Oct. 12, 1882] 



NATURE 



589 



the eye, could we see it without absorption, these figures appear 

 to show conclusively that it would be blue. Not to rely on any 

 assumption, however, we have by various methods at Alleghany, 

 reproduced this colour. 



Thus (to indicate roughly the principle used), taking three 

 Maxwell's discs, a red, green, and blue, so as to reproduce white, 

 we note the three corresponding ordinates at the earth's surface 

 spectrum, and comparing these with the same ordinates in the 

 curve giving the energy at the solar surface ; we re-arrange the 

 discs, so as to give the proportion of red, green, and blue which 

 would lie seen that, and obtain by their revolution a tint which 

 must approximately represent that at the photosphere, and 

 which is most similar to that of a blue near Frauenhofer's " F." 



The conclusion then is that while all radiations emanate from 

 the solar surface, including red and infra-red, in greater degree 

 than we receive them, that the blue end iss} enormously greater 

 in proportion, that the proper colour of the sun, as seen at the 

 photosphere, is blue — not only "bluish," but positively and 

 distinctly blue ; a statement which I have not ventured to make 

 from any conjecture, or on any less cau>e than on the sole ground 

 of long continued experiments, which, commenced some seven 

 years since, have within the past two years irresistibly tended to 

 the present conclusion. 



The mass of observations on which it rests must be reserved 

 for more detailed publication elsewhere, at present I can only 

 thank the Association for the courtesy which has given me the 

 much prized opportunity of laying before them this indication of 

 methods and results. 



UNDERGROUND TEMPERATURE* 

 II. 



E. YYe now proceed to a Comparison of Results. 

 HTHE localities at which definite results have been obtained 

 ■*■ may thus be classified : — 



1. Metallic mines. 2. Coal mines. 3. Wells and wet borings. 

 4. Tunnels. 



1. The mines at Przibram in Bohemia, with a depth of 1900 

 feet, are in very quartzose rock, and give a very slow rate of 

 increase, viz. 1° F in 135 feet. As all the shafts are in lofty 

 hills, an allowance of fa may be made for convexity, leaving 

 1° F. in 126 feet. Quartz is found by Prof. Herschel to have a 

 conductivity of about '0086. 



The mines at Schemnitz in Hungary, with a depth of 1368 

 feet, give an average rate of 1° F. in 74 feet, the rock being a 

 green hornblende-andesite (in German, Grihislcin-Trachyt), 

 which is a compact, fine-grained, crystalline, more or less vitreous 

 rock. Prof. Lebour estimates its conductivity as being probably 

 nearly the same as that of Calton Hill trap-rock, which Prof. 

 Herschel found to be about '0029. 



2. The principal results from coal mines are as follows ■ — 

 The mines of the Societe Cocqueril at Seraing (Belgium), with 



a depth of 1657 feet, give an average rate of 1° F. in 50 feet. 

 The rock is ccal shale. Prof. Herschel found for shale the low 

 conductivity '0019. 



The mines of Anzin, in the north of France, with a depth of 

 658 feet, gave in the deepest shaft an increase of 1° in 47 feet. 



Rosebridge Colliery, near Wigan, with a depth of 2445 feet, 

 gave a mean rate of 1° in 54 feet. 



The four following are in the East Manchester coalfield : — 



Astley Pit, Dukinfield, with a depth of 2700 feet, gave a 

 mean rate of 1° in 72 feet. 



Ashton Moss Colliery, with a depth of 2790 feet, gave 1° in 

 77 feet. 



Bredbury Colliery, with a depth of 1020 feet, gave 1° in 7S - s 

 feet. 



Nook Pit, with a depth of 1050 feet, gave 1° in 79 feet. 



South Hetton Colliery, Durham, with a depth of 1929 feet, 

 including a bore hole at bottom, gives very consistent observa- 

 tions at various depths, and an average rate of 1° in 57*5 feet. 



Boldon Colliery, between Newcastle and Sunderland, with a 

 depth of 1 5 '4 feet, and excellent conditions of observation, gives 

 an average rate of 1° in 49 feet. 



Kingswood Colliery, near Bristol, with a depth of 1769 feet, 

 and remarkable consistency between observations at various 

 points, gives 1° in 6S feet. 



