for a long time that it did not contain any.* Thus we may consider the polar ices and the water which runs from them as containing only a minimum of salt, perhaps less than the water of most rivers. As moreover it is detached from only a few sides of the icy platform in comparison with the mass of waters which melt every summer from the surface of these great platforms, and which contain no salt, we may without sensible error, consider the entire mass of the waters which flow each summer from the polar regions as a water without salt, and we can pronounce, without uncertainty, upon the direction of

these waters.

The sea water of these latitudes being like the water of the glaciers, at the temperature of 0, these two waters act, with regard to each other, as their specific gravity impels them, that is to say, the water from the glaciers will glide upon the surface of the sea towards the equator, without sinking at all; for although, during this move

I have proved that salt water in freezing retains a part of its salt. See my Grundiss der theoretischen Physik, T. II. and the Annalen der Physik, T. LVII. p 144. It follows from my experiments that the inferior parts of the ice of sea water must contain a little more salt than the superior.

ment, chemical action may cause salt to be taken up by this soft water, and the winds produce a mechanical mixture with the inferior adjacent beds, still the specific gravity of the chemical and mechanical mixture will be always less than that of the inferior water. Farther let us follow in imagination the pure water of the glaciers with its temperature of 0 to the equator, where the highest temperature of the sea water is 30° C., the pure water will not sink; for the specific difference between one temperature and the other is and the difference between the sea-water under the equátor and pure water, both at the temperature 0, is. Thus it is in no case possible, that the water from the polar icebergs, can get to the bottom of the ocean, to lower its temperature.

We conclude with certainty from all that has been said relative to the temperature of the sea, in its depths, that the bottom of the ocean is about the temperature of 0, of our thermometers, and that this temperature increases with the elevations above the bottom. We conclude that this phenomenon is diametrically contrary to what ought to take place if there existed below the bottom of the ocean a source of heat, which communicated to the ocean its actual temperature and that the ocean proves in this respect, in relation both to cause and effect, the contrary of what observation points out on the continents.

We have shown in the preceding chapters, so complete a difference between the observations made in the interior of the continent, and in the depths of the sea in the range of temperature, that it may be called a contradiction. But there is no real contradiction in nature, and when it is supposed that we have found one, it is our ignorance which makes it so by attributing to one and the same cause, two phenomena which the ocean offers us.

The temperature of the sea diminishes from the surface towards the bottom, at first rapidly, then slowly, and finally during the greatter part of the depth with extreme slowness; whence we conclude agreeably to the experiments of M. Lenz, that at the bottom of the ocean the temperature must be about the zero of our thermometers. We are further obliged to conclude from this, that temperatures are owing to the action of the solar rays, and that the proper temperature of the globe at this depth cannot exceed that indicated by the zero of the thermometer.

But as it has been demonstrated that the hypothesis of a globe ignited internally cannot resolve the marine problem, it is also certain that the temperatures on land which have been observed to increase

with the depth cannot be explained by the action of the solar rays. We are obliged to seek for another cause applicable to continents alone, and explanatory of these results.

This cause is stated in my Physique de la Terre, printed in 1815, namely the volcanic action which took place at the time of the formation of the crust of the globe, and with a much greater energy than is now developed. This ancient volcanic activity, is attested by the tearing and overturning of rocks, by the great number of ancient volcanoes which are now extinct, and which are scattered over almost all latitudes and longitudes, by volcanic productions in large and small masses, which are met with so frequently in places where volcanoes are not to be found, among the number of which may be mentioned basalt and its varieties; which is so scattered that the celebrated Werner was led to believe that basalts the last product of the general precipitation which formed continents that they had formerly entirely covered and were wanting only in places from which the breaking up and mechanical force of the revolutions had removed them.

The great force of volcanic action is demonstrated not only by ancient volcanoes which are still active, but by new volcanoes which are still forming upon continents and islands, in proximity to the shores, but this force is manifested particularly by earthquakes which so often indicate new theatres of action.

Volcanic action has then, ever since the formation of our globe, produced a very elevated temperature, capable of melting the rocks upon which it has exercised its immediate power in all parts of the present continents. One part of this temperature is spread in the interior of the globe in decreasing progression in the direction of the center, without our being able to know whether it has already reached the center, in any sensible quantity. The other part is spread towards the circumference, also in decreasing progression, and is dissipated more or less in the immensity of space. Volcanic operations are at present a remnant, a weak continuation of this great work which still produces unequally disseminated heat, the irregularity of which we might more clearly perceive if it did not take place at so great a depth.

