TERRESTRIAL TEMPERATURE. 



TERRESTRIAL TEMPERATURE. 



tion, the distribution of land and water, the itate of the countries 

 from which come the prevailing winds, the vicinity of the tea, the 

 elevation of the land, the electrical cUte of the atmosphere, and 

 numwou* other circumstances and phenomena. Under rUrtiOERA- 

 TIOX or THE GLOBE, the question of the Influence of the proper, or 

 internal, beat of the globe on iU superficial temperature is examined. 

 The condition and properties of the ATMOSI>UEBE are investigated 

 under that title. Under Snow, PERENNIAL, the decrement of tem- 

 perature on """"K'w into the atmosphere is noticed. Under SEA, 

 the temperature of the ocean la considered. Whilst the effects of 

 oceanic currents, glaciers, deserts, Ac., are treated under those heads 

 either in the present division, or in the NATURAL HISTORY and 

 GEOGRAPHICAL DIVISIONS of this Cyclopedia. The important subject 

 of Terrestrial Magnetism is reserved for a distinct article. Here it 

 only remains to speak of some of those general deductions of recent 

 investigators in climatology which have not been specifically mentioned ; 

 and especially have we to do so in reference to isothermal lines, or 

 lines of equal temperature, for an account of which, reference has 

 been made to this article from ISOTHERMAL LIKES ; and to the tempe- 

 rature of the atmosphere over the MO, a branch of the subject which 

 has been referred to the present article from SEA. 



In the primitive condition of the earth, when the globe was a fluid | 

 mass, or in remote geological periods, when its central fluid mass was 

 covered with a comparatively homogeneous crust, the effect of the 

 radiation of its heat on the superficial temperature must, as Mr. 

 Hopkins has shown, have been almost unlimited. But as by this 

 radiation it would continue to part with heat till the superficial tem- 

 perature approximated to that of the circumambient space, unless the 

 radiation were compensated by the generation of heat on the surface, 

 which is known not to hare been the case, it must necessarily happen 

 at some indefinite time (it has been shown to require many millions of 

 years), that the internal heat of the globe would cease to exert any 

 appreciable influence on its superficial temperature. And this is what 

 has now very nearly come to pass [REFRIGERATION or THE GLOBE] ; 

 this influence, according to the calculations of Mr. Hopkins, being now 

 reduced to less than the twentieth of a degree Fahrenheit. In fact, 

 whilst volcanoes, thermal springs, borings for artesian wells, the con- 

 tinuous increase of temperature in descending deep mines, and other 

 phenomena, afford irrefragable evidence of the higher temperature of 

 the interior of the globe, it is as clearly seen that for a short distance 

 from the surface the temperature of the earth is dependent on external 

 heat and moisture, and varies with the seasons of the year and the 

 hours of the day, whilst at a greater, but still small depth which 

 varies according to the latitude of the place, and the conducting power 

 of the rock, but nowhere probably exceeds 100 feet a point is reached 

 at which there is no sensible change of temperature, and which has 

 accordingly been designated the Invariable Stratum. The present 

 influence of the internal heat of the globe, although almost inappre- 

 ciable, may, however, acording to Dove, be regarded as constant ; 

 " lessening the extremes, but not affecting the periods of the variations 

 of temperature at the surface." 



When the influence of the internal heat ceased to be paramount on 

 the surface, changes of temperature must have been in a large measure 

 due to the altered conditions of land and water the elevation of 

 mountain regions, the subsidence of extensive areas, and the conse- 

 quent changes of oceanic currents to glacial action, &c. : circumstances 

 of which the results are clearly traceable in the animal and vegetable 

 remains preserved in the various strata which compose the crust of 

 the earth, and some of which (as notably in the case of the Gulf 

 Stream and the Arctic Current) are shown by the remarkable in- 

 flections of isothermal lines to be distinctly operating now. (Hopkins, 

 ' Trans, of GeoL Soo., 1 and ' Cambridge PhiL Trans.') 



In considering the present temperature of the earth, the sun must 

 be regarded as the only source of heat and the ultimate cause of all 

 climatic change, and hence we arrive at the possibility of ascertaining, 

 amidst all casual fluctuations, a regularly recurring periodicity, annual 

 as well as diurnal, for every variety of geographical position. The 

 bearing of this periodicity on a theory of the general distribution of 

 heat appears to have been first distinctly observed by Kirwan, who 

 (in vol. viii. of the Irish ' Transactions ') constructed a table of monthly 

 temperatures for all parallels of latitude between 10 and 80*. Before 

 the difference of temperature on the same parallel of latitude in the 

 old and new continents was known or regarded, a simple 'formula was 

 thought sufficient to express the temperature at any parallel of 

 terrestrial latitude. The celebrated Tobias Mayer, from such mean 

 temperatures as had in bis time been observed, found that the tem- 

 perature ( (on Fahrenheit's scale) at any place might be represented 

 by T 62* sin. 1 L, where T is the mean temperature at the equator, and 

 L the geographical latitude of the place ; and in 1819 M. Daubuisson 

 ('TraiU! de Oognosie') proposed the more accurate formula (=,27* 

 cos. 1 L (centigrade scale) ; which being adapted to Fahrenheit's scale, 

 considering the mean temperature at the equator to be 81, becomes 

 88* + 49* cos.* L. This formula has been found to serve for tempera- 

 tures in Europe as far north as the latitude of CO* ; but beyond that 

 parallel it is useless, as it supposes the temperature at the geographical 

 pole to be 32, which is much too high. 



