GRADES AND SELF-CLEANSING VELOCITIES 159 



and Filkins of Cornell University note that a piece of brick 

 nearly cubical in shape, weighing 22 ounces and having a 

 volume of over 16 ounces, was carried by a flushing- wave more 

 than 1000 feet, while under the same conditions a mere flake 

 of the same brick, having a volume of not more than 2 cubic 

 inches, could not be moved more than 600 feet. Also a piece 

 of limestone nearly cubical, weighing 7.75 ounces, was car- 

 ried 1400 feet, while a piece weighing 4.75 ounces, but nearly 

 flat in shape, was carried only 470 feet. 



It is seen, then, from the above old, meagre, and variable 

 data that a flow of water requires a certain velocity to carry 

 along solid material, and that the suspension of the material 

 depends also on its size, shape, and specific gravity. 



Material deposited at the same place will be lifted by a 

 flow of water and carried to different distances; those pieces 

 whose shapes are such as to withstand the current, offering 

 a thin and sloping edge to it, being last taken up, as the velocity 

 increases, and soonest dropped. In a similar way a large stone 

 too heavy to be carried along has been found to shelter smaller 

 ones which otherwise might have been taken up by the cur- 

 rent. Small irregularities in the channel serve as shelters for 

 the fine material, and piles of sand, etc., are likely to accumulate 

 behind projecting bits of mortar. It is plain, then, that 

 neither theoretical determinations of the velocity required to 

 carry matter in suspension, nor yet the results of experiments 

 on different materials of varying sizes and specific gravities, 

 are sufficiently like the conditions prevailing in sewers to deter- 

 mine the velocities required in the latter, and it is only from 

 experience in sewers themselves, where the material to be 

 transported is that natural to a sewer and where the condi- 

 tions of rugosity of bed and variation in the velocity in the 

 different laminae are those peculiar to a sewer, that any reli- 

 able recommendations must come. 



As an indication of the extreme lower limit of the range 

 of velocities, reference may be had to the recent work on sedi- 

 mentation where studies have been made on the velocity of 



