Not more than ten persons shall be hoisted or lowered at any one time in any shaft or slope, and whenever five persons shall arrive at the bottom of any shaft or slope in which persons are regularly hoisted or lowered they shall be furnished with an empty car or cage and be hoisted, except, however, in mines where there is provided a traveling way having an average pitch of fifteen degrees or less and not more than one thousand feet in length. This, however, shall not prohibit the hoisting or lowering of twenty persons at one time on slopes where two or more loaded cars are regularly hoisted: Provided, That not less than thirty workmen therein make such a request in writing, to the inspector of the district, and if, in his judgment, the hoisting appliances in every respect are of sufficient strength, he may comply with the request of the workmen: Provided, That in any coal mine or colliery where the hoisting appliances are not of sufficient strength to hoist or lower the number of persons named, the inspector shall have the power to reduce the number of persons to be hoisted or lowered.