Cord of Italian hemp, about ¼ in. thick, is excellent for small catapults. For large ones, horsehair rope, ½ in. thick, is the best and most elastic. Whatever is used, the material of the skein must be thoroughly soaked in neats-foot oil for some days previously, or it is sure to fray and cut under the friction of being very tightly twisted. Oil will also preserve the skein from damp and decay for many years. HOW TO WORK THE CATAPULT There is little to write under this heading; as the plans, details of construction and illustrations will, I trust, elucidate its management. The skein should never remain in a tightly twisted condition, but should be untwisted when the engine is not in use. Previous to using the catapult its winches should be turned with the long spanner, fig. 6, p. 12, first the winch on one side of the engine and then the one on the other side of it, and each to exactly the same amount. [Page 20] Small numerals painted on the surfaces of the large wheels near their edges, will show how much they have been revolved; in this way their rotation can be easily arranged to correspond. As the skein of cord is being twisted by the very powerful winches, the arm will gradually press with increasing force against the cross-beam between the uprights. The arm should be so tightly pressed against the fender, or cushion of straw, attached to the centre of this beam, that, whether large or small, it cannot be pulled back the least distance by hand. If the skein of my largest catapult is fully tightened up by the winches, three strong men are unable to draw the arm back with a rope even an inch from the cross-beam, though the windlass has to pull it down from six to seven feet when the engine is made ready for action. When the skein is as tight as it should be, attach the slip-hook to the ring-bolt in the arm and place the stone in the sling suspended from the top of the arm. The arm can now be drawn down by means of long spanners fitted to the windlass. Directly the arm is as low as it should be, or as is desired, it should be instantly released by pulling the cord fastened to the lever of the slip-hook. The least delay in doing this, and the resulting continuation of the immense strain on the arm, may cause it to fracture when it would not otherwise have done so. The plans I have given are those of my largest engine, which, ponderous as it seems--(it weighs two tons)--is, however, less than half the size of the catapult used by the ancients for throwing stones of from forty to fifty pounds in weight. As the plans are accurately drawn to scale, the engine can easily be reproduced in a smaller size. An interesting model can be constructed that has an arm 3 feet in length, and a skein of cord about 4 inches in diameter. It can be worked by one man and will throw a stone, the size of an orange, to a range of 300 yards.