88 INTERNAL HEAT OF THE EARTH, 



confined spaces, as in river-like fissures (and hence it often 

 happens that, of several borings in near proximity to each 

 other, only some are successful), while sometimes they may 

 form basins of wide extent in a horizontal direction, a dis- 

 tribution very favourable for yielding supplies of water, and 

 which only in very rare cases betrays an open communication 

 with the surface of the earth by the presence of eels, shells, 

 and vegetable remains. If from such causes as have been 

 mentioned, ascending springs are sometimes warmer than 

 might be anticipated from the small depth of the boring 

 through which they rise, on the other hand an opposite 

 effect is produced by colder waters flowing in laterally through 

 openings from cross -fissures. 



It has been remarked already that the maxima and minima 

 of the variations of atmospheric temperature, occasioned by 

 the height of the sun at the different hours of the day and 

 seasons of the year, reach points situated in the same vertical 

 line, at small depths below the surface, at very different 

 times. According to the always very exact observations of 

 Quetelet,( 37 ) the diurnal variations cease to be sensible at 

 Brussels at the small depth of 3 '8 French feet ; and in respect 

 to annual variations, a thermometer placed 24 French, or 

 25 '6 English, feet below the surface at Brussels, did not show 

 the maximum of temperature during the year until the 10th 

 of December, and the minimum until the 15th of June. 

 Also, in the fine series of experiments made by Forbes in the 

 \icinity of Edinburgh, on the conducting power of different 

 kinds of rock, the maximum temperature in the basaltic trap 

 of Calton Hill was observed to take place on the 8th of 

 January, at a depth of 23 feet.( 38 ) According to Arago's 

 series of many years' observations in the garden of the 



