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Thermal Springs. Volcanoes

only the temperature, but other circumstances,—the discharge of thousands of gallons of water daily from some of these springs, the water being equally heated, and the flow of it continuing uninterruptedly for centuries.* The water rises spontaneously, and in thermal springs the power which brings it to the surface is, according to Buff and Bunsen, the pressure of steam produced below by intense heat. At 509° steam has an elasticity equal to fifty atmospheres, and it would sustain a pressure of a water column 1700 feet in height. It is, in fact, in the condition of superheated steam, and would have the power necessary to lift up the liquid column pressing upon it. As it reached the surface it would pass to the temperature of 2120, or lower. This view is confirmed by the observations of Bunsen on the Geyser springs in Iceland. At a depth of 62 feet below the surface, the water column, during the interval between two following discharges of the Geyser, has been found to have a temperature of 261°; at 47 feet, 253°; at 32 feet, 248°; at 16 seet, 223°; and at the surface, 185°.

575. Of the evidence furnished by volcanoes nothing need be said. We have in these ample proofs of the existence of an intense heat in the fusion of such a substance as lava, and its ejection in the molten state. Volcanoes are found active from Iceland to the Antarctic regions, and in the extinct state in great numbers over the whole of the globe. The lava which is thrown up appears to be very similar in composition in all parts, and the phenomena attending an eruption are the same. These facts appear to show a common cause, a common source, and they point to a common origin at great depths below the surface. They serve as terrestrial pyrometers.

All calculations regarding the amount of heat in the substance of the earth can be only approximate. Professor Buff, of Giessen, has estimated that the increase of heat in the earth's crust, below the stratum at which solar influence is observed to penetrate, amounts to 1.8° Fahrenheit for every hundred feet of depth. On this datum, he says, at a depth of ten thousand feet we should find a temperature at which water would boil, and at 120,000 feet, i.e., abort twenty-three miles below the surface, and onlyth of the earth's radius, there would be a heat of nearly 2200° Fahrenheit, at which cast-iron melts and basalt runs like water.

Graham, relying on the experiments of Cordier, assigns an increase of one degree for every fifteen yards below the invariable

The well of Grenelle in Paris delivers daily, at a height of 86 feet above the surface, and at a temperature of 82°, 744,490 gallons of water.

Deep sea Temperatures.

391

stratum, equivalent to 116° for each mile. Admitting this rate of increase, we should have in a depth of about 30 miles, a temperature of 3580°, sufficient to render malleable iron fusible. From the very slight extent to which the crust of the earth has been penetrated we are bound to speak cautiously on this subject. The deepest mine, with an exception mentioned below, extends to about half a mile (2640 feet) below the level of the sea. This is not more than the sixteen-thousandth part of the earth's diameter.

576. This subject was considered to be of sufficient importance to be brought before the Parliamentary Committee appointed to inquire into the probable exhaustion of our coal-mines in 1871. It was alleged that when a depth of 3420 feet was reached, the temperature would be 98° (blood heat), and that this would put a stop to the further working of coal. At this time the two deepest English pits are reported to have been at Pendleton (2214 feet) and Rosebridge, near Wigan (2424 feet). In this, the deepest mine in England, the temperature of the coal was found to be 931°. At Charleroi, in Belgium, there is a mine 2640 feet (half a mile) deep, and at Verviers a shaft is reported to have reached a depth of 3511 feet, or nearly 100 feet below the supposed workable limit. The temperature of the strata at this great depth has not been stated, but nothing has occurred to prevent the working of the mine.

577. Deep-sea Temperatures.—It might be supposed that from the great depths at which the sea has been fathomed, stronger evidence would be obtained than in the exploration of mines; but from the fact that water has its greatest density at about 40°, and that heat is rapidly distributed by convection or circulation through the vast mass of the ocean, this source of evidence fails.

The most valuable observations on this subject have been either made or collected from authentic sources by Dr. Carpenter. Thermometers which are not protected, are subjected to enormous pressure, i.e., one ton per square inch for every 800 fathoms, or 4800 feet in depth.* Then there was an uncertainty in determining temperature at certain depths, as this might change during the time required to raise the instrument to the surface. Messrs. Negretti and Zambra have overcome this difficulty by constructing a deepsea thermometer, which will allow the temperature to be accurately taken for any known depth.†

* A column of sea water one inch in section and 800 fathoms high, weighs just a ton.

The principie upon which this instrument is constructed is as simple it is ingenious. The thermometer, well protected in the bulb, has a

392

Temperature of the Atlantic.

The pressure of sea-water, however, has hitherto formed a limit to the use of the thermometer. At a short distance north of St. Thomas's, in the West Indies, an extraordinary depression of the bottom of the Atlantic was noticed. The depth was 3800 fathoms (about 4 miles). The two protected thermometers which had previously withstood the pressure of nearly 4 tons on the square inch were both crushed by the pressure of 4 tons to which they were here subjected, and thus the temperature of this deep stratum could not be determined. As far down as the thermometer could be used it was 340.*

The subjoined engraving (fig. 161) represents a section of the Mid-Atlantic Ocean, from 38° N. to 38° S. :

This sectional chart is reduced from a larger engraving con

syphon-like form, with parallel legs. The thermometrical scale is pivoted on a centre, and it is inclosed within four strong bars. As it descends, the mercury acquires the temperature of the medium through which it is passing. So soon as the descent ceases, and a reverse motion is given to the line sc as to raise the thermometer, the instrument, by an ingenious mechanical arrangement, turns over on its centre, and makes a half revolution. The column of mercury then falls into the dilated syphon head at the top, and thence into the right hand tube, where it remains, indicating on a graduated scale, the exact temperature of the sea at the spot at which it was turned over. As the mercury breaks off at one particular point, no more can thus be transferred from one side to the other than that portion which indicates the exact temperature.

