238 EEPOEX — 1882. 



the London water-supply. The following is a section of the filter beds 

 of the Chelsea Waterworks, for which I am indebted to the manager, 

 Mr. Lott :— 



ft. iu. 



Fine sand 3 6 



Shells' 4 



Shingle 2 



Coarse shingle - 2 2 



Total' 8 



The chemical analysis of the top bed of the filter, which does the 

 work, gave the following : — 



Silica 90'05 



Alumina "40 



Oxide of iron 6'90 



Carbonaneous matter ... . . -70 



Carbonic acid 1'50 



Magnesia "01 



Alkalies trace 



Moisture "26 



99-97 



' "'The rate of filtration is 2 gallons per hour.'* By comparing the 

 chemical analysis of the filter-bed with those of the Millstone Grit and of 

 the Pennant Grit, it will be seen that to all practical purposes the analyses 

 are the same. There is nothing in the chemical composition of the filter 

 which can oxidise the organic impurities of the Thames water passing 

 through, but the oxidation is effected by air between the gi-ains of sand 

 with perfect effect. It is much the same with water percolating through 

 the rocks of the earth ; it comes in contact with air collected in the 

 interstices. With such rocks as the Mountain Limestone, however, where 

 the water yielded comes through fissures and joints in the strata, and does 

 not percolate, it is a question whether the purifying process would be 

 always satisfactory. 



Appendix V. — List of Queries circulated. 



1. Position of well or shafts with which you are acquainted ? la. State date at 

 which the well or shaft was originally sunk. Has it been deepened since by sinking 

 or boring ? and when ? 2. Approximate height of the surface of tlie ground above 

 Ordnance Datum (mean sea-level) ? 3. Depth from surface to bottom of shaft or 

 well with diameter ? Depth from surface to bottom of bore-hole, with diameter ? 

 3a. ^ Depth from the surface to the horizontal drift-ways if any ? What is their 

 len'oth and number ? 4. Height below the surface, at which water stands before 

 3JxA after pumping. Number of hours elapsing before ordinary level is restored after 

 pumping ? 4«. Height below the surface at which the water stood when the well 

 was first sunk, and height at which it stands now when not pumped ? 5. Quantity 

 capable of being pumped in gallons per day of 24 hours ? Average quantity daily 

 pumped ? 6. Does the water-level vary at different seasons of the year, and to what 



' To keep the upper layer from mixing with the lower. 



'^ Gradually increases in coarseness towards the base to prevent the pipes which 

 carry off the water from becoming clogged. 



3 The arenaceous material is taken from the Thames, and is well cleansed by a 

 powerful hose playing upon it. , , . , ^ 



* Colonel Bolton's Report for February, 1882, for which I am indebted to the 

 courtesy of Colonel Bolton. 



