The machinery of the Post-Office Department is a system of cog-fitting wheels, in all its component parts; and were it not so, in the necessarily limited period and space allotted, the work in postal-cars could not be successfully accomplished. The interior dimensions of postal-cars vary, from whole cars sixty feet in length, to apartments five feet five inches in length by two feet six inches in width. The most comprehensive conception of the practical working of the postal-car system, can be formed in a railway post-office from forty to sixty feet in length; with this in view, we will make a trip in one. A permit to ride in the car, signed by the superintendent of the division of the service, is necessary to allow us the privilege; and it is also required of clerks belonging to other lines. This rule is necessary, in order that the clerks may perform their work uninterruptedly and correctly; and also to exclude unauthorized persons from mail apartments. After a hasty exchange of salutations with the four clerks, the "clerk in charge" notes our names on his "trip report," and we are assigned a spot in the contracted space, where, we are assured, we will be undisturbed, at least for a while. The trip report mentioned is used in noting connections missed, and other irregularities that may occur. The interior of the car is fitted up with a carefully-studied economy of space, upon plans made under the supervision of the superintendent of the division, or chief clerk of the line. Occupying one end of the car are cases of pigeon-holes, or boxes, numbering from six hundred to one thousand, arranged in the shape of a horse-shoe, for the distribution of letters. These boxes are labeled with the names of the post-offices on the line of road, connecting lines, States, and prominent cities and towns throughout the country. A long, narrow aisle passes through the centre of the car, on both sides of which are racks for open sacks and pouches, into which packages of letters and pieces of other mail matter are thrown; on the sides above are rows of suspended pouches, with their hungry mouths open. By this plan, in this contracted space, upwards of two hundred different pouches and sacks can be distributed into between the termini. On one side of the aisle is a narrow counter, upon which the mail matter is emptied from the pouches and sacks; this is hinged to the pouch-rack, and can be swung back, to enable the clerks to get at the pouches more easily. The space beyond, divided by stanchions, is for the stowage of mails, and for their separation into piles. [Illustration: INTERIOR OF A RAILWAY POST-OFFICE.]