206 PROFESSOR FORBES ON THE TEMPERATURE OF THE EARTH. 



B. Ocrwral Observations on the Thermometric Curves. 



Some of the most important results depend upon the annual ranges of tem- 

 perature at different depths. But the determination of the extremes is no easy- 

 matter. I will first direct attention to the curves in Plate VII., which convey a 

 great deal of valuable information, which can here be only slightly touched upon. 

 They are reduced to one-sixth of the size of the original projections, in which one 

 degree occupied two-thirds of an inch vertically, and one day occupied one-tenth 

 of an inch horizontally. The corrected temperatures are those which have been 

 projected. 



The curves extend over five years ; and are placed in the order of depths 

 (vertically) to which they belong : the uppermost undulating curves shewing the 

 variations at the three stations 3 French feet below the surface, the lowest set 

 shewing the variations 24 French feet below the surface. 



The most obvious results are the following : — 



1. In the upper set of curves, though the irregularities are greatest, yet the 

 three curves follow one another with singular fidelity throughout these irregulari- 

 ties. The curves separate a little in summer, and regularly in the same direction 

 every summer, shewing the influence of exposure, the Experimental Garden being 

 most heated, then Craigleith, and lastly the Observatory, which is also the order 

 of the elevations of the stations above the sea. It may also be added, that the 

 diurnal change may possibly have some slight influence upon the Experimental 

 Garden, where the observations were made fully two hours later than at the other 

 stations. (See Section V.) 



2. As the local irregularities diminish at increasing depths, the range dimi- 

 nishes, and the times of maxima and minima ai-e continually retarded. 



3. At iucreasing depths, the curves, which followed one another so closely 

 and exactly amidst the irregularities of temperature near the surface, systemati- 

 cally separate from one another, both owing to a variation in the range or degree of 

 undulation of the curve, and owing to a gi-eater or less degree of retardation in 

 the maxima or minima of the different curves. 



4. The effect last described is least sensible in comparing the observations 

 at the Observatory and Experimental Garden, but most sensible if either of these 

 be compared with the Craigleith observations, for which last the range diminishes 

 more slowly, so that, at 24 French feet, it is about double that of either of the 

 others, and the retardation of the maxima and minima is much less. 



5. In the trap and loose sand, the range is diminished to one-tenth part in 

 descending from 3 feet to 24 feet ; but in the sandstone it is not quite diminished 

 to a fifth part. The epoch of maximum temperature is retarded in the two former 

 cases nearly five months, in the latter only three. 



From these statements it is easy to see that the influence of the conducting 



