ON ARCHITECTURE AND CARPENTRY. 127 



as the span becomes eleven fourteenths of the whole diameter. But if the 

 thickness of the dome be diminished 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 becomes 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 sta- 

 bility, 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 afford sufficient resistance to their pressure : they are 

 supported by walls of great 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 

 business 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, without 

 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 envelop- 

 ing 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, the 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.) 



The Gothic architects appear to have been superior to the Greeks in the 

 mechanical arrangement of the parts of their edifices, so as to produce the 

 most advantageous effect in preserving the general equilibrium. They 

 made every essential member of their buildings a constituent part of their 

 system of ornament, and even those embellishments, which, by a super- 

 ficial observer, might be deemed useless or prejudicial, are frequently cal- 

 culated, either by their strength or by their weight, to serve some bene- 

 ficial purposes. The pointed arch is not in all cases well calculated for 

 equilibrium, but when it has a pillar resting on its summit, it is exceedingly 

 strong. The most celebrated of modern architects have sometimes been 

 less successful than those of the middle ages ; and for want of paying suf- 

 ficient attention to mechanical principles, have committed such errors in 



