An interesting advance in the postal system of our country was made recently when the first of the pneumatic tubes which are to carry mail underground from one office to another was declared ready for use. Some three hundred prominent men were present to see the first package of mail matter sent. [Illustration: Self-Registering Mail Box] This tube extends from the Produce Exchange to the Post-Office Building, and the trip can be made from one office to the other in one minute and a quarter. Mr. Chauncey Depew was present at the opening ceremonies, and having made an appropriate speech, sent off the first carrier of mail matter that passed through the tube. In less time than it takes to tell the story the carrier returned, bringing a receipt for the mail that had been sent, and a pretty little kitten which arrived breathless from its spin through the tube. The carriers are two feet long and seven inches round, and are made to fit the tube closely. Other tubes are to be laid throughout the city, and before very long every post-office in the city will be connected with the general post-office by pneumatic tube, and letters will be posted in Harlem and sent flying down the seven miles to the City Hall in a few minutes. Another ingenious postal device which has just been put on trial is the scheme for registering letters yourself. The first thing to do is to put a ten-cent piece in the slot. The coin opens a small registering window, and reveals a pad on which you write the address of the registered letter, and also an aperture through which the letter is to be dropped. The letter must first have been stamped with a two-cent stamp. After the letter is mailed the sender pulls a handle until a gong rings, and a receipt is then pushed out toward the sender. This receipt is in fact the second half of the order which he himself has written. As soon as the receipt is given the machine locks itself, and nothing will unlock it but a fresh dime in the slot. Worn coins, or those that are not full size and weight, are instantly rejected by the machine. The coin, after entering the machine, passes over a very delicate balance, and if it is found to be light or bad when it is weighed, the machine throws it out on the floor in front of the would-be registerer. Three of these machines have been placed on trial: one in the Post-Office Building, one in the Equitable Building, and one in the branch office at Forty-second Street.