Page:Encyclopædia Britannica, Ninth Edition, v. 10.djvu/238

Rh 22-1 fully 900 miles from the Icelandic volcanoes on the one side, and 1100 miles from those of Italy aii_l Sicily on the other. (3.) Bo)-iiags, Wells, and .l[¢'nes. The inﬂuence of the seasonal changes of temperature extends downward from the surface to a depth which varies according to latitude, to the thermal coiidnctivity of the soils and rocks, and per- haps to other causes. The cold of winter and the heat of summer may be regarded as following each other in suc- cessive waves dowiiward, until they disappear along a limit at which the temperature remains constant. This zone of invariable temperature is commonly believed to lie some- where between 60 and 80 feet down in temperate regions. At Yakutsk in eastern Siberia (lat. 62° N .), however, the soil is permanently frozen to a depth of about 700 feet.‘ In Java, on the other hand, a constant temperature is said to be met with at a depth of only :2 or 3 feet? It is a remarkable fact, now veriﬁed by observation all over the world, that below the limit of the inﬂuence of ordinary seasonal cliauges the temperature, so far as we yet know, is nowhere found to diminish downwards. It always rises; and its rate of increment never falls much below the average. The only exceptional cases occur under cir- cumstances not difficult of explanation. On the one hand, the neighbourhood of liot-springs, of large masses of lava, or of other manifestations of volcanic activity, may raise the subterranean temperature much above its normal condition ; and this augmentation may not disappear for many thousand years after the volcanic activity has wholly ceased, since the cooling down of a subterranean mass of lava would necessarily be a very slow process. On the other hand, the spread of a thick mass of snow and ice over any considerable area of the earth's surface, and its continuance there for several thousand years, would so depress the subterranean iso- tlieriiials that for many centuries afterwards there might be a fall of temperature for a certain distance downwards. At the present day, in at least the more _northerly parts of the northern hemisphere, there are such evidences of a former more rigorous climate, as in the well sinking at Yakutsk already referred to.3 Sir William Thomsonf has calculated that any considerable area of the eartli’s surface covered for several thousand years by snow or ice, and retaining, after the disappearance of that frozen covering, an average surface temperature of 13° C., “would during 900 years show a decreasing temperature for some depth down from the surface, and 3600 years after the clearing away of the ice would still show residual effect of the ancient cold, in a half rate of augmentation of temperature downwards in the upper strata, gradually increasing to the whole normal rate, which would be sensibly reached at a depth of 600 metres.” But beneath the limit to which the lllll-JBIICC of the changes of the seasons extends, observations in most parts of the globe show that the temperature invariably rises as we penetrate towards the interior of the earth. According to present knowledge the average rate of increase amounts to 1° 1"-ahr. for every 50 or 60 feet of descent, and this rise is found whether the boring be made at the sea-level or on elevated ground. The subjoined table gives the results of temperature observations at widely separated localties” :— 1 Ilelmersen, Brit. Assoc. Report, 1871. 1 Jiinghiihn’s Jiwa, ii. p. 771. " Professor Prestwich (Inaugural Lecture, 1875, p. 45) has suggested that to the more rapid refrigeration of the earth's surface during this cold period, _and to the consequent depression of the subterraneoiis Ls-other_mal lines, the alleged present comparative quietiide of the volcanic forces is to be attributed, the internal heat not having yet recovered its dominion iii the outer crust. 4 I‘iri(.‘Assor. Ilrrpurts, 1376, Sections, p. 3. 