226 On a Mode of erecting light Vaults 



separate compartments are properly supported beneath, the 

 courses which rest on the same point perpetually keep each 

 other in equilibrium, and consequently no further contrivance 

 is needed than to execute the whole courses in the individual 

 horizontal planes at the same time, or nearly at the same time ; 

 that is, to carry the courses all the way round ; consequently 

 the process in such cross-vaulting is the same fundamentally 

 as in domes, when each course is locked by itself as a ring, 

 one ring is gradually laid upon another, and thus finally the 

 dome itself is locked, except that in these domes the upper 

 courses have steeper bed surfaces, and consequently the stones 

 will no longer remain in their places without the application of 

 other auxiliary means, but would slip down as soon as they 

 were laid, if not prevented in some other way. This is now 

 done in Vienna in a very simple manner, by means of some 

 strong ends of rope, which are fastened above, and somewhat 

 backwards, from the course to be vaulted, and hang down like 

 plummets, being loaded below by some stones tied to the rope. 

 As soon as a stone is laid, and by a moderate blow with the 

 hammer pressed against the preceding stone, one of these ropes 

 is brought over the stone, and the pressure produced by the 

 weight of the appended stone, combined with the adhesion of 

 the mortar, is sufficient to hold the stone till it is sufficiently 

 supported by the contact of the next stone ; and this in its turn 

 is prevented from slipping down by the pressure of the cord 

 upon it. 



We very often find, however, over ancient churches, cross- 

 vaults, where the diagonal lines consist of semicircles, and con- 

 sequently the transverse and longitudinal lines, and also the 

 lines of subdivision in compound vaults, form somewhat de- 

 pressed pointed arches, of which the radii are usually three- 

 fourths, but sometimes only two-thirds of the diameter; here 

 the same difficulty occurs in the upper courses, and probably 

 has been met by the same or similar methods. We sometimes 

 observe also a back-vaulting of those courses, of which we 

 shall have to speak again in describing the locking of the 

 vault. 



The only difference between these old cross vaults and the 

 usual ones, consists in this ; that the latter are formed by the 



