54 



MEAN TEMPERATURE OF THE EARTH. 



and of the chemical transformations which 

 solid and fluid bodies undergo under enormous 

 pressures. Most difficult of all, for our powers 

 of comprehension, is the conception of the 

 boundary line betwixt the fluid masses of the 

 interior and the concrete mineral species of the 

 outer crust of the earth, of the gradual increase 

 of solidity in the strata, and the state of tena- 

 cious semi-fluidity of earthy matters, to which 

 the known laws of hydraulics can only apply 

 under considerable modifications. The sun and 

 moon, which keep the ocean in a state of alter- 

 nate ebb and flow, act in all likelihood even 

 down to these depths. Beneath a vault of al- 

 ready consolidated mineral strata, periodical 

 rises and falls of a molten mass may, indeed, 

 be readily enough conceived as taking place, 

 and occasioning inequalities in the pressure ex- 

 erted against the vault. The amount and the 

 influence of such oscillations can, however, be 

 but small ; and if the relative position of the 

 attracting heavenly bodies must here also pro- 

 duce spring-tides, it is still certain that the con- 

 cussions of the earth's surface which take place, 

 are not to be ascribed to these, but to other 

 more powerful internal forces. There are 

 groups of phenomena, the existence of which 

 it is still useful to adduce in illustration of the 

 universality of the attractive influences of the 

 sun and moon upon the external and internal 

 life of the globe, however little we may feel 

 ourselves in a condition to determine numeri- 

 cally their amount. 



From experiments on Artesian wells, which 

 agree pretty closely, the temperature of the 

 upper crust of the earth appears, on an aver- 

 age, to increase 1° of the centigrade thermom- 

 eter for each 92 Paris feet in perpendicular 

 depth. Did this increase go on in arithmetical 

 progression, then, as I have already had occa- 

 sion to observeC^"^), would a granitic stratum 

 at the depth of 5f^ geographical miles (from 

 four to five times the depth of the highest peak 

 in the Himalaya range) be in a molten state. 



In the body of the earth there are three kinds 

 of motion of heat to be distinguished : the first 

 is periodical, and, according to the position of 

 the sun and the season of the year, alters the 

 temperature of the earth's strata according as 

 the heat penetrates from above downwards, or 

 as it passes in the same way from below up- 

 wards. The second kind of motion is likewise 

 an effect of the sun, and is of extraordinary 

 slowness : part of the heat which has pene- 

 trated the equatorial regions is propagated 

 along the interior of the crust of the earth 

 towards the poles, and there escapes into the 

 atmosphere and distant space. The third kind 

 of motion is the slowest of all : it consists in 

 the secular cooling of the body of the earth, in 

 the dissipation of the small amount of the prim- 

 itive heat of the planet which at the present 

 time is still given off from its surface. This 

 loss which the central heat suffers was very 

 considerable at the epochs of the oldest revolu- 

 tions of the globe ; since the commencement 

 of the historical period, however, it is scarcely 

 mensurable by our instruments. The surface 

 of the earth, from the foregoing view, is inter- 

 mediate between the red heat of the interior 

 strata, and the temperature of space, which is 

 probably below the congealing point of mercury. 



The periodical variations of temperature 

 which the altitude of the sun and the meteoro- 

 logical processes of the atmosphere occasion, 

 are propagated in the interior of the earth, but 

 only to very small depths. This slow conduc- 

 tion of heat by the ground, however, lessens 

 the loss of warmth in the winter, and is favour- 

 able to deeply-rooted trees. Points which lie 

 at different depths in a vertical line come to 

 the maximum and minimum of the communi- 

 cated temperature in very different times. The 

 more distant they are from the surface, the 

 smaller are the differences of these extremes. 

 On the continent of Europe, between the paral- 

 lels of 48° and 52°, the stratum of invariable 

 temperature occurs at from 55 to 60 feet deep ; 

 even at half this depth the oscillations of the 

 thermometer, in consequence of the influence 

 of the seasons, scarcely amount to half a de- 

 gree. In tropical climates, on the contrary, 

 the stratum of invariable temperature is met 

 with at no more than a foot below the surface ; 

 and this fact has been used by Boussingault, 

 in an able manner, as a convenient and, in his 

 opinion, accurate way of determining the mean 

 temperature of the air of a place(^'*'). This 

 mean temperature of the air at a determinate 

 point, or in a group of points of the surface ly- 

 ing near to one another, is, in a certain meas- 

 ure, the fundamental element of the climatic 

 relations, and also of the relations in reference 

 to civilization of a country ; but the mean tem- 

 perature of the whole surface is very different 

 from that of the earth itself The oft-repeated 

 questions, whether, in the course of centuries, 

 this has suffered any considerable change? 

 whether the climate of a country has become 

 deteriorated 1 whether the winters have not 

 become milder, and the summers in the same 

 proportion colder 1 can only be decided by the 

 thermometer ; and the discovery of this instru- 

 ment scarcely dates three half-centuries back ; 

 its rational application no more than about 120 

 years. The nature and novelty of the means, 

 therefore, prescribe very narrow bounds to in- 

 quiries into the temperature of the air. It is 

 quite otherwise with the solution of the groat 

 problem of the internal heat of the whole globe. 

 In the same way as from the unaltered rate of 

 a pendulum we can conclude on the unchanged 

 preservation of its temperature, so does the 

 unaltered velocity of rotation of the earth on 

 its axis inform us of the degree of stability of 

 its mean temperature. This perception of the 

 relations between the length of the day and the 

 earth's temperature, is one of the most brilliant 

 applications of a long knowledge of the heaven- 

 ly motions to the thermal condition of our plan- 

 et. The velocity of rotation of the earth, to 

 wit, depends on its volume : precisely as the 

 axis of rotation of the mass that was cooling 

 gradually by radiation would become shorter, 

 so through diminution in temperature must the 

 velocity of rotation be increased, and the length 

 of the day be abridged. Now by a comparison 

 of the secular inequalities of the moon's mo- 

 tions with the eclipses that have been observed 

 in the more ancient times, it appears that since 

 the age of Hipparchus, for full 2000 years there- 

 fore, the length of the day has not varied by the 

 one-hundredth part of a second. From this, 

 again, and, within the utmost limits of the de- 



