The bearing-rings are made of fibrous woven material, especially prepared, and held in place by being clamped between two metal rings, one of which is riveted to the body of the carrier. Of course these rings wear out and have to be replaced occasionally, but their usual life is about one thousand miles. The rear end of the carrier is closed by a hinged lid and secured by a special lock. The lock consists of three radial bolts that pass through the body of the carrier and the rim of the lid. These bolts are thrown by three cams, attached to a short shaft that passes through the lid and has a handle or lever attached to it upon the outside of the lid. This cam-shaft is located out of the geometrical centre of the lid in such a position that when the lever or handle is swung around in the unlocked position, it projects beyond the periphery of the lid, and in this position the carrier will not enter the tube. When the lid is closed and locked, the lever lies across the lid in the position shown in Fig. 19, and when the carrier is in the tube it cannot become unlocked, for the lever cannot swing around without coming in contact with the wall of the tube. This insures against the possibility of the carriers opening during transit through the tube. The empty carriers weigh about nine pounds, and when filled with mail, from twelve to fifteen pounds. They have a capacity for two hundred ordinary letters, packed in the usual manner. =Operation of the Tubes.=—The tubes are kept in constant operation during the day, and six days of the week. The air-compressor is started at nine o’clock in the morning and runs until seven in the evening, except during the noon hour, the air flowing in a constant steady current through the tubes. When a carrier is placed in the tube it is carried along in the current without appreciably affecting the load on the compressor. Carriers may be despatched at six-second intervals, and when they are despatched thus frequently at each office, there will be eighteen carriers in the tube at the same time. If ten carriers per minute are despatched from each office, and each carrier contains two hundred letters, the tube has a carrying capacity of two hundred and forty thousand letters per hour, which is far beyond the requirements of this office. About five hundred carriers a day are despatched from each office. This varies considerably on different days and at different seasons of the year. Experience has taught that a certain period of time should elapse between the despatching of carriers, in order that they may not come in contact with each other, and that the receivers may have time to act. With the present plant this period is made about six seconds. In order to make it impossible for carriers to be despatched more frequently than this, time-locks are attached to the sending apparatus. One of these locks may be seen in Fig. 13, connected to the handle of the sending apparatus. It is so arranged