201 



bases, where they remain, unimpaired and uninjured 

 for ages, though exposed to all the vicissitudes of tem- 

 perature ; and at the same time composed of the very 

 materials as the rocks in the quarries, hills, or moun- 

 tains, which are said to be rapidly progressing in a 

 state of decomposition. This is a fact 1 wish may be 

 kept in view. 



These effects may appear strange, but the fact is no 

 less true, that a single tuft of grass, implanted upon the 

 top of a wall of masonry, will, in time, if not disturb- 

 ed, injure its texture and break up its materials. 

 What then may we not expect, where the ivy and other 

 vines are spreading their luxuriant branches over the 

 deserted walls, and annually forcing their numberless 

 little wedges or tendrils, into every hole and crevice^ 

 and gradually expanding, and uniting their forces, in 

 the destruction of the noblest works of man, who, with 

 so much care, has reared them to increase his glory 

 and perpetuate his fame. 



Thirdlv. Rocks of a description similar to those 

 last mentioned and such as are composed of irregular 

 and amorphous masses lying in a juxta position, yet 

 imperfectly united, are liable to disruptions, to be shat- 

 tered and broken down, not only by the two latter 

 causes, but by the alternations of heat and cold, wet and 

 dry. 



Almost all substances, rocks and stones as well as 

 other things, are, on exposure to heat, liable to ex- 

 pansion in a degree proportioned to the degree of 

 beat absorbed ; and to a shrinking on the reverse of 



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