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Article INTERNAL TEMPERATURE OF THE EARTH. ← Page 2 of 3 →
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Internal Temperature Of The Earth.
that when the same quantity of heat passes through superimposed strata of different conductive powers , the increase of depth corresponding to a given increase of temperature ( as 1 ° is in any stratum proportional to the conductive power . Consequently , if the conductive power of the lower portions of the earth ' s solid crust be greater than that of the thin upper portion of it through which man has been able to
penetrate , the depth to which we must proceed to arrive at a certain temperature ( as that of fusion for the lower rocks ) will be proportionally greater . The precise , nature of the rocks situated at a great depth can only be judged of by analogy with those which are accessible to us ; but those geologists who adopt the conclusion of
the extreme thinness of the earth ' s crust will doubtless admit that its inferior part must be of igneous ori gin , and must therefore be allowed to bear a certain resemblance to igneous rocks on the surface of tho earth . Mr . Hopkins had recently made a great number of experiments on the conductive powers of various rocks . That of the softer sedimentary rocks , which arc great absorbents of water , is very much
increased by the quantity of moisture they contain ; but taking chalk , one of the best absorbents , its conductive power , even when saturated , is not half so great as that of some of the igneous rocks on which Mr . Hopkins had experimented . Calcareous , argillaceous , and siliceous substances , reduced to fine powder , stand , with reference to their conductive powers ,
in the order in which they are now mentioned , the conductivity of tho first being the least ; and when in a compact state , all that contributes to give a hard aud crystalline character to the substance , and continuity to the mass through which the heat is conducted , increases the conductive power . These considerations lead to the conclusion that the
conductivity of the inferior portions of the earth ' s solid crust must be much greater , and may be very much greater , than that of the less consolidated and mere superficial sedimentary beds . Moreover , tho temperature of fusion of certain substances , as Mr . Hopkins had shown by experiment , is much increased by great pressure ; and by analogy it may be concluded that such would , at least in some considerable degree , be the case with the mineral matter of the earth ' s crust .
The chalk is that formation in which the most numerous and some of the best observations on terrestrial temperatures have been made ; and is would seem impossible to conclude , from actual experiment and the considerations above stated , that its conductive power' can exceed one-third of that of the inferior rocks , and may not improbably be a considerably smaller
fraction of it . Now the increase of depth in the chalk corresponding to au . increase of one degree Farenheit is well ascertained to be very nearly sixty feet , and therefore the rate of increase in the inferior rocks must probably be at least three times as great as in the chalk , and may be very considerabl greater still . Hencesupposing that the
thicky , ness of the solid crust would be about sixt y miles , if the conductive power of its lower portion were equal to that of chalk , its actual thickness must probably be at least about two hundred miles , and may be considerably greater , even if we admit no other source of terrestrial heat than the central heat here contemplated .
^ There is also another way of investigating the thickness 01 the earth ' s crust , assuming the whole terrestrial mass to consist of a fluid nucleus inclosed in a solid envelope . If the earth were accurately spiherical , instead of being spheroidal , . *^ of rotation would always remain exactly parallel to self , on the same principle as that on which the gyroscope
preserves , in whatever position it may be held , the parallelism ° i the axis about which it rotates . But the attraction of the Jii . and moon on the protuberant equatorial portions of the th * ' S ™ C £ rtlses a progressive change in the position of ne earth ' s axis , by virtue of which the north pole , or that i wt in the heavens to which the northern extremity of the
earths axis is directed , instead of being stationary , describes a circle on the surface of the heavenly sphere about a fixed point in it called the pole of the ecliptic , with a radius of nearly twenty-three and a half degrees , equal to the inclination of the equator to the ecliptic , or tho obliquity . The whole of this revolution is completed in about twenty-five thousand years ; but , as follows from what has just been stated ,
without any change , beyond small periodical ones , in the obliquity . A corresponding change of position must manifestly take place also in the position of the equinoxes , wliich have thus a motion along the ecliptic in a direction opposite to that in which the signs of the zodiac arc reckoned , completing a revolution in the period above mentioned of twentyfive thousand years . It is called the precession of tho
equinoxes . This processional motion has been completely accounted for under the hypothesis of the earth ' s entire solidity , and that of a certain law according to which the earth's density increases in approaching its centre ; but some years ago Mr . Hopkins investigated the problem with the view of ascertaining how far the observed amount of precession might be consistent
