400 



NA TURE 



[Feb. 26, i ! 



been made, and full particulars published in the Annual Reports 

 of 1868-83. 



But notwithstanding the precautions taken, and the accuracy 

 of the experiments, they present very wide differences in the 

 thermometric gradient, ranging from under 30 to above 1 20 feet 

 per degree F. Consequently different writers have adopted 

 different mean values. On the Continent one of 30 m. per 

 degree C. has been commonly adopted, while in this country 

 some writers have taken a mean of 50 feet per degree, and others 

 of 60 feet or more. The object which the author has in view is 

 to see whether it is not possible to eliminate the more doubtful 

 instances, and to bring the probable true normal gradient within 

 narrower limits. In so doing he confines himself solely to the 

 geological side of the inquiry. 



In a general list, Table I., he gives all the recorded observa- 

 tions in the order of date. The list embraces observations at 

 530 stations in 248 localities. The most reliable of these he 

 classifies under three heads in Tables II., III., and IV. 



(1) Coal mines. 



(2) Mines other than coal. 



(3) Artesian wells and bore-holes. 



To which tunnels are added in a supplement. 



The author then proceeds to point out that the gradients given 

 in many of the earlier observations were wrong in consequence 

 of neglecting the height of the surface, and from the exact mean 

 annual temperature of the locality not being known. They also 

 differed amongst them elves from taking different surface tem- 

 peratures, and starting from different datum levels. To these 

 he endeavours to assign a uniform value. 



The essential differences in the results in several tables depend, 

 however, upon dissimilar geological conditions, which unequally 

 affect the conductivity of the strata, and disturbing causes of 

 different orders. In the mines the latter are : — 



(1) The currents established by ventilation and convection. 



(2) The circulation of underground waters. 



(3) Chemical reactions. 



(4) The working operations. 

 And in artesian wells — 



(1) The pressure of the water on the thermometers. 



(2) Convection currents in the column of water. 



In the latter experiments pressure has been thoroughly guarded 

 against, but against the subtle influence of the other causes, 

 though long known, it is more difficult to guard. 



Coal Mines. — The author then proceeds seriatim with each 

 subject, commencing with coal-mines. In these he shows that 

 ventilation and convectio 1 currents have rendered many of the 

 results unreliable, as he shows to have been the case in the well- 

 known instance of the Dukinfield coal pit. The circulation of 

 air in coal pits varies from 5000 to 150,000 cubic feet per 

 minute, and tables are given to show how this variously affects 

 the temperature of the coal at different distances from the shaft, 

 though on the same level. As a rule, the deeper the pit the 

 more active is the ventilation, and therefore the more rapid the 

 cooling of the underground strata. In some pits the indraughted 

 air has been known to form ice, not only in the shaft, but icicles 

 in the mine near the shaft. 



The cooling effects of ventilation are shown to begin imme- 

 diately that the faces of the rock and coal are exposed, and as 

 the hotter (and deeper) the pit, and the more gassy the coal, the 

 more active is the ventilation, so these surfaces rapidly undergo 

 a cooling until an equilibrium is established between the normal 

 underground temperature and the temperature of the air in the 

 gallery. Judging by the effects of the diurnal variations on the 

 surface of the ground, it is clear that an exposure of a few days 

 must, when there is a difference of 10° to 12° or more between 

 the air in the gallery and the normal temperature of the rock, 

 tell on the exposed coal and rock to the depth of 3 to 4 feet 

 ■ — the usual depth of the holes in which the thermometers are 

 placed. The designation of "fresh open faces "is no security, 

 as that may mean a day or a week, or more. The author con- 

 siders also that so far from the length and permanence of the 

 experiment affording security, he is satisfied on the contrary that 

 those experiments in which it is stated that the thermometer has 

 been left in the rock for a period of a week, a month, or two 

 years without any change of temperature, affords prima facie 

 evidence of error, inasmuch as it shows that the rock has so far 

 lost heat as to remain in a state of equilibrium with the air at 

 the lower temperature in constant circulation. 



