The time fixed by this Act as the time during which the F1workers in a mine may be below ground for the purpose of their work and of going to and from their work may be extended as respects any mine by the . . . F2 manager of the mine, F3on not more than sixty days in any calendar year by not more than F4one hourF4half an hour a day, and on any day on which an extension of time is made in accordance with this section as respects any mine the time as so extended shall be substituted for the purposes of this Act as respects that mine for the time as fixed by this Act.

The . . . F2 manager of every mine shall cause a register to be kept in such manner as the Secretary of State may direct of the cases in which any extension of time has been given under this section, and the register shall be open to inspection by the inspector.