n8 HARBOUR CONSTRUCTION. 



of an adjustable ratchet arrangement. The water is regulated 

 by means of a screw-cock. 



The machine is driven by means of a vertical engine, which, 

 together with its boiler, etc., is mounted upon the same frame 

 and carriage as the mixer, the whole travelling upon an ordinary 

 line of rails, and being self-propelling. 



The mixing of concrete by any of the foregoing machines is 

 much more expeditious and perfect than mixing by manual 

 labour, and in large works concrete-mixing machines, of one 

 kind or another, are now almost always employed. 



At the Newhaven harbour-works (Sussex), a very large 

 " Carey-Latham " machine was in use, it being capable of mixing 

 and delivering about 300 tons of concrete per hour. 



A convenient size of machine for general use is one which is 

 termed a " 20-yard mixer," by which is meant one that will mix 

 20 cubic yards per hour. In practice, however, taking stoppages 

 and hindrances into account, a machine of this size cannot be 

 relied upon to turn out more than from 14 to 16 cubic yards per 

 hour. 



Concrete block making. Concrete blocks are usually cast in 

 moulds formed of 3-inch or 4-inch planking, according to the 

 size of the blocks to be made. The sides of the moulds are so 

 put together that they can be easily taken apart for the purpose 

 of freeing the blocks when sufficiently hard. Moulds for the 

 larger sizes of blocks are generally stayed, either by means of 

 bolts passing completely through them, or by horizontal side- 

 stiffening ribs. Where bolts are used, they should be turned 

 the day after the concrete has been put in, so as to facilitate 

 their withdrawal, which should take place at the time of the 

 removal of the moulds say three to four days after the blocks 

 have been made. 



Sometimes pipes are placed in the blocks, through which the 

 stay-bolts or small chains are passed. I, however, much prefer 

 making the sides of the boxes so stiff that the use of through- 

 stays is altogether obviated. 



The holes for the T-headed bars, which are commonly used for 

 lifting the blocks (Fig. 71, p. 254), are best made by means of long 

 tapered folding wedges, or " cores," which are easily withdrawn 

 after the concrete has set. At the bottom of these holes chambers 

 are formed of sufficient size to admit of the T-ends of the lifting- 

 bars being freely turned. These chambers are most conveniently 



