TEMPERATURE VARIATIONS. 287 



tures are not uniform, either in area, depth or time, and whatever the 

 surface temperature may be, we know nothing from direct observation of 

 their penetration or variation in the particular rocks under discussion. 



The observations of Forbes* at Edinburgh, of Queteletf at Brussels, 

 of Bequerel J at Paris and of Malaguti and Duroche § show that the high 

 and low temperatures do not penetrate the rocks far. || At a depth of 

 three feet the total variation according to Forbes is 15 degrees, at 6 feet 

 it is 10 degrees, at 12 feet it is 5 degrees and at 24 feet it is IJ degrees. 



Curves platted from the data furnished by Forbes and Quetelet show 

 that the annual changes are clearly perceptible only to a depth of about 

 40 feet.^ The temperatures we have to deal with in the temperate re- 

 gions, therefore, are surface temperatures almost literally ; but the depth 

 to which a given temperature penetrates varies with the square root of 

 the period of exposure,** and as the rocks are exposed to a pretty even 

 annual temperature, and as the periods during Avhich they are exposed 

 to the high diurnal temperatures are but short, it follows that in Brazil 

 the ranges of 10 and 15 degrees lie still nearer the surface than they do 

 in England. 



A set of observations was made in lateritesft at Trevandrum, India 

 (north latitude 8° 30' 32'^, east longitude 5^ 7"" 59'), with thermometers at 

 depths of 3, 6 and 12 feet. 



Forbes concludes from the results obtained by Caldecott " that the 

 phenomena of the propagation of heat into the ground near the equator 

 resemble those of temperate latitudes, though modified in extent and 

 character." J J It should not be overlooked, however, that Caldecott's ob- 

 servations were practically on soil and not on solid rock. 



Some important observations that bear on this subject were made by 

 Messrs Newberry and Julien near Bronxville, New York, in July, 1890, 

 upon a boulder of granitoid gneiss. §§ The character of the rock treated, 



* Ti'ans. Roy. Soc. Edin., vol. xvi, 1849, pp. 189-236. See also vol. xxix, pp. 637-656 ; vol. xxii, pp. 

 405-427; vol. xxxv, pt. 1, 1887-88, pp. 287-312. 



f Comptes Rendus, ii, 1836, 357; Amer. Jour. Sci., vol. viii, 1849, p. 44; Annaies de PObservatoire 

 de Bruxelles, iv, 1845. 



t Comptes Rendus, Ixvii, 1868, pp. 1150, 1151 ; Ixxiii, 1871, p. 1136. 



§ Comptes Rendus, 38, 1854, p. 785. 



Ij Lectures on some Recent Advances in Physical Science. P. G. Tait. 2d ed., London, 1876, p. 274. 



^Examination of the temperature readings suggest that there is a layer between about 33 feet 

 and about 60 feet, through which the slight changes of temperature are constantly endeavoring to 

 adjust themselves. 



** Theory of Heat. J. Clerk Maxwell. 3d ed., London, 1872, p. 245. 



ft Observations on the temperature of the ground at Trevandrum, in India, from May, 1842, to 

 December, 1845. John Caldecott, astronomer to the Rajah of Travancore. Trans. Roy. Soc. Edin., 

 vol. xvi, 1849, pp. 379-391. 



IX Remarks on the preceding observations. J. D. Forbes. Trans. Roy. Soc. Edin., vol. xvi, p. 392. 



ggStudy of the New York obelisk as a decayed boulder. Alexis A. Julien. Annals New York 

 Acad. Sci , vol. viii, 1893, pp. 93-16t>. See especially page 155. 



XXXII— Bull. Geol. Soc. Am., Vol. 7, 1895. 



