448 Mr. Underwood on the Coliseum at Rome. [June, 



Article IX. 



On the Coliseum at Rome. By T. R. Underwood, Esq. MGS. 



(To the Editor of the Annals of Philosophy.) 



SIR, . P<>™, Maijn, 182*. 



As I am not aware that the following circumstance has been 

 hitherto noticed, I shall beg the favour of the insertion of it in 

 your journal. 



It is generally supposed that the ground plan of the external 

 wall of the Coliseum at Rome is an ellipsis. This, however, is 

 not the case, as I discovered, when examining it in September, 

 1802, that the plan of the external surface of the walls between 

 the columns was a straight line, and not a curve ; so that this 

 vast amphitheatre is in fact a polygon of 80 sides. The differ- 

 ence in the effect of the whole, between this and an ellipsis, is 

 not sensible in a general view, the angles of the polygon being 

 covered by the columns ; but the saving of time and expence in 

 constructing the arches circular in a curved plan must, as every 

 architect well knows, have been very considerable indeed. The 

 entablatures, however, over the columns are ellipses, which is 

 the source of the error respecting this building. 



On my return from Italy such was the force of prejudice, even 

 in professional men, English as well as French, in favour of the 

 supposed ellipsis, that I could not obtain any credit for this 

 statement, nor even induce any one who visited Rome, by accu- 

 rate examination, to confirm or confute it, tillM. A. Brongniart, 

 Member of the French Institute, on his visit to Italy in 1820, 

 undertook, at my request, the inquiry, and his careful examina- 

 tion has confirmed its accuracy. 



I have the honour to be, your humble servant, 



T. R. Underwood. 



Article X. 



A nalusis of the Argillaceous Iron Ore. 

 By R.Phillips, FRS.&c. 



It is well known that the greater part of the immense quan- 

 tity of iron yielded by the mines of this kingdom is obtained 

 from what is called the argillaceous iron ore. A specimen 

 which I analyzed was of that variety, which is called at Low 

 Moor Iron Works near Bradford, Yorkshire, Black Iron Stone. 



Its colour, as its name imports, is nearly black ; its sp. gr. 



