So truly are those grooves formed, that the slight tap of a hammer is sufficient to fix the coak in its place.
By the seventh operation, the central hole in the coak for the pin, on which the sheave turns, is drilled out.
The coak which remains in the retorts constitutes about 60 per cent.
When the coak is used for heating the retorts, about one half of the whole is required.
If we estimate the coak by its comparative heating power, it represents 65 per cent.