Postmasters may, in addition to the leave of absence provided by law, grant a leave of absence without pay to carriers in cases of illness, disability received in the service, or other urgent necessity, such leave not to exceed thirty days. An application for a leave of absence to cover a longer period of time must be made to the First Assistant Postmaster General (Division of Free Delivery) for suspension with loss of pay, or for removal from the service, as the circumstances may require. When regular carriers are absent from duty for any cause, their places shall be supplied by substitute carriers. The Postmaster General is authorized to employ, when necessary, during the time such fifteen days’ leave of absence is granted to a letter carrier, such number of substitute letter carriers as may be deemed advisable, who shall be paid for the services rendered at the rate of six hundred dollars per annum. Substitute carriers shall be assigned to duty by the postmaster, or his representative, and must never be called into service by carriers, except in cases of urgent necessity, when it is plainly impossible to notify the postmaster in time, either by telephone or messenger. Substitute letter carriers will be paid one dollar per annum, payable quarterly. They also receive pay at the rate of $600 per annum when serving in place of carriers who have been granted annual leave of absence. The pro rata salary of carriers for whom they serve who are absent without pay is also given to them, as well as the pro rata salary of carriers who are granted leave of absence with full pay in order that they may serve as members of local Civil Service Boards in conducting examinations, as witnesses for the Government in United States courts, or as members of the United States Militia of the District of Columbia. When substitutes serve for regular carriers on Sunday and perform the same amount of work the regular carrier would have performed on that day, they receive a full day’s pay. Hours of Service for Carriers. Eight hours constitutes a day’s work for letter carriers in cities or postal districts connected therewith, for which they receive the same pay as is paid for a day’s work of a greater number of hours. If any letter carrier is employed a greater number of hours per day than eight he is paid extra for the same in proportion to the salary fixed by law.