414 EEPOKT— 1882. 



by sedimentary and crystalline deposits. Deep mines are often very 

 dry ones. 



As an example of water-supply obtained from deep down in the chalk, 

 we may take the supply drawn from the London basin. Below London 

 we have a great thickness of both the upper and the lower chalk, amounting 

 together to more than GOO feet in places. As this chalk, so far as we 

 know, rests in a basin of gault clay, and rises in continuous beds to form 

 the high ground to the north and south of the London basin, it is in the 

 best position to be saturated with water, and such was its condition before 

 it was made use of as a source of water-supply for London and its neigh- 

 bourhood. As the supply from the shallow -wells made in the tertiary 

 beds became exhausted, deeper wells were sunk into the chalk beneath, 

 and water was obtained in plenty. But the supply soon ceased to be equal 

 to the demand ; the level of the water in the chalk was depressed, so that 

 wells had to be deepened, and new adits made to cut new fissures. Li 

 1838 the quantity pumped' in London was estimated at 6,000,000 gallons 

 a day ; in 1850 the quantity had increased to not more than 12,000,000 

 gallons a day. In the meantime the level of the water in the chalk had 

 been depi^essed in some places 50 feet. Thus the larger quantity could 

 only be obtained at a greater depth, and by increasing the fall in the 

 fissures delivering the water to the pumps. Now this chalk, from which 

 only a limited supply could be obtained at a given depth over a large area, 

 was connected on all sides with an unexhausted i-eservoir, at a much 

 higher level, formed by the saturated masses of chalk round the London 

 basin, and yet the water-level in it was reduced by a compai'atively small 

 amount of pumping.^ 



Before leaving the subject of the quantity of water found in carrying 

 out particular works, 1 will refer to two tunnels now in progress, although 

 they are not being made in the chalk formation. 



The Mersey Tunnel, which is being made under the direction of Mr. 

 Brunlees, one of the engineers of the Channel Tunnel Company, will 

 connect Liverpool and Birkenhead by means of a railway. It is 2j miles 

 long, of which nearly three-quarters of a mile is beneath the Mersey. 

 Shafts have been sunk, and a heading has been driven for a quarter of a 

 mile, and the main tunnel for 600 yards, through the pebble beds of the 

 New Red Sandstone. Mr. Brunlees tells me that the greatest quantity of 

 water pumped amounted to 6,000 gallons a minute. This quantity is 

 being gradually reduced by lining and tubbing. 



The Severn Tunnel is in a more advanced state ; and as it illustrates 

 many of the points to which I have referred, and as I have been continually 

 on the works during the last three years, I shall refer to it at greater 

 length. The Severn Tunnel will be the longest railway tunnel in England, 

 in all 4i miles in length ; and it is the only railway tunnel which can claim 

 to be submarine. It passes, for 2^ miles, below a tideway where there is 



' Minutes of Proceedings of Institution of Civil Eiigineers, vol. ix. p. 161 (Homer- 

 sham). 



^ Minittcs of Proceediinis of InMitntion of Civil Engineers, vol. ix. p. 165 

 (Braithwaite), p. 155 (Clutterback), and p. 168 Braithwaite, states Messrs. Combe 

 bored 300 feet into chalk, and only obtained 25 gallons a minute ; Meux & Co. bored 

 160 feet, obtaining only 10 gallons a minute ; Messrs. Keid & Co. laid open an area 

 of 1,600 feet of chalk, and only obtained 200 gallons a minute ; p. 172 Homersham 

 admits depression of water-level under London ; p. 175 Taberner admits depression 

 of water-level under London ; p. 176 Home says, water at St. Luke's depressed 21 

 feet between 1841 and 1850. 