with the existence of a fluid nucleus . Tho result was , that such could only be the case provided the thickness of tho solid shell were much greater than that which , as above stated , has been supposed by many geologists . The numerical result was , that the least admissible thickness of the crust must be about one-fifth of the earth ' s radius ; but , without assigning any great importance to an exact numerical result , Mr . Hopkins had a full confidence in the investigation , as showing that the thickness of the crust could not be so small as two
or three hundred miles , and consequently that no geological theory can be admitted which rests on the hypothesis of tho crust being nearly as thin as it has been frequently assumed to bo . The influence of the interior fluidity on the processional motion above described is due to the difference between the
motions wliich the attractions of tho sun and moon tend to produce on a solid mass in one case , and a fluid mass on tho other . It has been recently stated , as an objection to this investigation , that the interior fluid mass of the earth may move in the same manner as if it were solid . The only reply which could be given to such an objection was , Mr . Hopkins
conceived that it was mechanically impossible that these motions should be the same , though the resulting processional motion for the solid crust , under certain conditions , to lie determined only by the complete mathematical solution of the problem , might be the same as if tho whole mass were solid . The effect of the attractions of the sun and moon also depends on the ellip ticity of the inner surface of the solid shell ; and it has been said that since that ellipticity depends on the law
of the earth s density , which can only be imperfectly known , no result can be depended on which involves that ellipticity . This was not a correct statement of tho problem . It was assumed , in the solution referred to , that the ellipticity of tho inner surface would depend partly on the law of density and partly on the forms of the isothermal surfaces .
Mr . Hopkins had supposed it possible , at the timo he was engaged iu this investigation , that a surface of equal solidity might approximate to a surface of equal pressure ; ho has now experimental reasons for believing that it must approximate much more nearly to an internal surface of equal temperature . Now for depths greater , probably much greater ,
than thoso wliich have often been supposed to correspond to the thickness of the earth ' s solid crust , there is no doubt that the internal isothermal surfaces have a greater ellipticity than , the external surface itself ; a conclusion wliich is independent ofthe law of density . Hence a like conclusion will hold with reference to the internal surface of the shell , if it approximate
sufficiently to the surface of equal temperature , and this is the conclusion most unfavourable to the thin shell supposed by some geologists .
Note: This text has been automatically extracted via Optical Character Recognition (OCR) software.
Internal Temperature Of The Earth.
that when the same quantity of heat passes through superimposed strata of different conductive powers , the increase of depth corresponding to a given increase of temperature ( as 1 ° is in any stratum proportional to the conductive power . Consequently , if the conductive power of the lower portions of the earth ' s solid crust be greater than that of the thin upper portion of it through which man has been able to
penetrate , the depth to which we must proceed to arrive at a certain temperature ( as that of fusion for the lower rocks ) will be proportionally greater . The precise , nature of the rocks situated at a great depth can only be judged of by analogy with those which are accessible to us ; but those geologists who adopt the conclusion of
the extreme thinness of the earth ' s crust will doubtless admit that its inferior part must be of igneous ori gin , and must therefore be allowed to bear a certain resemblance to igneous rocks on the surface of tho earth . Mr . Hopkins had recently made a great number of experiments on the conductive powers of various rocks . That of the softer sedimentary rocks , which arc great absorbents of water , is very much
increased by the quantity of moisture they contain ; but taking chalk , one of the best absorbents , its conductive power , even when saturated , is not half so great as that of some of the igneous rocks on which Mr . Hopkins had experimented . Calcareous , argillaceous , and siliceous substances , reduced to fine powder , stand , with reference to their conductive powers ,
in the order in which they are now mentioned , the conductivity of tho first being the least ; and when in a compact state , all that contributes to give a hard aud crystalline character to the substance , and continuity to the mass through which the heat is conducted , increases the conductive power . These considerations lead to the conclusion that the
conductivity of the inferior portions of the earth ' s solid crust must be much greater , and may be very much greater , than that of the less consolidated and mere superficial sedimentary beds . Moreover , tho temperature of fusion of certain substances , as Mr . Hopkins had shown by experiment , is much increased by great pressure ; and by analogy it may be concluded that such would , at least in some considerable degree , be the case with the mineral matter of the earth ' s crust .
