71A. In this Chapter, unless there is anything repugnant in the subject or context,- (a) the employment of the railway servant is said to be “essentially intermittent” when it has been declared to be so by the authority empowered in this behalf, on the ground that it involves long periods of inaction; during which the railway servant is on duty but is not called upon to display either physical activity or sustained attention; and ²[* * *]