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



From these figures I have plotted the curve (Fig. 2), showing 

 rate of penetration of heat into this stone. 



Teriod oj hesti^g /^ 



Fig. 2. 



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f-h,. 



Hejith 

 in 

 Cenii-meiers. 



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imCi 



iinOKii 



(airnf^ 



^ 



'^'^: 





fn. 



'm 



<^^ 



(^s. 



7 



From a consideration of the figures in the table, and this plotted 

 curve, the following conclusions may be drawn : — 



(1). That the progress of the heat-wave into the stone is curiously 

 intermittent, with alternations of slow advances and rapid plunges, 

 lessening however in contrast, in proportion to the increasing depth. 



As the stone, though gneissoid in structure, is comparatively 

 homogeneous, and the direction of penetration is normal to the 

 lamination-planes, we may reasonabl}'^ attribute this character of 

 the curve mainly to the moisture locked up, in var3'1ng proportions, 

 in the interstices of the successive layers. The increment of heat 

 seems to be repeatedly absorbed, during a period of one-half to one 

 hour, during the vaporization of moisture in a layer of about one 

 centimeter in depth, and its advance thereby delayed. Then the 

 balance of forces is suddenly broken, possibly by a lateral escape of 

 vapor through some crevice, and a rapid advance of the heat-wave 

 ensues during a few minutes, at first to a depth of two or more 

 centimeters. Then comes the resistance of gathering vapor as 

 before. 



(2). The determination of the rate of increment of heat, in this 

 experiment, has been affected by several sources of disturbance and 

 variation. The acquirement of exact and uniform figures would 

 involve the prevalence of the following theoretical conditions : the 