The chalk is that formation in which the most numerous and some of the best observations on terrestrial temperatures have been made ; and is would seem impossible to conclude , from actual experiment and the considerations above stated , that its conductive power' can exceed one-third of that of the inferior rocks , and may not improbably be a considerably smaller
fraction of it . Now the increase of depth in the chalk corresponding to au . increase of one degree Farenheit is well ascertained to be very nearly sixty feet , and therefore the rate of increase in the inferior rocks must probably be at least three times as great as in the chalk , and may be very considerabl greater still . Hencesupposing that the
thicky , ness of the solid crust would be about sixt y miles , if the conductive power of its lower portion were equal to that of chalk , its actual thickness must probably be at least about two hundred miles , and may be considerably greater , even if we admit no other source of terrestrial heat than the central heat here contemplated .
^ There is also another way of investigating the thickness 01 the earth ' s crust , assuming the whole terrestrial mass to consist of a fluid nucleus inclosed in a solid envelope . If the earth were accurately spiherical , instead of being spheroidal , . *^ of rotation would always remain exactly parallel to self , on the same principle as that on which the gyroscope
preserves , in whatever position it may be held , the parallelism ° i the axis about which it rotates . But the attraction of the Jii . and moon on the protuberant equatorial portions of the th * ' S ™ C £ rtlses a progressive change in the position of ne earth ' s axis , by virtue of which the north pole , or that i wt in the heavens to which the northern extremity of the
earths axis is directed , instead of being stationary , describes a circle on the surface of the heavenly sphere about a fixed point in it called the pole of the ecliptic , with a radius of nearly twenty-three and a half degrees , equal to the inclination of the equator to the ecliptic , or tho obliquity . The whole of this revolution is completed in about twenty-five thousand years ; but , as follows from what has just been stated ,
without any change , beyond small periodical ones , in the obliquity . A corresponding change of position must manifestly take place also in the position of the equinoxes , wliich have thus a motion along the ecliptic in a direction opposite to that in which the signs of the zodiac arc reckoned , completing a revolution in the period above mentioned of twentyfive thousand years . It is called the precession of tho
equinoxes . This processional motion has been completely accounted for under the hypothesis of the earth ' s entire solidity , and that of a certain law according to which the earth's density increases in approaching its centre ; but some years ago Mr . Hopkins investigated the problem with the view of ascertaining how far the observed amount of precession might be consistent
with the existence of a fluid nucleus . Tho result was , that such could only be the case provided the thickness of tho solid shell were much greater than that which , as above stated , has been supposed by many geologists . The numerical result was , that the least admissible thickness of the crust must be about one-fifth of the earth ' s radius ; but , without assigning any great importance to an exact numerical result , Mr . Hopkins had a full confidence in the investigation , as showing that the thickness of the crust could not be so small as two
or three hundred miles , and consequently that no geological theory can be admitted which rests on the hypothesis of tho crust being nearly as thin as it has been frequently assumed to bo . The influence of the interior fluidity on the processional motion above described is due to the difference between the
motions wliich the attractions of tho sun and moon tend to produce on a solid mass in one case , and a fluid mass on tho other . It has been recently stated , as an objection to this investigation , that the interior fluid mass of the earth may move in the same manner as if it were solid . The only reply which could be given to such an objection was , Mr . Hopkins
conceived that it was mechanically impossible that these motions should be the same , though the resulting processional motion for the solid crust , under certain conditions , to lie determined only by the complete mathematical solution of the problem , might be the same as if tho whole mass were solid . The effect of the attractions of the sun and moon also depends on the ellip ticity of the inner surface of the solid shell ; and it has been said that since that ellipticity depends on the law
of the earth s density , which can only be imperfectly known , no result can be depended on which involves that ellipticity . This was not a correct statement of tho problem . It was assumed , in the solution referred to , that the ellipticity of tho inner surface would depend partly on the law of density and partly on the forms of the isothermal surfaces .
Mr . Hopkins had supposed it possible , at the timo he was engaged iu this investigation , that a surface of equal solidity might approximate to a surface of equal pressure ; ho has now experimental reasons for believing that it must approximate much more nearly to an internal surface of equal temperature . Now for depths greater , probably much greater ,
than thoso wliich have often been supposed to correspond to the thickness of the earth ' s solid crust , there is no doubt that the internal isothermal surfaces have a greater ellipticity than , the external surface itself ; a conclusion wliich is independent ofthe law of density . Hence a like conclusion will hold with reference to the internal surface of the shell , if it approximate
sufficiently to the surface of equal temperature , and this is the conclusion most unfavourable to the thin shell supposed by some geologists .