Prof. 1 hillips' observations in Monkwearmouth Collier) - , pub- 

 lished in Phil. Mag. for December 1834, showed a temperature 



1 Continued from p. 567. 



of 7 1 "2 in a hole bored in the floor of a recently exposed part at 

 the depth of 1584 feet. The surface of the ground is 87 feet 

 above high water, and the mean temperature of the air is assumed 

 by Prof. Phillips to be 476. If, as usual, we add i° to get the 

 soil temperature, instead of assuming, as Prof. Phillips does, 

 that the temperature 100 feet deep is identical with the air 

 temperature at the surface, we obtain a rate of increase of 1° in 

 70 feet. 



3. The following are the most trustworthy results from wells 

 and borings : — 



The Sperenberg bore, near Berlin, in rock salt, with 3 depth 

 of 3492 English feet, to the deepest reliable observation, gave an 

 average of 1° in 51-5 feet. This result is entitled to special 

 weight, not only on account of the great depth, but also on 

 account of the powerful means employed to exclude convection. 



Rock salt, according to Prof. Herschel, has the very high 

 conductivity '0113. 



Three artesian well in the chalk of the Paris ;Basin gave the 

 following results : — 



St. Andre, depth of observation ... 830 ... 1 in 56^4 



Grenelle 1312 ... 1 in 56 - 9 



Military School 568 ... I in 56-2 



An artesian well at St. Petersburg, in the Lower Silurian 

 strata, with a depth of 656 feet, gave about 1° in 44 feet. 



A well sunk at Vakoutsk, in Siberia, to the depth of 540 feet, 

 disclosed the fact that the ground was permanently frozen to this 

 depth, and probably to the depth of 700 feet. The rate of 

 increase of temperature was 1° in 52 feet. 



Of the English wells in which observations have been taken, 

 the most important is that at Kentish Town, in which Mr. G. 

 J. Symons, F.R.S., has taken observations to the depth of 1 100 

 feet. The temperatures at different depths form a smooth series, 

 and have been confirmed by observations repeated at long 

 intervals. The only question that can arise as to the accuracy 

 of the results is the possibility of their being affected by 

 convection. 



The well is 8 feet in diameter, with brickwork to the depth of 

 540 feet, and this part of it is traversed by an iron tube 8 inches 

 in diameter, which is continued to the depth of more than 1300 

 feet from the surface. The tube is choked with mud to the 

 depth of about 10S0 feet, so that the deepest observations were 

 taken under 20 feet of mud. The temperature at 1 100 feet was 

 69°'9, and by combining this with the surface temperature of 

 49° 9 observed at the Botanic Gardens, Regent's Park, we 

 obtain a rate of 1° in 55 f ee t- These data would give at 250 

 feet a calculated tempeiature of 54'5> whereas the temperature 

 actually observed at this depth was 56*1, or 1° 6 higher ; the 

 temperature at 300 feet and at 350 feet being also 56'i. This 

 seems to indicate convection, but it can be accounted for by con- 

 vection in the 8-foot well which surrounds the tube, and does 

 not imply convection currents within the tube. Convection 

 currents are much more easily formed in water columns of large 

 diameter than in small ones, and the 20 feet of mud at the 

 b >ttom give some security against convection at the deepest point 

 of observation. It is important to remark that the increase from 

 1050 to 1 100 feet is rather less than the average instead of 

 being decidedly greater, as it would be if there were convection 

 above, but not in, the mud. The rate of 1° in 55 feet may 

 therefore be adopted as correct. 



The strata consist of tertiary strata, chalk (586 feet thick), 

 upper greensand, and gault. 



The Kentish Town temperature at the depth of 400 feet (58°) 

 is confirmed by observations in Mr. Sich's well at Chiswick, 

 which is 395 feet deep, and has a temperature varying from 58° 

 to 57°-5- 



T he Bootle well, belonging to the Liverpool Waterworks, is 

 1302 feet deep, and the observations were tiken in it during the 

 sinking. The diameter of the bore is 24 inches, and convection 

 might have been suspected but for the circumstance that there 

 was a gradual upward flow of water from the bottom, which 

 escaped from the upper part of the well by percolation to an 

 underground reservoir near at hand. This would check the 

 tendency to downflow of colder water from the top ; and as the 

 observations of temperature were always made at the bottom, 

 they would thus be protected against convective disturbance. 



The temperature at 226 feet was 52°, at 750 feet 56°, at 1302 

 feet 59°, giving by comparison of the first and last of these a 

 mean rate of 1° in 154 feet. The circumstance that the boring 



