Correspondence is conveyed by either railway or steamer; on the roads it is forwarded to its destination by special native couriers. The weight of the despatches enclosing letters and printed matter may not exceed, for transport by land, 10 kilogrammes.[21] The porters required for this service are furnished by the chiefs of posts. The transmission of correspondence into the interior of the country is, besides, regulated by instructions, to which the local authorities frequently draw the serious attention of the territorial chiefs. Thus, in all parts of the State territory, the couriers must leave on a fixed day, and they have a certain time, which has been calculated after much experience as sufficient, for the journey from one point to another. It is expressly forbidden to the authorities to detain the native couriers after the date fixed for their departure, or to entrust them with correspondence not sealed. All postal packages must be paid for (with the exception that certain officials have the right to post free) and enclosed in a sealed envelope having the address clearly shown. Each postal despatch contains a ticket of advice which is to be returned to the originating office, dated and signed by the agent of the office that received it, after he has found the contents exact. The carrier of the mail is also in possession of a route ticket which informs him of the number of sacks and envelopes composing the mail. It must be checked and dated, and must show in a special column, for the way out and back and for each station, the hour of the arrival and departure of the couriers. The sub-controllers of the post-offices must forward each month, for the purpose of verification, to the Controller of the post-office at Boma, the route tickets of the couriers sent during the previous month. The Director of Finance sends, as often as possible, the Controller of the Post-Office to examine the accounts of the various offices which are run by selected agents appointed from the Belgian administration. In districts where sub-post-offices are established, the District Commissioner sees to the strict observance of the instructions regulating the important postal service.