5 See ‘ Reports of Committee on Undergroiinrl Tcmperatu1'e," J},-,'(_ Assoc. Rep. froiu 1868 to 1877. GEOLOGY [ii. cEoc:_'osv. Diikinﬁeld, near Maneliester (‘2040 f t., eoiil measures) 1‘ Fiilir. for every Rose llridge. Wigun (2445 ft.. coal ir.easures)  ,, " 54-3 South llulgniy, Glasgow (525 ft.. coul ineusures) ,, H .41 Kentlsli Town. London (1100 ft., London elny, chalk. giiult, ttzc.) ,, " 54-6 Ln Cliupelle. Paris (660 metres, chalk, ttzc.) ,, ,, 84 Grenelle Well, Paris (179.16 ft, do ,, ,, 56-9 Sr Andre. do. (263 metres, do.)  ,. ,. 56-4 Neu Sulzwerk boring. Westphuliii ('_".'.'l fr.)   ,, ,, -'2-l-o.~i Mcndorff bore, near Luxeinbonrg (2394 ft.)  ,,, 5'.‘-o Bore neur   .. ,, ,, 5.3 Mont Cenis tunnel (5280 ft. below suiiiinit of .lount P us, metamorphic rocks)   ,, ,, ('3) 81 Ynkutsk, Siberia, (656 ft., limestone, &c.. and graiiiie) .... .. ,, ,, 61) (4.) I rre_r]ulcu'ities in the ])ownw(mI fmxrenzent of I[eat.——- While these examples prove a progressive increase of temperature, they show also that this rate of increase is not strictly uniform. The more detailed observations which have been made in recent years have brought to light the important fact that considerable variations in thc rate of increase take place even in the same bore. If, foriiistaiii-e, we examine the temperatures obtained at different depths in the Rose Bridge colliery shaft cited in the foregoing list, we ﬁnd them to read as in the following columns :— Depth in Tempcriitiire Depth in Tenipernture Yards. (Fuhiz). Yards. (F:ihi‘.). 558 ............. .. 78 745 ............. .. 89 605 ............. .. so 761 ............. .. 90.5 630 ............. .. 83 775 ............. .. 91.5 663 ............. .. 85 ............. .. 92 671 ............. .. 86 800 ............. .. 93 679 ............. .. 87 806 ............. .. 93.3 734 ............. .. 881. S15 ............. .. 94 At La Cliapelle, in an important well made for the water- supply of Paris, observations have been taken of the tempera- ture at different depths, as shown in the subjoined table“ :—— Depth in Teniperature Depth in T('lllp(.'l'f|[lll'C Metres (Fulir.). Metres. (l-‘:ilir.). 100 ............. .. 595 500 ............. .. 79:6 200 ............. .. 61 ‘8 G00 ............. .. 75 '0 300 ............. .. 656 660 ............. .. 760 400 ............. .. 69'0 In drawing attention to the temperature-observations at the Rose Bridge colliery—the deepest mine in Great Britain—Professor Everett points out that, assuming the surface temperature to be 49° l*‘ahi'., in the first 558 yards the rate of rise of temperature is 1° for 577 feet; in the next 257 yards it is 1° in 48'2 feet; in the portion between 605 and 671 yards~—a distance of only 198 feet—it is 1° - in 33 feet; in the lowest portion of 432 feet it is 1° in 54 I feet.7 When such irregularities occur in the same vertical shaft, it is not surprising that the average should vary so much in different places. There can be little doubt that one main cause of these ‘variations is to be sought in the different thermal condiic— tivities of the rocks of the earth’s crust. The first accur-ate measurements of the conducting powers of rocks were made by the late Professor J. 1). Forbes at Edinburgh (1837-1845). He selected three sites for his thermometers, one in “ trap-rock” (a porpliyrite of Lower Carboniferous ago), one in loose sand, and one in sandstone, each instru- ment being sunk to a depth of 21- French feet from the surface. He found that the wave of summer heat reached the first instrument on 4th January, the second on 25th December, and the third on 3d November, the trap-rock ‘being by far the worst conductor, and the solid sandstone by far the best.3 The British Association has recently appointed a coin- mittee to investigate this subject in greater detail. Already some important determinations have been iiiade by it re- garding the absolute conductivity of various rocks. As a rule the lighter and more porous rocks offer the greatest 3 “ Report of Committee on Underground Temperature,” I}:-it. Assoc. I{cp., 1873, p. 254. _ 7 “ Report of (‘ominittee on Underground Temperature," Brit. Assoc. Rep. for 1870, p. 31. 5 Trans. Roy. Soc. }:'¢l[n., xvi. 180.