LIMITING DEPTH OF CHANGE. 



129 



Tlie dates of occurrence of the mean values of 

 the maxima and of the minima were : — 



Depth 















in Parisian 



Annual Mean. 



Maximum. 



Minimum. 



feet. 



















April 24 and Oct. 21 



July 



26 



Jan. 



14 



6 



May 20 



, Nov. 11 



Aug. 



11—20 



Feb. 



11—20 



12 



June 19 



, Dec. 18 



Sept. 



18—19 



March 



18—19 



18 



July 19 



, Jan. 18 



Oct. 



18—19 



April 



18—19 



24 



Aug. 15 



, Feb. 18 



Nov. 



15—18 



May 



15—18 



30 



Sept, 13 



, Mar. 18 



Dec. 



13—18 



June 



13—18 



36 



Oct. 11 



April 7 



Jan. 



7-11 



July 



7-11 



Bischof states that the annual variations of 

 temperature do not vanish completely within the 

 depth of 60 feet. Quetelet arrived at the same 

 result by his observations made at Brussels. 

 But the temperature observed at such a depth as 

 this does not correspond to the mean at the sur- 

 face, but is raised by the influence of the internal 

 heat of the earth, which, as may be deduced from 

 the yearly mean-numbers above quoted, is already 

 perceptible at but little distance from the surface. 

 For this reason we must not reckon, as true ex- 

 pressions of the heating due to the solar rays, 

 any indications of temperature by thermometers 

 placed more than about six feet below the sur- 

 face. 



The knowledge of all these facts leads us now 

 to the following simple method of determining 

 the mean temperature of the earth's surface at 



