The most abundant deleterious gas met with in wells is carbonic acid, which extinguishes flame and is fatal to animal life. Carbonic acid is most frequently met with in the chalk, where it has been found to exist in greater quantity in the lower than in the upper portion of the formation, and in that division to be unequally distributed. Fatal effects from it at Epsom, 200 feet down, and in Norbury Park, near Dorking, 400 feet down, have been recorded. At Bexley Heath, after sinking through 140 feet of gravel and sand and 30 feet of chalk, it rushed out and extinguished the candles of the workmen. Air mixed with one-tenth of this gas will extinguish lights; it is very poisonous, and when the atmosphere contains 8 per cent. or more there is danger of suffocation. When present it is found most abundantly in the lower parts of a well from its great specific gravity. Sulphuretted hydrogen is also occasionally met with, and is supposed to be generated from the decomposition of water and iron pyrites. In districts in which the chalk is covered with sand and London clay, carburetted hydrogen is occasionally emitted, but more frequently sulphuretted hydrogen. Carburetted hydrogen seldom inflames in wells, but in making the Thames Tunnel it sometimes issued in such abundance as to explode by the lights and scorch the workmen. Sulphuretted hydrogen also streamed out in the same place, but in no instance with fatal effects. At Ash, near Farnham, a well was dug in sand to the depth of 36 feet, and one of the workmen descending into it was instantly suffocated. Fatal effects have also resulted elsewhere from the accumulation of this gas in wells. INDEX. Abridge, well at, 190 Accident tools, Mather and Platt's, 138-143 Acton, wells at, 190 Africa, rainfall in, 30 Air freshening in wells, 53 Albany Street, well at, 190 Aldershot Place, wells at, 190 Alluvion, 5, 7 America, North, rainfall, 30, 31 ---- South, 31 American tube well, 81 Amwell End, well at, 190 Apothecaries' Hall, well at, 191 Apparatus for boring, 68, 71, 72 Arlesey, well at, 190 Artesian well, definition, 2 ---- ---- causes of failure, 2-4 Ash, well at, 190 Asia, rainfall in, 29, 30 Augers, 62-64 Available rainfall, 27 Bagshot Sands, 5 ---- well at, 190 Balance-beam, Kind's, 91 Balham Hill, well at, 190 Ball-clack, 91 Bank of England, well at, 190 Bare outcrop, 18-21 Barking, well at, 190 Barnet, wells at, 190 Battersea, wells at, 190 Bearwood, well at, 191 Beaumont Green, well at, 191 Bell-box, 64 Belleisle, well at, 191 Berkeley Square, well at, 191 Bermondsey, wells at, 191 Berry Green, well at, 191 Bexley, well at, 191 Bexley Heath, wells at, 21 Bickford's fuse, 50 Birkenhead, wells at, 155 Birmingham, wells at, 156 Bishop Stortford, wells at, 167, 191 Blackfriars, well at, 191 Blackheath, well at, 191 Blasting, sinking by, 44