ON ARCHITECTURE AND CARPENTRY. l€^ 



the dome be diminislied as it rises, it will not require to be bound so high: 

 thus, if the increase of thickness in descending begin at about 30 degrees 

 from the summit, and be continued until, at about 60 degrees, the dome be- 

 comes a little more than twice as thick as at first, the equilibrium will be 

 so far secure; and at this distance it would be proper to employ either a 

 chain, or some external pressure, to preserve the stability, since the weight 

 itself would require to be increased without limit, if it were the only source 

 of pressure on the lower parts. (Plate XII. Fig. 157.) 



The dome of St. Paul's cathedral is elliptical, and is built of wood, and 

 confined by strong chains, consisting of iron bars; that of the Pantheon at 

 Rome is nearly circular, and its lower parts are so much thicker than its 

 upper parts, as to aflPord sufficient resistance to their pressure : they are sup- 

 ported by walls of gVeat thickness, and furnished with many projections 

 which answer the purpose of abutments and buttresses. (Plate XII. Fig. 

 158, 159.) 



A knowledge of the parts and proportions usually assigned to columns, and 

 to buildings in general, and of their technical names and divisions, belongs 

 rather to the subject of ornamental than to that of useful architecture; and 

 the consideration of symmetry and elegance is in great measure foreign to 

 that of the mechanical properties of bodies, which it is our present busi- 

 ness to investigate. The five orders of ancient architecture are found to 

 differ considerably in their proportions, in the different remains of Greek 

 and Roman edifices ; but there always remain some characteristic distinctions :" 

 the Tuscan is known by its strength and simplicity, Avithout any peculiar 

 ornament; the Doric by its triglyphs, or triangular grooves, above each 

 column, imagined to represent the ends of beams; the Ionic by the large 

 volutes, and the Corinthian by the foliage, respectively enveloping their 

 capitals; and the Composite usually by the combination of both these 

 characters; each order being lighter than the preceding, and being sometimes 

 employed with it in the upper parts of the same building. In general, tlie 

 length of the Tuscan column, with its capital, is equal to about seven 

 diameters of the base, that of the Doric eight, of the Ionic nine, 

 and of the Corinthian and Composite ten diameters. (Plate XII. Fig. 

 160 . . 164.) 



