158 Study of the New York Obelisk as a Decayed Boulder. 



and 50 centimeters, which also were most affected by lateral radia- 

 tion and loss of heat through the north face of the boulder, the 

 average increment lessened to a rate of over 2 minutes for 1° of 

 temperature per centimeter of depth. 



It would have been interesting to repeat the experiment from 

 the north side of the boulder, on a series of thermometers, with 

 bulbs lying at successive depths along the direction of lamination 

 or strike of the boulder. Our experiment has at least thrown 

 light on some conditions and precautions, which would require 

 attention, in properly carrying on a series of such experiments on 

 the conduction of heat through various species of rock, in directions 

 varying in reference to planes of structure. 



The curve presents at a glance the practical result of our experi- 

 ment, that the temperature of the melting-point of the paraffin-com- 

 pound (63° C.) reached a depth of 5 to 6 centimeters in about 2 

 hours. 



21. Absorption-coefficients of Syene granite. 



"With a view to determine the exact changes in physical condition 

 in the interior of Syene granite, under the influence of long weather- 

 ing, both by the conditions of the climate of Egypt and of that of 

 New York, I have made the following experiments, with particular 

 reference to absorptive power. The essential features of my method 

 are founded on a distinction between two modes of absorption of 

 liquid by a porous solid : 



(a). Lateral absorption, i e., from one surface; such as occurs 

 in construction, when ashlar is moistened by rain upon its face. 

 The soaking up of water is here but partial, effected almost entirely 

 by interstices between the constituent grains, which may be distin- 

 guished as the rock-pores: 



(b). Total saturation, where water is forced into all the interstices 

 of the rock, including the more minute interstices within the con- 

 stituent mineral-grains, which may be distinguished as the mineral- 

 clefts. This therefore includes the- amount of liquid in the rock- 

 pores, and the difference enables us to estimate the volume of the 

 second class of voids. 



All kinds of mechanical strain to which a rock may be subjected 

 (such as tension, jar, frost, etc.) are likely to develop mainly an 

 increased volume in the rock-pores; while the irregular contractions 

 and expansions, incident to the combinations, losses, and solutions 



