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



presume that any injury could have been done to the surface of the 

 Obelisk, in the waterproofing process, by warming at a temperature 

 which rarely approached 85° C, during a period not exceeding 2 

 or 3 minutes. 



11. Effects of the sun''s heat on granite. 



In regard to the action upon granite of high natural tempera- 

 tures, it should be noted that those of rock surfaces, exposed to the 

 sun during the heats of sumnier, often rise to 150° F. (66° C.) and 

 over, especially if the rock is dark-colored ; and that of the sands 

 of African deserts sometimes reaches 200° F. 



An interesting application of this natural warming of surfaces of 

 stone occurred during the hottest period of August, this last sum- 

 mer (1892), at Sandy Hook, N.J. The casemates of the fortifica- 

 tions are constructed of a dark concrete, in large part composed of 

 fragments of "bluestone" (flagstone from the base of the Catskill 

 Mountains). On account of the porosity of the concrete and its 

 permeability by rain-water, these constructions had been under- 

 going for some months the same waterproofing treatment with 

 paraffin as that applied to our Obelisk in 1885. On certain hot 

 afternoons, it was found that the surfaces of those bomb-proofs 

 which lay exposed to the sun had already become heated to such 

 a degree that artificial heating could be dispensed with and the 

 melted paraffin directly applied. 



It is a question of some interest, in reference to the durabilit}'^ of 

 building-stones used in New York City, to determine how often the 

 direct heat of the sun reaches its maximum in this climate. By a 

 collation of the observations of Mr Daniel Draper,' the Director of 

 the Meteorological Observatory in Central Park, the following table 

 has been prepared, presenting for ten years the maxima in the sun 

 of 140° F. or over and of 146° F. or over. The latter temperature 

 (63° C.) is that of the melting-point of the particular paraffin re- 

 ferred to above. 



I Abstract of Registers, 1880 to 1889. 