It was however reserved for Humboldt to determine (1817) from the 

 registers of observed temperatures in Europe, and from the numerous 



observations made by himself and other travellers in different regions 

 of the world, the constancy of the mean annual temperature of places, 

 and to "connect graphically by line* those points where accurate 

 observations indicated equality of mean periodic temperature." Those 

 lines (or curves supposed to be traced on the surface of the earth) 

 which connect the places where the mean annual temperature is the 

 same he called Itotlimnal tine*. In order to ascertain with the utmoet 

 possible precision the mean temperature of any place from the tables 

 there kept, Humboldt divided the sum of all the temperatures observed 

 in each day at intervals of one hour by the number of observations ; 

 and the sum of all these mean daily temperatures being divided by 

 S65, gave the mean annual temperature. And in determining the 

 series of points for his lines of equal temperature, when there existed 

 no observations on which he could depend, he interpolated the tempe- 

 rature and geographical position between the values of those elements 

 at two or more places where they were well known. 



The diagram on next page represents an orthographical projection, 

 on the plane of the equator, of the principal meridians and parallels of 

 latitude in the northern hemisphere of the earth ; and the strongly 

 marked curves represent the nine isothermal lines whose forms were 

 determined by Humboldt. Their distances from one another are such as 

 correspond on the earth to a change .of mean annual temperature equal 

 to 2'5 degrees of the centigrade thermometer (4'5 of Fahr.), and the 

 most northern curve is that on which the mean temperature is ex- 

 pressed by zero on the former, or 32 on the latter scale. The number 

 on each curve in the diagram expresses, according to Fahrenheit's ther- 

 mometer, the mean annual temperature, at the level of the sea, of all 

 the places through which the curve passes. The centre p represents 

 the pole of the earth, and the longitudes of the meridian lines are 

 numbered eastward and westward from the meridian of Greenwich. 



The isothermal line of 32 passes about 4 southward of Nain, a 

 Moravian settlement on the coast of Labrador ; and under the influence 

 of the gulf stream, makes a remarkable inflexion, ascending as high as 

 North Cape in Lapland, where it abruptly returns southward, and 

 attains its lowest limit in the eastern parts of Asia, about 50* N. lat. 

 Proceeding westward from Labrador the curve crosses the lower 

 extremity of Hudson's Bay, from whence it again tends northwards to 

 the Great Slave Lake, reaching its northern maximum in about 70* N. 

 lat. The positions of the other curves seem to be affected in a greater 

 or less measure by the same influences as act upon the curve just 

 mentioned. In their progress from the western coast of Europe to the 

 eastern coast of America they incline towards the terrestrial equator, 

 yet BO that the southern curves approach near to parallelism with that 

 great circle of the earth. Within the territory of the United States 

 they assume a form which is convex to the equator, and farther 

 west they appear to reascend towards the north. In the isothermal 

 lines of 50 and 60 westward, and those of 40* to 10 eastward of the 

 meridian of Greenwich the curves have their convexities turned north- 

 wards ; and farther eastward they descend towards the equator. The 

 isothermal line of 54'5 is one that has been traced nearly round the 

 earth : commencing at the mouth of the Columbia, on the western 

 coast of North America, it passes south of Council Bluffs, and near tbe 

 city of Washington with its convexity towards the south ; and after 

 crossing the Atlantic it runs between Paris and Bordeaux, from whence 

 it continues to a point a little north of the city of Pekin, where it is 

 again convex towards the equator. 



But it must be remembered that in every country the mean tem- 

 perature varies with the height of the place above the level of the 

 sea; and Humboldt, from observations made as well on the Cordilleras 

 as in Europe, having determined that at every 343 feet the mean 

 temperature of the air is diminished by a quantity equal to that dimi- 

 nution which is consequent on an augmentation of latitude equal to 

 one degree, calculated a table of the corrections which should be made 

 in the curvatures of the isothermal lines at the level of the sea, in 

 order to obtain the forms of those which appertain to points at any 

 given elevation, but this calculation, as pointed out under CLIUATK 

 (col. 968) does not hold with strict accuracy for places situated 

 without the tropics. 



The differences between the mean summer and mean winter tempera- 

 tures Humboldt found to be very considerable at places whose mean 

 annual temperature is the same ; and these differences are not equal in 

 the Old and New Continent. On the isothermal line of 32 in 

 Europe, that difference proved to be equal to 39'6, and in America to 

 54; and on the isothermal line of 68 the differences were respec- 

 tively 21 '6 and 27. He also remarked that the differences between 

 summer and winter are least near the northern, and greatest near the 

 southern bends of the curves. To indicate these variations, Humboldt 

 laid down other lines. The curves formed by connecting, on the 

 isothermal lines, points at which the mean temperature of summer is 

 the same he called IxAheral lines; and those formed by connecting 

 points at which the mean winter temperature is the same Iiochtimal 

 lines : both these systems of lines he found to deviate more than the 

 isothermal lines from the parallels of terrestrial latitude. 



Humboldt, in the first volume of his 'Cosmos,' expressed a hope that 

 not only a clearer insight had thus been gained into the distribution 

 of heat in the atmosphere, but that this " system of Isothermal, 

 Isotbcral, and Isocheimal lines, if gradually perfected by the united 

 efforts of investigators, may prove one of the chief foundations of a 



