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, thougb 

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

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



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

 Sj 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 

 1787 a*. Naas, near Kildare in Ireland. 