We may add to this that volcanic explosions frequently eject pyrites which have often formed beds upon layers of existing rocks and which have then been covered with new rocks. These beds of pyrites, continually acted on by the water of the atmosphere

percolating through crevices of the rocks, become new sources of partial heat, which being found incomparably nearer the surface than the centers of volcanoes, may sensibly increase, in certain places, the interior temperature at depths which have been reached with the thermometer. These are the waters which furnish us with mineral springs, warm if the water has only a short distance to flow before it comes to light, and if the action of the water upon the pyrites is energetic and the quantity of water small,-cold if the intensity of the action is weak, the course long and the quantity of flowing water considerable. I do not rank in this class the Geyser, the Ricum and other spouting fountains of Iceland, which I regard as immediate products of the volcanoes of that island.

We must also reckon among the number of causes of continental heat, fossil coal, mineral coal of all kinds, the antique remains of an immense vegetation which had existed in the latter periods of the formation of the crust, inflammable substances which are not (even at a moderate temperature) indifferent to the oxygen, either of the atmosphere, or of the water which finds access to them, and whose action may increase even to a kind of volcanic activity, such as are offered us by the sacred fires of Bakou, and other analogous appearances along the shore of the Caspian and Dead Seas; to which we may add those parts of the earth where petroleum and bitumen are formed, as in France, Italy, Germany, Bohemia, China, and North America.

I do not instance the heat produced by the great work of general precipitation, for the first idea of which we are indebted to M. de Humboldt, because this temperature, occurring over the whole surface of the globe, cannot serve to explain the local anomalies, which observation proves to exist.

These considerations explain the great anomalies which are observed in mines relative to the progression of temperature in the interior of the earth. We may add to the principal causes now cited, the slow oxidation of metals in the metallic state, and the chemical changes which some oxides, and even rocks, may undergo, by air, water, and carbonic acid. From these it will necessarily result that the temperature of the exterior strata of continents should be a little higher than the corresponding beds of the ocean, even independently of the difference of action of the solar rays upon liquids and solids, and of the greater evaporation from the surface of the sea than from the surface of the earth, and that the difference of temperature should

be much greater as we advance further into the interior. These considerations would explain to us (even if the exterior refrigeration during winter does not already do it) why M. Scoresby found a slight increase of temperature at increasing depths in the sea between Greenland and Spitzbergen, the ground of these two masses of land, and indeed the bottom of the sea between them, being entirely volcanic.

These same considerations discover to us the possibility of a difference of proper heat between one place and another, at the depth where the thermometer is stationary upon the continents; thus the thermometric observations given in the tables at the commencement of our first chapter, show very palpable differences, at depths where all influence of climate ceases, and which can be attributed only to local circumstances. But as we know no other causes than those just expressed, and as these suffice to account for the phenomena, we think we have solved in all its points, the problem which constitutes the subject of this chapter.*

ART. III.-An Enquiry into the Cause of the Voltaic Currents produced by the action of magnets and electro-dynamic cylinders upon coils and revolving plates; by JOHN P. EMMET, M. D., Prof. of Chemistry in the University of Virginia.—Jan. 1834.

EVERY person, conversant with the history of electro-magnetism, knows how long it was before it became satisfactorily proved that magnetism constantly accompanies the voltaic current ; and, that after Oersted furnished, by a highly important discovery, the most conclusive evidence, it was nearly six years before he arrived at satis

* The partisans of the system of an incandescent globe would perhaps attempt to explain these irregularities of temperature by the difference of conducting power of the different rocks, and of their appendages. But it will be immediately perceived, first, that this explanation will not apply to observations in places near each other, as two pits in one and the same mine, and it will not agree with the results which M. Fourier has drawn from his calculations, relative to the extreme slowness with which heat advances at present from the depth of 30 leagues to the surface. In order to obtain, at the depth of some hundred metres, the remarkable differences of temperature, and the increase of temperature at the same level, it is necessary that the cause of heat should be incomparably nearer these points than would be the incandescent globe; and it is on these accounts that we present the causes of heat that have just been enumerated.