The American tents are of many varieties in shape and size. The Sibley tent gives 1,052 feet to seventeen or eighteen, and sometimes to twenty men, being 53 to 62 feet for each. The Fremont tent is somewhat larger, and, as used in the cavalry camp at Readville, gave the men more air than the Sibley. Both of these have means of ventilation. The wedge-tent, being the simplest in structure, is most easily pitched, struck, and packed by the soldiers, and therefore used by 58 per cent, of the regiments of the Union army, six me sleeping in each. But as occupied by two of the regiments in Massachusetts, in the summer of 1861, it was the most crowded and unhealthy. Those used by the Second Regiment at West Roxbury, and the Ninth at Long Island, (in Boston Harbor,) were twelve and a half feet long, eight feet wide, and six feet high to the ridge, and held twelve men. Each sleeper had 8-1/3 square feet of floor to rest upon, and 25 cubic feet of air to breathe through the night, with no ventilation, except what air passed in through the door-way, when left open, and through the porous cloth that covered the tent. Some of the tents of one of the regiments encamped at Worcester had 56 feet of floor-surface, and 160 feet of air, which was divided among six men, giving each 27 feet of air. In all the camps of Massachusetts, and of most armies everywhere, economy, not only of room within the tents, but of ground where they are placed, seems to be deemed very important, even on those fields where there is opportunity for indefinite expansion of the encampment. The British army-regulations prescribe three plans of arranging the tents. The most liberal and loose arrangement gives to each soldier eighty square feet of ground, the next gives forty-two, and the most compact allows twenty-seven feet, without and within his tent. These are densities of population equal to having 348,000, 664,000, and 1,008,829 people on a square mile. But enormous and incredible as this condensation of humanity may seem, we, in Massachusetts, have beaten it, in one instance at least. In the camp of the Ninth Regiment at Long Island, the tents were placed in compact rows, and touched each other on the two sides and at the back. Between the alternate rows there were narrow lanes, barely wide enough for carriages to pass. Thus arranged, the men, when in their tents, were packed at the rate of 1,152,000 on a square mile, or one man on every twenty--two square feet, including the lanes between, as well as the ground under, the tents.