212 



PROFESSOR FORBES ON THE TEMPERATURE OF THE EARTH. 



imperfectly reduced to theory, that we cannot satisfactorily compare it with ex- 

 periment ;* but one law of great simplicity has long been known from theory 

 to be approximately true, namely, that the retardation of epochs is uniformly 

 greater as the depth increases. This is also easily verified graphically. By tak- 

 ino- the depths in a vertical direction, and setting off the day of greatest or least 

 temperature horizontally, a series of points is obtained through which a straight 

 line should pass. I have not engraved these projections for each year, but that 

 for the mean of the whole wiU be seen in the lower part of Plate X., where the 

 interpolating hues in general pass so nearly thz-ough the dots that they cannot be 

 distinguished. From these projections the mean rate of propagation downwards 

 is easily determined, and affords a palpable illustration of the conducting powers 

 of the soil. 



Table XIV. Shewing the Number of Days required by the impression 

 OF Heat to pass through One Foot of Soil. 



It must be added, that in the several years the law of uniform progression is 

 by no means accurate, although, in the mean of five, the accidents are nearly 

 compensated. And here, again, we find the good conductor, the sandstone, gives 

 by far the most regular and consistent results. 



E. — On the Form of the Annual Chtrues. 

 With a view to approximate more nearly to the form of the annual curves of 

 temperature at different depths, I have had the mean temperatm-e for each week 

 of the year taken by the mean of five years, which has the effect of disposing of 

 the more irregular fluctuations, as may be seen in Plate VIII., the curves in which 

 are taken from the following Table : 



See the Appendix. 



