72 A TREATISE OX DREDGES AND DREDGING 



from the pistons of vertical engines. This method is commonly 

 employed, as it works well through any material, and the speed of 

 the motion on the ladder can be varied independently of the engine. 



The efficiency of the ladder dredge is determined by the number 

 of buckets that pass over the driving tumbler every minute, which 

 as a rule varies between 15 and 20. But the efficiency really depends 

 upon the material to be dredged and the capacity of buckets em- 

 ployed. Smaller resistance is encountered in dredging loose soils and 

 consequently the driving tumbler may run at greater speed and more 

 buckets will pass over it every minute; yet in clayey soils it is neces- 

 sary to run the tumbler at reduced speed in order to give time to 

 the sticky clay to detach itself from the buckets. In removing rock 

 it is necessary to attack the material with great force, consequently 

 the buckets are run at high speed, hence a greater number will pass 

 every minute over the driving tumbler. However, the buckets, 

 in order to be stronger so as to easily break the rock, will be of 

 smaller capacity than those employed for dredging loose soils. 

 Thus the efficiency of the dredge depends upon the material to be 

 dredged and the capacity of the buckets, which, however, should be 

 filled up only to J of their capacity. In general, when the dredge 

 is working in shallow water, the buckets are traveling at a slight 

 inclination only and consequently the buckets will be only par- 

 tially filled, while they will be full when working in deep water, 

 when the ladder will be in almost a vertical position. Consequently 

 the inclination of the ladder should also be taken into consideration 

 in determining the efficiency of a dredge. 



In the ladder dredge a large amount of power is wasted in over- 

 coming the great friction of the various parts of the machinery. 

 Mr. Webster, after an accurate series of experiments, was able to 

 deduce some practical rule, determining the power required to 

 work a ladder dredge through different materials. The result of 

 his experiments are expressed in the general formula: 



in which C is a coefficient varying with the different materials; 

 W= number of tons per hour to be dredged; 

 H= height of the upper tumbler from the bottom of the 

 surface ground to be dredged. 



The different values of C are 0.04 for very stiff clay and mud, 



