56 report — 1859. 



If Fourier's conditions of uniformity, stated above, were fulfilled strictly, the num- 

 bers shown in the second column would be all equal among one another, and equal 

 to those in the third column. The differences between the actual numbers are sur- 

 prisingly small, but are so consistent that they cannot be attributed to errors of 

 observation. It is possible they may be due to a want of perfect agreement in the 

 values of a degree on the different thermometric scales ; but it seems more probable 

 that they represent true discrepancies from theory, and' are therefore excessively in- 

 teresting, and possibly of high importance with a view to estimating the effects of 

 inequalities of surface and of interior conductivity. The final means of the numbers 

 in the second and third columns are, for 



CaltonHill -11702 



Experimental Gardens . . . -11061 

 Craigleith Quarry .... -06988 



The thermal capacities of specimens of the trap rock, the sand, and the sandstone 

 of the three localities were, at the request of Professor Forbes, measured by Uegnault, 

 and found to be respectively 



•5283, -3006, and -4623. 



Hence, according to position (3), stated above, the thermal conductivities are as 



follows : — 



Trap rock of Calton Hill 121'2 



Sand of Experimental Gardens . . 77*19 

 Sandstone of Craigleith Quarry. . . 273*6 



These numbers do not differ much from those given by Professor Forbes, who for 

 the first time derived determinations of thermal conductivity in absolute measure 

 from observations of terrestrial temperature. In consequence of the peculiar mode 

 of reduction followed in the present investigation, it may be assumed that the esti- 

 mates of conductivity now given are closer approximations to the truth. To reduce 

 to the English foot as unit of length, we must multiply by the square of 1*06575 ; to 

 reduce, further, to the quantity of heat required to raise 1 lb. of water by 1° as unit 

 of heat, we must multiply by C6 - 447 ; and lastly, to reduce to a day as unit of time, 

 we must divide by 365j. We thus find the following results : — 



Trap rock of Calton Hill 23'5 



Sand of Experimental Gardens . . . 15'0 

 Sandstone of Craigleith Quarry . . . 53"5 



These numbers show the quantities of heat per square foot conducted in a day 

 through a layer of the material 1 foot thick, kept with its two surfaces at a difference 

 of temperature of 1 degree, — the unit of heat being, for instance, the quantity required 

 to raise 1000 bis. of water by ^Vot-h of a degree in temperature. 



On the Establishment of Thermometric Stations on Mont Blanc. 

 By Professor Tyndall, F.R.S. 



I proposed to the Royal Society some months ago to establish a series of stations 

 between the top and the bottom of Mont Blanc, and to place suitable thermometers 

 at each of them. The Council of the Society thought it right to place a sum of money 

 at my disposal for the purchase of instruments and the payment of guides; while I 

 agreed to devote a portion of my vacation to the execution of the project. At Cha- 

 mouni I had a number of wooden piles prepared, each of them shod with iron, to 

 facilitate the driving of it into the snow. The one intended for the summit was 12 

 feet long and 3 inches square; the others, each 10 feet long, were intended for five 

 stations between the top of the mountain and the bottom of the Glacier de Bossons. 

 Each post was furnished with a small cross-piece, to which a horizontal minimum 

 thermometer might be attached. Six-and-twenty porters were found necessary to 

 carry all our apparatus to the Grands Mulets, whence fourteen of them were imme- 

 diately sent back. The other twelve, with one exception, reached the summit, whence 

 six of them were sent back. Six therefore remained. In addition to these we had 

 three guides, Auguste Balmat being the principal one ; these, with my friend. Dr, 



