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The water issuing from the back of the crosscut at Dolcoath deposits in considerable quantities a soft reddish-grey precipitate which frequently assumes stalactitic forms. Similar incrustations, although generally less abundant, are found in nearly all deep mines wherever water issues from the vicinity of a vein and flows over the surface of the adjacent rock.

Three analyses made of air-dried specimens of this substance afforded the following results :

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Nos. I. and II. are duplicate analyses of the same specimen, made in my laboratory; No. III. is an analysis of another specimen, made, at my request, by my friend Mr. Dugald Campbell.

It may appear somewhat remarkable that a plentiful deposit of arsenate of iron should be formed from water in which only traces of arsenic could be detected, and in which the amount of iron present is so inconsiderable. On examining, however, the jars in which the water had been collected for analysis, a small amount of a flocculent brownish deposit was found at the bottom of each; this, on being thrown on a filter and subsequently analyzed, was found to contain 31.52 per cent. of arsenic anhydride and 25.27 per cent. of ferric oxide. The ferric arsenate, together with a little clay, had been deposited before the clear water was siphoned off for analysis; this accounts for the small amounts of iron and arsenic retained in solution.

These waters differ materially from those of the lithium spring at Huel Seton, which contain above a thousand grains of solid matter per gallon, while those from the Phoenix Mines and from Dolcoath contain respectively but 14.91 and 46.97 grains per gallon. The Huel Seton water is believed to be derived from the sea by percolation through a fault; the waters, of which analyses are now given, are probably the result of the infiltration of surface-water through the workings of the mines, and through fissures in the respective veins.

XXIII. On Ocean-currents.-Part III. On the Physical Cause of Ocean-currents. By JAMES CROLL, of the Geological Survey of Scotland.

Ν

[With a Plate.]

[Concluded from p. 122.]

The Gibraltar Current.

IN my last paper I proved that it was only the water lying

above the level of the submarine ridge which crosses the Strait of Gibraltar that could exercise any influence in producing circulation between the Atlantic and the Mediterranean*. The water of the Atlantic below the level of this ridge might be as light as air, and that of the Mediterranean as heavy as molten lead; but this could produce no disturbance of equilibrium. It is only the waters lying above the level of this ridge in the two seas that require to balance each other; and if there is no difference of density between the Atlantic and the Mediterranean waters from the surface down to the level of the top of the ridge, then there is nothing that can produce the circulation which Dr. Carpenter supposes. This submarine ridge comes up to within very nearly 100 fathoms of the surface; and according to Dr. Carpenter's own admission, little or no difference of density exists down to that deptht; it follows therefore that there is nothing to produce disturbance of equilibrium, or any such circulation as that which he infers. It is true that in his last expedition he found the bottom-water on the ridge somewhat denser than Atlantic water at the same depth, the former being 1.0292 and the latter 1.0265; but it also proved to be denser than Mediterranean water at the same depth. He found, for example, that "the dense Mediterranean water lies about 100 fathoms nearer the surface over a 300-fathoms bottom, than it does where the bottom sinks to more than 500 fathoms" (§ 51). But any excess of density which might exist at the ridge could have no tendency whatever to make the Mediterranean column preponderate over the Atlantic column, any more than could a weight placed over the fulcrum of a balance have a tendency to make the one scale weigh down the other.

Although Dr. Carpenter has done me the honour to discuss nearly all the objections which I have advanced against his theory, he nevertheless makes no reference whatever to this objection; and this is the more singular, seeing that the expedition, of which his memoir is a report, was chiefly if not solely undertaken for the purpose of establishing the correctness of his theory of the Gibraltar current. If, therefore, there was any one objection advanced by me which he might have been expected to *Phil. Mag., October 1871, pp. 269–272.

+ Proc. Roy. Geog. Soc. Jan. 9, 1871, § 13.

discuss, it was surely that relating to the Gibraltar current; for if the objection referred to be sound, it shows the mechanical impossibility of his theory. It proves that whether there be an under current or not, or whether the dense water lying in the deep trough of the Mediterranean be carried over the submarine ridge into the Atlantic or not, the explanation offered by Dr. Carpenter is one which cannot be admitted. It is incumbent on him to explain either (1) how the almost infinitesimal difference of density which exists between the Atlantic and Mediterranean columns down to the level of the ridge can produce the upper and under currents carrying the deep and dense water of the Mediterranean over the ridge, or (2) how all this can be done by means of the difference of density which exists below the level of the ridge.

We shall now pass to the consideration of Dr. Carpenter's objections to my conclusions regarding the influence of the Gulfstream on climate.

Dr. Carpenter's Objections to my estimate of the Thermal power of the Gulf-stream.

"The doctrine of the extension of the Gulf-stream proper to the polar area, carrying with it a vast amount of equatorial heat, has been advocated with great ability by Mr. James Croll; who, employing the modern method of computing units of heat, essays to prove that the quantity of heat carried from the equatorial area by the Gulf-stream is so enormous, as to be competent not only to do all that Dr. Petermann attributes to it, but a great deal more" (§ 99).

"Without attempting to follow Mr. Croll through his calculations, I may stop to point out what appear to me to be the fallacies of his method; since if this can be proved erroneous, Mr. Croll's great array of figures is utterly valueless " (§ 100).

