(b)the employment of a railway servant is said to be “essentially intermittent” when it has been declared to be so by the prescribed authority on the ground that the daily hours of duty of the railway servant normally include periods of inaction aggregating to fifty per cent. or more (including at least one such period of not less than one hour or two such periods of not less than half an hour each) in a tour of twelve hours duty (on the average over seventy-two consecutive hours), during which the railway servant may be on duty, but is not called upon to display either physical activity or sustained attention;
