METHODS OF CONSTRUCTING BREAKWATERS. 253 



motion other than that imparted by the movement of the main 

 arm. 



A large machine of the rigid cantilever type, capable of deal- 

 ing with working loads of 50 tons at a radius of 100 feet, is now 

 in use at the Harbour of Refuge works at Peterhead, N.B. ; and 

 two machines of the braced type, of somewhat similar power 

 and dimensions, which were designed by Mr. P. J. Messent, 

 M. Inst. C.E., have been employed for a number of years at the 

 mouth of the River Tyne, constructing the piers there under his 

 direction. 



The setting of blocks or the laying of bags, either by a 

 "radial" or "all-round" machine, is not so easy as with one 

 having only longitudinal and transverse motions, inasmuch as, 

 excepting when the main arm of the machine is parallel to the 

 centre line of the work, the load moves diagonally, and when 

 adjusting its line in one direction it is thrown out in another. 

 This feature is especially inconvenient in setting work under 

 water, where it is out of sight. 



The range or overhang of a setting machine must necessarily 

 depend to a great extent upon the design of the work upon 

 which it is to be employed ; but it will be found convenient to 

 have it sufficiently large to lay three or four tiers of the bottom 

 course of blocks in advance of the scar-end, because the blocks 

 can, in this way, be laid and levelled with greater facility and 

 regularity than when an advance of only one block at a time is 

 made. 



Block-setting, When blocks are to be used in built work, 

 they are generally lifted by means of T-headed bars (Fig. 71, 

 p. 254). These pass through oblong holes, terminating in recesses 

 at the bottom of the blocks. The object of these recesses is to 



allow of the bars being turned round, so that the r ^ 



T-heads may take hold of the blocks, as in Fig. 



70. After a block has been set, the bars can be L- b^rj 



released immediately by giving them a quarter- ( 



turn. 



When blocks are large, it is necessary, in I IL~_~_~_. 



order to prevent the concrete being crushed by FlG - ro - 



the T-heads, to build into them pieces of finely dressed hard 

 stone, or of hard wood or iron for the T-heads to bear upon. 



In blocks for sloping coursed work, the holes for the lifting 

 bolts should be placed at such an angle that the block, when 



