PRINCIPLES OF MECHANICS. 543 



tion once made, the cost of the ingenuity is nothing. Of course, the 

 mechanical principles involved in such structures are obvious to the 

 mathematician, when the problem has been practically solved. And 

 in this case, as in the previous cardinal inventions in structure, though 

 the event has taken place within a few years, no single person, so fur 

 as I am aware, can be named as the inventor. 5 



5 Since this was written, I have been referred to Rees's Cyclopaedia, Article Oblique 

 Arches, where this invention is correctly explained, and is claimed for an engineer 

 named Chapman. It is there said, that the first arch of this kind was erected in 

 1767 c* Xaas, near Kildare in Ireland, 