Another cause of the loss of heat which requires some notice 

 is the escape of the gas, which exists in the coal either in a 



highly compressed, or, as the author thinks more probable, in a 

 liquid state. A strong blower of gas has been observed to 

 render the coal sensibly cooler to the touch. In another case 

 whereas the temperature of the coal at the depth of 1269 feet 

 was 74° F., at the greater depth of 1588 feet in a hole with a 

 blower of gas it was only 62°. One witness observed that "the 

 coal gives out heat quicker than the rock." There is generally 

 a difference of 2° or 3 between them. 



On the other hand, the coal and rocks when crushed and in 

 " creeps " acquire a higher temperature owing to the liberation 

 of heat. 



The effects of irregu'arities of the surface on the underground 

 isotherms, although unimportant in many of our coal-fields, pro- 

 duce very decided results in the observations on the same level 

 in the mines among the hills of South Wales. Sections are 

 given to show how the temperature rises under hills and falls 

 under valley-', showing that it is often essential to know not only 

 the depth of the shaft but the depth beneath the surface at each 

 station where the experiments are made. 



The author therefore considers that to assign a value to an 

 observation we should know (1) height of pit above sea-level ; 

 (2) the exact mean annual temperature of the place ; (3) depth 

 beneath the surface of each station ; (4) distance of the stations 

 from the shaft ; (5) temperature and columns of air in circula- 

 tion ; (6) length of exposure of face ; (7) whether or not the coal 

 is gassy. The dip of the strata and the quantity of water are 

 also to be noted. 



Very few of the recorded observations come up to this standard, 

 and the author has felt himself obliged to make a very restricted 

 selection of cases on which to establish the probable thermo- 

 metric gradient for the coal strata. Amongst the best observa- 

 tions are those made at Boldon, North Seaton, South Hetton, 

 Rosebridge, Wakefield, Liege, and Mons. These give a mean 

 gradient of 49J feet for each degree F. The bore-holes at 

 Blythswood, South Balgray, and Creuzot give a mean of 50 '8 

 feet. 



Mines other than Coal. — The causes affecting the thermal con- 

 ditions of these mines are on the whole very different to those 

 which obtain in coal mines. Ventilation affects both, but in 

 very unequal degrees. In mineral mines it is much less active, 

 and the cooling effects are proportionately less. On the other 

 hand the loss of heat by the underground waters in mineral 

 mines is very important. In some mines in Cornwall, the quan- 

 tity of water pumped up does not exceed 5 gallons, while in 

 others it amounts to 200 gallons per minute. The Dolcoath 

 mine used to furnish half a million gallons of water in the twenty- 

 four hours, while at the Huel Abraham mine it reached the large 

 quantity of above 2,000,000 gallons daily. The rainfall in 

 Cornwall is about 46 inches annually, and of this about 9 inches 

 pass underground. In the Gwennap district, where 5500 acres 

 were combined for drainage purposes, above 20,000,000 gallons 

 have been discharged in the twenty-four hours from a depth of 

 1200 feet. This water issues at temperatures of from 60° to 68°, 

 or more than 12° above the mean of the climate, showing how 

 large must be the abstraction of heat from the rocks through 

 which the waters percolate. 



Hot springs are not uncommon in these mines. They are due 

 to chemical decomposition, and to water rising in the lodes and 

 fissures from greater depths. The decomposition which goes on 

 in the lodes near the surface, and whereby the sulphides of iron 

 and copper are reduced ultimately to the state of peroxides and 

 carbonates of those metals, is a permanent cause of heat, espe- 

 cially apparent in the shallower mines. On the other hand, 

 where the surface waters pass rapidly through the rocks, they 

 lower the temperature and give too low readings. 



While ventilation, therefore, reduces the rock temperature, the 

 water which percolates through the rock, and more especially 

 through the veins and cross-courses, sometimes raise, and at 

 other times lower, the temperature of the underground springs. 

 Mr. Were Fox, who for many years made observations on the 

 underground temperature of the Cornish mines, gave the prefer- 

 ence to the rocks ; while Mr. Henwood, an observer equally 

 experienced and assiduous, considered that the underground 

 springs gave surer results. Both were of course fully alive to 

 all the precautions that in either case it was necessary to take to 

 guard against these in'erferences. 



Taking ten of the most reliable of Mr. Henwood's observa- 

 tions at depths of from 800 to 2000 feet, the mean gives a ther- 

 mometric gradient of 42 '4 feet per degree, but Mr. Henwood 

 himself gives us the mean of 134 observations to the depth of 



