THE ARCH. 119 



rough stonowoi-k and a mncli larger proportion of mortar ; 

 but the mortar is much tho weaker material of the two. 

 Perronot, indeed, who built in Paris, about the end of tho 

 last century, some of the most bold and beautiful bridges 

 yet constructed, did dreas his stones for tho full thickness 

 of tho arch from front to back all through; for he had 

 access to splendid stone quarries, and had the State for his 

 paymaster. Prom his drawings it can bo soon that he 

 commonly used stones 5 feot long, 4 feet deep (the thick- 

 ness of his arch), and over a foot in thickness, in the middle 

 parts of liis arch. But now comes tlie second renson, you 

 cannot led these large stones throughout in mortar or cement. 

 The usual way of sotting these large blocks of stone is by 

 slinging them over the spot from a crane, spreading the 

 mortar with a trowel as evenly as you can on the proposed 

 bod, and then letting the stone down into its place ; but the 

 stone cannot be thoroughly bedded in this way— it bears 

 hard in places, but does not touch in others, as may be 

 proved experimentally if the stono be raised again a day or 

 two after, when tho places whore it did not touch will bo- 

 come apparent. It is in fact a most difficult operation, to 

 thoroughly and completely bed a large block of stone, and 

 m such a case as this, with highly inclined beds, practically 

 impossible. Engineers of the greatest experience say that 

 it is impossible to bed a large block of stone with a trowel. 



Now, considering that the best cement will not bear a 

 crushing weight of more than two tons on the square inch, 

 while the best class of stones will bear double that weight, 

 it is clear that the cement is the weakest part of tho arch, 

 and that consequently it is of the greatest importance that 

 it should have a complete and thorough bearing. 



On the other hand, let us see how it is with brickwork. 



In discussing the comparative strength of a brick arch, it 



