These difficulties have to a great extent been overcome by the folding machine. For many years printers have made use of machines for folding, but it was not until a comparatively recent date that a machine adapted to office use was perfected. Now there are several practical machines on the market. One of these machines will fold from 4000 to 7000 letters or circulars an hour, depending on the size and weight of the paper, and the number of folds. This is equal to eight persons folding by hand, and the machine and operator occupy no more space than one hand folder. The best machines are so made that they can be operated by hand or electric power. Wherever electric lights are available, the power machine can be used. Including the cost of power, the maximum cost of folding with one of the machines is from 3 cents to 5 cents a thousand. One of the well-known types, with motor attached, is shown in Fig. 24. =Mailing Machines.= To a concern mailing large quantities of circular matter, an envelope sealer or mailing machine is indispensable. Envelope sealers which worked in a more or less satisfactory manner have been on the market for several years. These machines have been improved, and their best features combined with new ideas, until now machines are to be had which work perfectly. Several of the machines seal envelopes only; one both seals the envelopes and attaches the stamp. The combination sealing and stamping machine seals, stamps, and counts envelopes at an average rate of 8000 an hour, doing the work of from eight to ten clerks. When sealed by the machine, the flaps of the envelopes are folded tightly and securely sealed. The moistening device is so constructed that the amount of moisture passing under each flap is regulated to a nicety, preventing envelopes sticking together on account of too much water being applied. Coincident with the sealing of an envelope, the stamp is applied. Every envelope passing through the machine is stamped; as a result, none are returned from the postoffice for lack of postage. [Illustration: Fig. 25. The Mailometer Mailing Machine _Mailometer Co._] A special counting device automatically counts the stamps as applied, the counter being in full view of the operator. This means an absolute check on the postage account. The stamps are locked in a glass case and, by moving a lever, the stamp mechanism can be thrown out of operation. It can only be made to apply stamps again by unlocking the case, which places the stamps absolutely under the control of one person. After being sealed and counted, the envelopes pass to a stacker, all facing one way so that they can be tied in bundles for sending to the postoffice. This is a point worthy of consideration where dispatch in getting mail out is of importance. Envelopes of any size up to twelve inches are handled, and by throwing off the sealing device, mailing cards can be stamped.