134 On theReduction of Observations of Under ground Temperature . 



length of a French foot being 1-06575 of the British standard 

 foot, we must therefore multiply the preceding numbers by 

 1" 13581 to reduce them to convenient terms. 



41. We may, lastly, express them in terms of the most common 

 unit, which is the quantity of heat required to raise the tempera- 

 ture of a grain of water by 1°; and to do this Ave have only to 

 multiply each of them by 7000 x 6.2-447, being the weight of a 

 cubic foot in grains. 



42. The following Table contains a summary of our results 

 as to conductivity expressed in several different ways, one or 

 other of which will generally be found convenient : — 



Table X. — Thermal Conductivities of Edinburgh Strata, in 

 British Absolute Units [Unit of Length, the English Foot] . 



Description 

 of terrestrial 

 substance. 



Trap-rock ofl 

 Calton Hill, j 



Sand of Ex- j 

 perimental \ 

 Gardens... | 



Sandstone of] 

 Craigleith > 

 Quarry... J 



Conductivities in terms of 



thermal capacity of unit 



bulk of substance 



Conductivities in terms of 



thermal capacity of unit bulk 



of water 



Conduc- 

 tivities in 



terms of 



thermal 

 capacity of 

 one grain 



of water. 



Per ann. 



267-0 

 295-9 



784-5 



Per 24 h. 



•7310 



•8100 



2-1478 



Per second. 

 •000008461 



•000009375 

 •00002486 



Per ann. 

 1411 



88-9 

 362-7 



Per 24 h. 



•3863 

 •2435 



•9929 



Per second. 

 000004471 



000002818 

 ■00001149 



Per second. 

 1-9544 



1-2319 

 5-0225 



43. The statements (§§ 36 and 37) by which the signification 



of - has been defined and illustrated, require only to have cubic 



feet of water substituted for cubic feet of rock, in their calorime- 

 tric specifications, to be applicable similarly to define and illus- 

 trate the meaning of the conductivity denoted by k. The fluidity 

 of the water allows a modified and somewhat simpler explana- 

 tion, equivalent to that of § 36, to be now given as follows : — 



44. If a long rectangular plate of rock one foot thick, in a 

 position slightly inclined to the horizontal, have water one foot 

 deep flowing over it in a direction parallel to its length, and if 

 the lower surface of the plate be everywhere kept 1° higher in 

 temperature than the upper, the water must flow at the rate of k 

 times the length of the plate per unit of time in order that, the 

 heat conducted through the plate may raise it just 1° in tempe- 



directly as the cube of the unit length, and therefore the numbers expressing 

 the quantities of heat compared will be inversely as the cubes of the lengths 

 chosen for unity, and directly as these simple lengths ; that is to say, finally, 

 they will be inversely as the squares of these lengths. 



