Beckman, in his History of Inventions, says, that a machine of this description was invented by a citizen of Amsterdam, in the year 1690, and was by him called the _water camel_. It consisted of two half ships, and on the deck of each were placed horizontal windlasses from which proceeded ropes made to pass under the keel of the vessel intended to be raised. The two sides of the camel having been sunk by the admission of water, the ropes were drawn tight, and the pumps being put in motion, the vessel was gradually raised to the surface. It appears to have been principally employed in crossing the bar of the Zuyderzee. No. XCIV. A little engine, portable in one's pocket, which placed to any door, without any noise, but on crack, openeth any door or gate. NOTE. The simple engine called a _Jack_, used for the purpose of raising great weights, with small manual exertion, appears to be admirably calculated for this purpose; and its even uniform motion is evidently described by the noble author. Ramelli has also given a description of several very curious instruments for the same purpose. Vide _Artificiose Machini_, p. 255, &c. No. XCV. A double cross-bow, neat, handsome, and strong, to shoot two arrows, either together, or one after the other so immediately, that a deer cannot run two steps, but, if he miss of one arrow, he may be reached with the other, whether the deer run forward, sideward, or start backward. NOTE. The cross-bow, though long since superseded in point of general utility by the invention of fire-arms, might still be found a useful auxiliary in the sports of the field, and as such, it has been thought advisable to notice what appears to be the plan on which this instrument must be constructed. To fire two arrows in immediate succession, it will be necessary either to attach a second bow to the under side of the stock, which, after discharging one arrow, may immediately be reversed, and the second fired. Or, where a bow of sufficient length is used, the string may communicate the required degree of impetus to two arrows in succession, a stud being previously prepared for its reception, about half-way down the stock, from which it may readily be liberated for the second discharge. No. XCVI. A way to make a sea-bank so firm and geometrically strong, that a stream can have no power over it; excellent likewise to save the pillar of a bridge, being far cheaper and stronger than stone walls. NOTE.