Now, in order to show that my "array of figures is utterly valueless," it is necessary to prove either that I have overestimated the amount of heat conveyed by the Gulf-stream, or the effects produced by that heat. This is evident; for if I am correct both as to the amount of heat conveyed and the effects produced by that heat, the figures must possess all the value which I claim for them.

We shall now consider how Dr. Carpenter manages to render my estimate of the heat conveyed by the stream "utterly valueless."

"In the first place, in Mr. Croll's preliminary comparison of the temperatures of the northern and southern hemispheres, he altogether ignores the influence on the distribution of heat over the globe which is exerted by the great relative preponde

rance of land in the northern hemisphere . . . . . And to affirm, as Mr. Croll does, that the lower mean temperature of the southern hemisphere is due to the amount of heat transferred over from that hemisphere to the northern by ocean-currents, is to repudiate all that has been established by the researches of meteorologists, as to the relative effects of land and sea," &c. (§ 101).

But what has all this to do with my estimate of the quantity of heat conveyed by the Gulf-stream? Supposing it to be true that I "altogether ignore the influence on the distribution of heat over the globe which is exerted by the great relative preponderance of land in the northern hemisphere," and supposing it to be true that the lower mean temperature of the southern hemisphere is not due, as I have concluded, to the amount of heat transferred over from that hemisphere to the northern by ocean-currents, this cannot in any way affect the value of my figures regarding the amount of heat conveyed by the Gulfstream. It is not true, however, that I ignore and repudiate all that has been established as to the effect of land and sea in producing the difference of mean temperature between the two hemispheres. So far from this being the case, I have devoted an entire paper (Phil. Mag. Sept. 1869) to an examination of the arguments which have been advanced to explain the lower mean temperature of the southern hemisphere. I have given my reasons for concluding that an enormous amount of heat is transferred from the southern hemisphere to the northern by means of currents. These reasons may or may not be satisfactory; but nevertheless they are reasons, not assumptions. It would be needless as well as out of place to repeat these arguments; but I may be permitted simply to refer to one of them, viz. my reason for concluding that a great portion of the heat possessed by the Gulf-stream is derived from the southern hemisphere. If all that heat came from the northern hemisphere, it could only come from that portion of the Atlantic, Caribbean Sea, and Gulf of Mexico which lies to the north of the equator. The entire area of these seas, extending to the tropic of Cancer, is about 7,700,000 square miles. Were the heat which passes through the Straits of Florida derived exclusively from this area, the following Table would then represent the relative quantity per unit surface possessed by the Atlantic in the three zones, assuming that one half of the heat of the Gulf-stream passes into the arctic regions, and the other half remains to warm the temperate regions*:

From the Equator to the Tropic of Cancer.
From the Tropic of Cancer to the Arctic Circle
From the Arctic Circle to the North Pole . .

* See Phil. Mag. for October 1870, p. 258.

773

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848

610

If a very large proportion of the heat possessed by the Gulfstream be not derived from the southern hemisphere, these figures show that the Atlantic, from the equator to the tropic of Cancer, should be as cold as from the tropic of Cancer to the North Pole. But independently of this, a mere glance at a chart of ocean-currents will show that the Gulf-stream is chiefly fed by a current from the southern hemisphere. Without such a transference of heat it would be impossible to account for the N. Atlantic being actually 5 degrees warmer than the S. Atlantic. Again, Dr. Carpenter remarks:-"In computing the heat imparted by the sun to the equatorial area from which the Gulfstream is fed, Mr. Croll assumes that the heat, being wholly taken up by the water of the ocean, is transferred by its currents towards the polar regions; whilst of the heat which falls upon the land, a very large proportion is lost by radiation, passing off into the stellar spaces" (§ 102).

But this cannot in any way affect the correctness of the result of my computation of the amount of heat conveyed by the Gulfstream. What I have maintained in my papers is, that the quantity of heat conveyed by the winds from intertropical land is trifling to that which is conveyed by currents from intertropical seas. Dr. Carpenter says that "the heat lost by evaporation from the sea must be far greater than that lost by radiation from the land." According to the laws of radiation and absorption, all the heat received from the sun by the land must be either reflected or radiated from its surface, with the exception of the small portion which is communicated to the air in contact with that surface. In fact it is by radiation that the sea as well as the land loses the greater part of its heat-the only difference in the two cases being, that heat radiated from the sea is absorbed more readily by the air than heat radiated from the land, and consequently produces a greater influence on climate.

Dr. Carpenter continues :-" Mr. Croll leaves almost entirely out of the question the N.E. transportation of an enormous amount of heat from the general surface of the Atlantic by the agency of the aqueous vapour thus raised; although the importance of this agency has been insisted on by the most eminent authorities in meteorology" (§ 103).

Here again, however, my estimate of the heat conveyed by the Gulf-stream, or the effects which it produces, cannot possibly be affected by the above consideration. It seems to be forgotten in this objection, that, were it not for the Gulf-stream, the quantity of heat which could possibly be derived from the Atlantic would be so much the less by an amount equal to that conveyed by the stream. Besides all this, there may be other

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