* A still deeper sounding was made in the Challenger expedition, near the north coast of New Guinea. A strong hempen line was employed, and

Deep-sea Soundings.

393

structed by Dr. Carpenter. It shows the lines of equal oceanic temperature (isothermal) from the surface to a depth of 2600 fathoms, or 15,600 feet below the level of the Atlantic Ocean.

The temperature observed at this great depth was 35'6°. The thermometer rose as it was drawn to the surface-at 1800 fathoms it was 36°; at 700 fathoms, 40°; at 300 fathoms, 60°; and at the surface, 71°. This was in latitude 38° N. At the Equator the bottom was sounded at 2500 fathoms, and the temperature of the water was 324°, the water at the surface being 78°. In latitude 38° S., at 2150 fathoms, the temperature was 33'5°, that of the surface being 54°. This curious chart shows that there are isothermal lines in the depths of the sea as on the surface of the globe, and that similar temperatures are found at different depths, varying with the latitude north and south.*

578. In these results we find no evidence whatever of terrestrial

this was run out to the extent of 4500 fathoms, or more than five miles, the greatest depth to which the ocean has been sounded with any accuracy. Three thermometers out of four were crushed in this experiment by the enormous pressure.

*It is a remarkable fact that the low temperature and great pressure which exist at the bottom of the Atlantic, are not inconsistent with the existence of invertebrate animals. Dr. Carpenter has very properly pointed out that an undue importance has been attached to the supposed effect of pressure on animal life. As he justly observes, the pressure is equal on all sides, and the pores of these animals are filled with water, not air, so that the pressure is neutralized.

Professor Thompson, one of the seventeen members of the Challenger Expedition, found that life extended to very great depths in the Atlantic Ocean. It was represented by all the marine invertebrate groups. He procured from a depth of 14,490 feet a dentalium, one or two crustaceans, several annelids, and a new crinoid with a stem four inches long, several starfishes, two hydroid zoophytes, and many foraminifera. The fauna at this depth, owing to the cold, has a dwarfed and Arctic character, no doubt from the influence of the great Arctic current. At from 4800 to 5400 feet below the surface, the temperature being about 40°, the fauna was specially characterized by the great abundance of vitreous sponges

It is highly probable, as Dr. Carpenter has suggested, that the presence of life in the deep Atlantic, is mainly owing to the thermal circulation of the waters. It is this which ensures that degree of oxygenation which is really necessary to the maintenance of animal life. By variations of temperature and by increased specific gravity from evaporation, an oceanic

394

Temperature of the Mediterranean.

heat. The greatest depth at which accurate observations were made may be taken at three miles, six times as far below the sea level as any mine below the surface of the earth. But even this great depth would represent a proportion of only the two thousand six hundredth part of the earth's diameter.

There is reason to believe that the temperature in the depths of the Atlantic, is kept low by the Polar current, for in the Mediterranean, an inland sea, this low temperature is not found.* During the winter months the western basin of this sea averages about 54° at the surface, and this temperature is maintained with extraordinary uniformity to the bottom, at 1500 or 1600 fathoms (nearly 10,000 feet), while in the Atlantic, at an equal depth, the temperature was about 35°. In the summer months it is only the surface layer that is affected by solar radiation, the temperature from 50 fathoms downwards remaining constant at 54° throughout the year.

From the observations made by Dr. Carpenter it would appear that the line of 39° (maximum density of water) was generally reached at 1000 fathoms, and that it is an error to suppose that by reason of increase of density, this temperature is always found in the lowest stratum of sea-water.

Although, therefore, owing to the peculiar laws which govern the

circulation is established which brings every drop of water in turn to the surface. Carbonic acid is thereby removed and is replaced by oxygen.*

* The Mediterranean is strongly contrasted with the Atlantic in this respect. The bottom temperature ranges from 54° to 56°, showing the entire absence of a Poiar current, hence there is no thermal circulation and no life is found below 250 fathoms. It is a great inland basin from 1600 to 2000 fathoms deep (12,000 feet), and the submarine ridge at the Straits of Gibraltar, which is only 200 fathoms or 1200 feet deep, cuts this sea completely off from the great Polar stream passing over the bottom of the Atlantic. For further information on these remarkable thermal conditions of the ocean, the reader may refer to Dr. Carpenter's essay above quoted. Vertebrate animals could not exist at the great depths at which the invertebrate are found. Captain Scoresby noticed in one of his expeditions that a sperm whale ran out 700 fathoms, or 4200 feet of line. It plunged vertically into the sea, but it is most probable that it did not descend below one fourth of this depth. This would represent an increased pressure of nearly 500 pounds on every square inch of its body.

*On the Conditions which determine the Presence or Absence of Life on the Deep Sea Bottom,' by Dr. W. B. Carpenter, pp. 20, 25.