1852.] On Filtering the Waters of Tanks. 473 



On Filtering the Waters of Tanks in large quantities, for the use 

 of Towns. — By Henry Piddington. Curator Museum of Econo- 

 mic Geology. 



About four or five years ago I had occasion in reply to some 

 queries addressed to me from Oudeypore by Mr. Brandreth, C. S., to 

 consider this subject a little. I have kept no copy of my letter to 

 that gentleman, but the matter has again been brought to my mind 

 by the observations of Dr. Chuckerbutty lately published, and as the 

 question is one of great sanitary importance, a record of any proposal 

 for accomplishing this, cheaply, effectively, and abundantly, may be 

 worth preserving. Every project of the kind will necessarily be sub- 

 ject to modifications in practice, though the principles may be found 

 always to hold good, and the means to be perhaps more efficacious and 

 cheaper than is commonly supposed. 



There are two objects in filtering water, which are : 



1. To obtain clear water, or mechanical filtering. 



2. To obtain clear and pure water, or chemical filtering. 



And these should be kept in mind as being distinct ; though as I shall 

 subsequently shew, they may perhaps be combined and accomplished 

 at one operation. We will first consider that we have the common river 

 or tank water of Calcutta to deal with, and wish only to make clear — 

 that is wholesome- looking — water of it. 



The processes of nature in filtering her waters are — upwards or 

 downwards, or diagonally, or horizontally, as between the close layers 

 of stratified rocks ; and she uses an infinity of various materials and 

 mixtures of these materials for her filters, some of which act simply to 

 clear the water, and others to purify it. Many of these of both kinds 

 either natural or artificially compounded are within our reach, and I 

 set down here such as occur to me. 



Clearing Materials, Purifiers, 



or Mechanical filters. or Chemical filters. 



1. Common sandy earths. 1. Coarse kunkur gravel of the 



2. Sands, coarse and fine. limestone kunkurs, which is 



