TYI'KS OK CKNTKKs A Mi MOLDS '-7 



Only skilled and exj>erienced workmen should lie employed in 

 erecting the centers ; and they should work under the immedi- 

 ate direction of the engineer, who must establish the axis and 

 level of each center by transit and level. 



Lagging. By the lagging is meant the covering of narrow 

 longitudinal boards resting upon the upper curved chords of the 

 centers, and spanning the opening between consecutive centers. 

 This lagging forms the em-veil surface or mold upon which the 

 arch masonry is laid. When the roof arch is of ashlar masonry 

 the strips of lagging are seldom placed nearer together tlian 

 the joints of the consecutive ring stones, but in brick arches 

 they are laid close together. llesides the weight of the arch 

 masonry, the lagging titulars support the sln.it props winch 

 keep the poliii-l)oards in place after the strutting is removed 

 and until the arch masonry is eomplet.-.l. 



Strihin;i tin- (,'///,/*. The centers are usually brought i<> 

 thf proper elevation by means of wooden wedges inserted be- 

 tween the sill of the ernter and its support, or U-tween the 

 in of the posts carrying the center and the tunnel Moor. 

 These wedges are usually made of ban! wood, and are alnmt 

 : ins. wide by I ins. thick by IS ins. lon^. To strike the center 

 att'-r the aivh masonry is completed, these wedges are with- 

 drawn, thus allowing the renter to fall clear of the masonry. 

 Usually the center is not removed immediately after striking, 

 so that if the an-h masonry fails the ruins will remain ujwm the 

 center. The method of striking the iron miter devised by Mr. 

 K/iha has been described in the previous chapter on strutting. 



