i873-J Notices of Books. 533 



sand, gravel, and broken stone or shingle, the depth of the whole 

 varying from 5 to 6tV feet ; a layer of cells has sometimes been 

 used placed between the stratum of gravel, but this is not found 

 essential, and is now generally omitted. In some cases this 

 filter bed rests upon a foundation of puddled clay or concrete, 

 where there is a loose and porous sub-stratum beneath. Each 

 filter bed, at short intervals varying with the condition of the 

 water, must have the deposit which accumulates on the surface 

 of the sand cleared off or removed, and while any one is under- 

 going this process, the other remaining filters must be competent 

 to deliver the required supply without overstraining their functions. 

 If, then, there are six filters, five of them must be competent 

 to the full duties of the service, and if eight filters, seven of 

 them must be competent to this duty, on the supposition always 

 that not more than one filter will at any time be off duty. Should 

 the circumstances in effect render two unserviceable, the re- 

 mainder must have area enough to meet the requirements of the 

 case. 



Space will not admit of our entering further into detail re- 

 lative to the peculiarities of filter beds in different localities and 

 under varying circumstances, but these will be found carefully 

 considered in the pages of the Report itself, to which we must 

 refer our readers if they desire more reliable information upon 

 the subject. 



VOL. III. (N.S.) 3 Z 



