and from this time, till they arrive at maturity, they go on always augmenting the diameter of their stems, but at the same time decreasing in number. It is calculated that, if sixteen hundred trees of four inches in diameter can stand and thrive on an acre of ground, there will not be more than four hundred of them when the trees have grown to eight inches, two hundred when they have reached twelve inches, and between one hundred and one hundred and forty when they have attained sixteen inches in diameter. Little is to be done in the earlier stages of a forest's growth except to keep the heads of the most valuable species from being overtopped by those which stand near them: and this can be done best, not by removing the others, but by cutting off or breaking the. tops; for it is desirable at this stage, for the sake of the natural pruning, to have the trees growing as thickly together as possible. At a later stage, thinnings can be judiciously arranged so as to pass through the entire forest at intervals of from ten to fifteen years, enabling the whole area to be operated on in turn. In executing these, the most difficult of all forest operations, it will be well to remember that the object is to give room to the heads of the trees, and not to their stems; for the stems will never be too close together as long as the heads have room properly to develop themselves. The favoring of the most promising trees, and the removal of the weaker ones, together with the preservation of continuous shade to the surface of the ground, while all the trees have sufficient room to grow, should be the particular ends aimed at.
A New Plan for Armored Vessels.—The Naval Committee of the United States House of Representatives has given favorable consideration to a new plan for building armored vessels which has been devised by a retired invalid engineer. The principal armorial application consists of a submerged "turtle-back," about four inches thick, and extending from side to side and from stem to stern of the vessel and below the waterline, so arranged that an enemy's shot from any direction can hit it only at a deflecting angle, so as to be thrown off rather than go through. The sides of the vessel above the turtle-back are filled in with cotton or cork, in which a breach made by the passage of a ball will be self-closed by the elastic action of the substance. They may, moreover, be shot to pieces without destroying the buoyant power of the ship. The guns are mounted upon heavy, impenetrable, centrally arranged, cylindrical armor, which extends to the bottom of the ship, and is there seated on an hydraulic cushion. The breech of the gun is also inclosed in an oval armor, so arranged as to deflect a ball, striking it from any point, in a harmless direction. The gun is operated by an hydraulic loading apparatus, which is worked by one gunner, and hydraulic buffers are provided to take up the recoil.
The Timber-Line of Mountains.—Mr. Henry Gannett, noticing, in "The American Journal of Science," Dr. Rothrock's statement that, as a whole, there is little or no increase in the altitude of the timber-line toward the equator in the Western hemisphere, south of the forty-first parallel of north latitude, observes that the height of the timber-line is purely a question of temperature, and that that is a function of the latitude, the elevation, and the mass, of the country in the neighborhood. A great mass of country, if raised to a considerable height above the sea, carries with it the isothermals. Therefore, in considering the height of the timber-line, "we must regard the mountain-ranges in connection with the plateaus on which they stand, their latitudes, heights, and masses, or what, in a measure, sums up these three, their temperatures, as it is by these that its height is determined." The actual elevation above sea-level of the timber-line in the Cordilleras of North America ranges from six or seven to twelve thousand feet. It is lowest in the Coast and Cascade Ranges of Washington Territory, and rises as we go southward through Oregon and California. On the high Sierras of Eastern-Central California, forests grow to 10,000 or 12,000 feet, while the ranges of Southern California do not reach the upper limits of forests Few of the ranges of Nevada reach the timber-line, which varies from the height of 9,000 feet in the northern to probably 11,000 feet in the southern part of the State. In Ari-
