204 PROFESSOR FORBES ON THE TEMPERATURE OF THE EARTH. 
VIL. Results of the Observations. 
A. Mean Temperature at different depths, 
It has been observed at Brussels and elsewhere,* that, even at depths less than 
25 feet, the mean annual temperature indicated by the lower thermometers is 
greater than that of those nearer the surface. This appears to be also clearly 
established by the observations at the three stations near Edinburgh, as contained 
in the following Table. 
TABLE V. MEAN TEMPERATURE FOR FIvE YEARS. 












3 French Feet, (No.4). 6 French Feet, (No. 3). 12 French Feet, (No. 2.) 24 French Feet, (No. 1), 

Experi- 
mental 
Garden. 
Experi- 
mental 
Garden. 
Experi- 
mental 
Garden. 
Experi- 
mental 
Garden. 
Observa- 
tory. 
Observa- 
tory. 
Craig- 
leith. 
Observa- 
tory. 
Craig- 
Craig- 
: leith. 
leith. 
Observa- 
tory. 
Craig- 
leith. 









46°46 
45:97 
45°87 
46°18 
45°87 
47°28 
47°05 
46°98 
47°04 
47-08 
47°29 
46°89 
46°68 
46°77 
46°79 
46°87 
46°90 | 
46°44 
| 46°67 
| 46-91 
46°89 
46°65 
46°12 
46°18 
46°43 
46-44 
46°70 
45°90 
46°43 
46°58 
46-51 
46°23 
45°35 
45°75 
45°99 
45°99 
46°26 
45°22 
45°83 
45-97 
46:13 
46°54 
45-44 
46°14 
46:26 
46-29 
45-94 
44°81 
45°48 
45°56 
45°66 
45°49 | 46-13 
1837-1838 
1838-1839 
1839-1840 
1840-1841 
1841-1842 
Means 




















45°88 | 45°86 | 46:42 46°36 | 46°76 47-09 | 46:07 








Mean of Observatory, 
Experimental Garden, 
Craigleith, . ; 


46°14 Elevation, 
46°60 ss 
45°95 ” 

350 feet. 
70 feet. 
150 feet. 



Adie’s Observations, 45:28 

Mean Temperature of Air from Mr 240 feet. 
Mean of 3 Feet Thermometers, 45°83 
nO ' 46-07 
sol As 46°36 
24. 46:68 
The cause of the increased mean temperature below, is by no means clear. 
From its irregularity, it is most probably due to several causes, of which the cen- 
tral heat of the earth is perhaps one; its effect at 25 feet need not be insignifi- 
cant, since the average rate of increase at great depths is 1° Fahr. for from 40 to 
50 English feet. In the present case, the increase is not uniform, and it is also 
decidedly different in the different soils, and will appear by the sequel to be inti- 
mately connected with the conducting power of the strata. The order of magni- 
tude of the increase is this, 1. Observatory, 2. Experimental Garden, 3. Craigleith ; 
—or, Trap, Sand, Sandstone, which is also the order of the conducting power. The 
following table shews that, in every instance but one (from 6 feet to 12 feet at 
Craigleith), the increase was apparent. 
* QueETeELET, Annales de l'Observatoire Royale de Bruxelles, iv. 150. 

