For night firing, or when there is much fog, a luminous aiming-point is required. This may be readily obtained by the use of a bull's-eye lantern. Soften the light by placing paper or other material over the glass, and draw thereon two right lines intersecting at right angles at the centre of the glass. At a convenient distance in front of the gun and in the plane of fire place the lantern on a post so that the point of intersection of the lines shall be at such a height that when the rear sight is at zero, _and the axis of the gun horizontal_, it can be directly aimed at. Establish a plumb-line between the lantern and the gun so that the plumb-line, the point of intersection of the lines on the lantern, and the object aimed at are in the same vertical plane. (The above requirements must of course be completed during the day, when the object can be seen.) Give the rear sight the elevation corresponding to the range, and aim at the point where the plumb-line covers the point of intersection of the two lines on the glass of the lantern. If a gunner's quadrant or pointing-arc be available, simply determine and mark accurately the direction for each piece, and give elevation by either instrument. CHAPTER XII. Cordage. How Preserved. Strength. Blocks and Tackle. Knots, Hitches, etc. Lashings. Bridges, Trestles, Piers, etc. Single-sling and Treble-sling Bridges. Stringer Bridges. Fords. Flying and Floating Bridges. Rafts and Casks. CORDAGE. Ropes are distinguished as to size by their circumference in inches. Their length is given in fathoms. Hemp rope when new stretches freely, and deteriorates very much after a few months' wear. It is about one third stronger than manilla rope. To ascertain the strain in pounds a rope will bear without breaking, square the circumference and multiply the result by the tabular unit in the following table: ---------+--------------+-------------------+------------------- Kind. |Circumference.| White. | Tarred. | +---------+---------+---------+--------- | |3-strand.|4-strand.|3-strand.|4-strand. ---------+--------------+---------+---------+---------+--------- Hemp {| 2.5 to 6 in. | 1140 | 1330 | 850 | 1000 {| 6 " 8 " | 1090 | 1360 | 825 | 940 Manilla {| 2.5 to 6 in. | 810 | 950 | .... | .... {| 6 " 12 " | 760 | 835 | .... | .... ---------+--------------+---------+---------+---------+--------- For ropes in daily use reduce the tabular unit one third to meet reduction in strength by wear and exposure. A safe general rule for all ropes is, one fourth the square of the circumference gives the breaking weight in short tons. TABLE OF WEIGHTS THAT MANILLA ROPE IN DAILY USE WILL SUSTAIN.