232 On a Mode of erecting liyht Vaults 



are thicker than those at the summit. e Voutes extradossees 

 moiti de niveau et moitie d'inegale epaisseur.' Art. de Batir, 

 vol. iii., p. 328 to 332, and p. 380 to the end of the vol. 

 The author knows several old churches, with semicircular cross 

 or domical vaulting, where the side-walls have declined con- 

 siderably from the perpendicular ; and where the vaults, having 

 separated in the middle, the chinks have been at various 

 times again filled in ; and thus the semicircle has gradually 

 become a depressed arch. If it had, in these cases, been the 

 arch which pushed the walls asunder, it must necessarily have 

 fallen in as soon as the cracks began to gape ; that is, when 

 their edges no longer touched each other. That this did not 

 happen, gives the most convincing proof that no push at all 

 had taken place ; but that rather each half of the vault was 

 detached, and hung to the wall, and that its yielding was pro- 

 duced by other causes. These lie often in a deficient construc- 

 tion of the roof, but most commonly in the small care which 

 is generally taken in carrying off the water which falls on the 

 roof; in consequence of which, the rain-water for the most 

 part drips upon the outside of the foundation, and necessarily 

 causes in this, the most dangerous place, a continued settling, 

 and consequently a gradual outward yielding of the walls. 



In order to make a small thickness of wall suffice, the skilful 

 ancients applied also another effective means namely, to build 

 very slowly, and to put on the vault late, after the complete 

 drying and hardening of the mortar ; and till then, for the tem- 

 porary use of the church, to cover the roof simply with boards, 

 or perhaps, after the manner of so many basilicse, to leave it 

 quite open. This, however, cannot be done at present, when 

 a building must be completed inside and out by a fixed day. 



The material of ancient church vaults is, on the Lower 

 Rhine, everywhere, the well-known tuf, which is manufactured 

 as Trass, modelled to the size of common tiles, and three or 

 four inches thick. On the Upper Rhine, beginning from 

 about Bingen, it is brick of small size : the thickness of the 

 vaults varies from four to eight inches : the execution is often 

 very slovenly, and different in almost every vault ; that is, the 

 concavity is sometimes very flat, sometimes very strong, some- 

 times again uncommonly neat and resembling an assemblage 



