As it is best to have the bottom of your house level, you must manage to nail the A blocks the same distance above the ground on each side of each tree--that is, if the ground is level; if not, you must allow for the slant of the earth. Spike the blocks securely to the trees with six-inch nails, using about three nails to each block. [Illustration: FIG. 3.--End View of Corbel Resting on B Sticks.] The foundation of the house may be higher than your ladder will reach. In this case cut two more poles and four more blocks, and at the point where the top of your ladder reaches spike on the blocks, and then rest The B Poles on them on each side of the two trees, as in Fig. 1. Nail the B poles securely to the tree, and with plank or half-round sticks floor the space between the trees, and you will have a good landing below your house (see Fig. 5) from which a ladder may be run to the proposed foundation. After the upper rods have been nailed to the trees and a ladder adjusted, and for security nailed fast to the tree and lower platform, you are ready to begin the serious work of building. Take a good strong plank, two inches thick, and cut two pieces about six feet long, and shaped as shown by The Corbel Piece D in Fig. 2; then cut four struts (E, E, in Fig. 2) and two king-posts (C, Fig. 2). Shave off the ends of the struts, as shown in the diagram, to fit the notches cut in the corbel pieces and the king-posts. [Illustration: FIG. 4.--Perspective View of Corbels Resting on B Sticks.] It is not necessary to spike this frame together--the big nails might split the timber--they may be fastened together slightly with wire nails and strengthened by a piece of hoop-iron nailed on with small nails, as shown in Fig. 2 at F, F, F, F, and this will keep the pieces from accidentally slipping out of their bearings, or holes may be bored and the parts held together with screws. The real strain being an up-and-down thrust on the notches, the weight will not bear upon the iron bands or screws. Great care must be taken to make neat-fitting joints. [Illustration: FIG. 5.--A Two-Tree House.] How to Build the Foundation. When the two pieces of the form of Fig. 2 are completed, make fast a line to them and haul them up the tree; then slip the ends of the rods B and B under the corbels D, until the king-post C lies flat against the side of the tree-trunk. Spike C securely to the tree-trunk, as shown in Fig. 3; do the same with the other frame on the far side of the other tree, and you have a firm foundation that will hold more weight than you are liable to put upon it. Now cut two more pieces of two-inch plank, say, ten feet long by four inches broad; hoist them up and spike them to the top of the corbel pieces D, D, so that they will project the same distance beyond the tree at each end, as in Fig. 4.