After the principles of hydraulics had thus been reviewed and discussed by the philosophers of the 17th and 18th centuries and applied, to the extent indicated, further application of them was made, and especially for the propelling of vessels. In 1718 La Hire revived and improved the double-acting pump of Ctesibius, but to what extent he put it into use does not appear. However, it was the double-acting pump having two chambers and two valves, and in which the piston acted to throw the water out at each stroke. In 1730 Dr. John Allen of England designed a vessel having a tunnel or pipe open at the stern thereof through which water was to be pumped into the air or sea--the reaction thus occasioned driving the vessel forward. He put such a vessel at work in a canal, working the pumps by manual labor, and suggested the employment of a steam engine. A vessel of this kind was patented by David Ramsey of England in 1738. Rumsey of America in 1782 also invented a similar vessel, built one 50 feet long, and ran it experimentally on the Potomac river. Dr. Franklin also planned a boat of this kind in 1785 and illustrated the same by sketches. His plan has since been tried on the Scheldt, but two turbines were substituted for his simple force pump. Further mention will be made later on of a few more elaborate inventions of this kind. It also having been discovered that the fall of a column of water in a tube would cause a portion of it to rise higher than its source by reason of the force of momentum, a machine was devised by which successive impulses of this force were used, in combination with atmospheric pressure, to raise a portion of the water at each impulse. This was the well-known _ram_, and the first inventor of such a machine was John Whitehurst of Cheapside, England, who constructed one in 1772. From a reservoir, spring, or cistern of water, the water was discharged downward into a long pipe of small diameter, and from thence into a shorter pipe governed by a stop-cock. On the opening of the stop-cock the water was given a quick momentum, and on closing the cock water was forced by the continuing momentum through another pipe into an air chamber. A valve in the latter-mentioned pipe opened into the air chamber. The air pressure served to overcome the momentum and to close the chamber and at the same time forced the water received into the air chamber up an adjacent pipe. Another impulse was obtained and another injection of water into the chamber by again opening the stop-cock, and thus by successive impulses water was forced into the chamber and pressed by the air up through the discharge pipe and thence through a building or other receptacle. But the fact that the stop-valve had to be opened and closed by hand to obtain the desired number of lifts rendered the machine ineffective.