The following rules and symbols were adopted for determining the relative position of points on the aerodrome, some of them during 1891, and some of them since. All are given here for convenience of reference, though their chief application is to the larger steam aerodromes described later. Those which immediately follow were meant to give some of the notation of descriptive geometry in untechnical language for the use of the workmen employed. Let ‹X›, ‹Y› and ‹Z› be three lines at right angles to each other, and passing through the same point in space, ‹O›, lying at any convenient distance above the floor of the work-shop. The line ‹X› lies North and South; the line ‹Y› lies East and West, and the line ‹Z› points to the zenith. Now place the aerodrome on the floor so that its principal axis lies horizontally in the plane ‹XZ›, with its head pointing North, and in such a position that a line passing through the center of the propellers shall coincide with the line ‹Y›. [Illustration: FIG. 2. Diagram showing mensural coordinates.] When measurements are made on or parallel to the line ‹X›, the point of intersection ‹O› will be marked 1500 centimetres, and distances toward the South will be less than, and distances toward the North greater than 1500 centimetres. When measurements are made on or parallel to the line ‹Z›, the point ‹O› will be considered to be marked 2500 centimetres, and distances above will be greater than, and distances below will be less than 2500 centimetres. [p015] Lastly, when measurements are made on or parallel to the line ‹Y›, the point ‹O› will be marked 3500 centimetres, and distances toward the East will be greater than, and distances toward the West will be less than 3500 centimetres. Measurements in these latter directions will be comparatively infrequent because the center of gravity and center of pressure both lie in the plane ‹XZ›. EXAMPLE In the figure the point ‹T› in the tail, if 15 centimetres to the South of ‹O›, would be graduated 1485 centimetres. A weight (‹W›) 25 centimetres below the axis, would be graduated 2475 centimetres. A point 50 centimetres above the axis would be graduated 2550 centimetres, etc. ‹CG› represents the Center of Gravity of the aerodrome, or (with subscript letters) of any specially designated part, or with reference to some indicated condition. ‹CG›_1 ‹CG›_2 represent the Center of Gravity as referred to the first, or horizontal, and to the second, or vertical plane, respectively. ‹CP› represents the Center of Pressure[14] of the whole aerodrome, or (with a subscript) of any specially designated part. ‹CF› represents the Center of Figure of the aerodrome, or of any specially designated part. Subscripts: “‹fw›” refers to the front wings. “1” refers to the plane ‹XY›. “‹rw›” refers to the rear wings. “2” refers to the plane ‹XZ›. “‹r›” refers to a state of rest. “3” refers to the plane ‹YZ›. “‹m›” refers to a state of motion.