92 



Proceedings of the British Association. 



already stated, too imperfectly observed for the upper thermometers to 

 be of much service. The remaining observations afford the following 

 results : — 



Soil and 

 Locality, 



Time of propagation of heat through One foot 

 (French), deduced from 



Maximum, 

 1837. 



Minimum, 

 1838. 



Maximum, 

 1838. 



Mean. 



Trap (Observato-) 

 ry) J 



Sand (Exp. Gar-\ 

 den) J 



Sandstone \ 

 (Craigleith) / 



7.5 days 

 7.1 — 



4.9 — 



6.5 days 

 5.8 — 



3.6 — 



6.8 days 

 6.8 — 

 3.6 — 



6.9 days. 

 6.6 — 

 4.0 — 



These results confirm the relation of conducting powers indicated by 

 the constant B already found ; but the numerical comparison of these 

 independent results is a matter of extreme complication — (see Poisson, 

 Thiorie de la Chaleur, chap, xii.) 



The President congratulated the members on the results likely to flow 

 from experiments conducted on so well-digested a system. Any person 

 who was conversant with the writings of Fourier, and other foreign 

 writers on this branch of mixed mathematics, must be aware how 

 necessary it was to be in possession of a sufficient store of well-ascer- 

 tained facts, on which any theory, if it be sound, must rest and be 

 dependent. The facts now collected were almost as complete as could 

 be expected or desired ; that those are not merely accidental variations 

 of temperature which are indicated, must be observed on the most 

 superficial examination of the three curves. The general conformity, 

 while the thermometers, of whose indications they were as it were the 

 types, were placed at such distances and in substances differing so mate- 

 rially in structure and physical character, together with the repro- 

 duction of curves in successive years so coincident in their general 

 characters, were circumstances tending to stamp with the character 

 of truth the results, and to show the soundness of the system on which 

 these researches had been conducted. Theory had long been in advance 

 of practical knowledge on this subject, but practice was now coming up 

 and beginning to take her proper place as the handmaid and sure assis- 

 tant of theory. Heretofore, the scale upon which experiment had been 

 performed on the conducting power of the several substances of which 

 the crust of our earth is composed, were on so small a scale that the 

 analyst scarcely knew whether he was safe in using their results. In 

 every point Qf view, then, they were most important. — Mr. Snow Harris 

 observed, that an inspection of these curves would lead to the conclu- 

 sion that, as the depth increased, their curvature diminished, and that 

 therefore at some certain depth they would turn into straight lines, and 

 the temperature at that depth become constant. — Prof. Forbes said that 

 not only did this appear obviously from an inspection of the curves, but 



