133 
of Fairlop Fair it was 398 feet thick, and uniform throughout. In 
the Dengey and Rochford hundreds, where the clay is from 300 to 
400 feet in thickness, it is divided by beds of sand into three or 
four parts. A bed of sand also usually occurs between the clay and 
the chalk. These alternations Dr. Mitchell is of opinion, indicate 
successive periods of turbulence and tranquillity. 
A sufficient supply of water is sometimes obtained in the first bed 
of sand, but it is more often necessary to sink to that resting imme- 
diately on the chalk, on reaching which a vast volume of water 
rushes up, and compels the well-digger to ascend precipitately to 
the surface. Cement-stones are sources of great impediment, par- 
ticularly to well-borers, a week or fortnight being occasionally spent 
in punching through a single mass. At the bottom of the clay a 
layer frequently occurs, and is technically called the water-rock, 
because, being penetrated, a powerful spring rushes up. 
The water is sometimes, but not very often, combined with a saline 
substance, probably sulphate of magnesia, as that salt is abundant in 
the waters of the London clay in Surrey, and solid magnesia occurs 
at Stamford Hill, near London. Foul air is not unknown in the 
wells, though it has done little harm in Essex. Its nature has not 
been ascertained, but Dr. Mitchell conceives, that it is probably 
sulphuretted hydrogen, as in Middlesex and Hertfordshire that gas 
has been most destructive. Inthe chalk of Surrey carbonic acid gas 
is very troublesome, and has sometimes produced fatal effects. 
There is, perhaps, no part of the world where artesian wells are 
more general, or are more useful than in Essex. In the vale of the 
Lea they have been bored with the greatest facility and at a small 
expense. In Waltham Abbey the cost is usually about 16/. In 
the district of Bulpham Fen, seven miles south from Brentwood, they 
yield a large supply of water. In the marshes, as well as along the 
coast, and in the islands of Essex, they have proved of the greatest 
utility. Formerly, in some seasons, when the ditches became dry, 
the cattle suffered, the fishes died, and the farmer lost severely on 
his stock ; but by the aid of artesian wells the ditches are now kept 
full all the year, and the farmer and landlord are accordingly bene- 
fited. In Foulness Island there are no natural springs, and until. 
lately no water, except atmospheric, collected in the ditches. In hot 
seasons this water became putrid, but the inhabitants and the cattle 
continued to partake of it as long as it lasted; and supplies were 
then obtained, at the distance of seven miles, from the east end of 
the island. Artesian wells now keep the ditches full of fresh and 
sweet water, labourers are obtained at reduced wages, and farmers 
of a higher class are beginning to reside on the island. Wallisea, 
Mersea, and other islands have profited in a similar manner. 
A great addition is made annually to the land along the coast of 
Essex, and valuable districts, one amounting to five hundred acres, 
and another to between one hundred and two hundred, have been 
recently protected by embankments. Outside of these inclosures 
are tracts of sand, estimated equal to 33,000 acres, not yet covered 
with vegetable mould, but dry eight hours out of every tide. To- 
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