Width at bottom = .7 height; at middle = .5 height; and at top = .3 height.

Tunnels - (From Actual Practice In Brickwork.)

PURPOSE.

Formation of Strata.

Extreme Height.

Extreme Width.

Depth at Crown.

Feet.

Ins

Feet.

Ins.

Feet.

Ins.

Canal................

Various..........

16

2

17

1

3

Canal..........

Clay.......................

21

6

20

1

6

Thames Tunnel....

Clay..............

22

3

37

6

2

6

Railway...........

Chalk....................

26

6

27

1

6

"

Various

27

6

27

1

10½

"

Shale.....................

30

30

1

10½

"

Green sand.........

30

6

30

2

3

"

Freestone..........

36

36

2

3

Canal....................

Chalk and earth..

39

35

6

1

2