In 1796 Montgolfier, a Frenchman and one of the inventors of the balloon, substituted for the stop-cock of the Whitehurst machine a loose impulse valve in the waste pipe, whereby the valve was raised by the rush of the water, made to set itself, check the outflow and turn the current into the air chamber. This simple alteration changed the character of the machine entirely, rendered it automatic in action and converted it into a highly successful water-raising machine. For this invention Montgolfier obtained a Gold Medal from the French Exposition of 1802. Where a head can be had from four to six feet, water can be raised to the height of 30 feet. Bodies of water greater in amount than is desired to be raised can thus be utilised, and this simple machine has come into very extensive use during the present century. Allusion was made in the last chapter to the powerful hydraulic press of Joseph Bramah invented in 1795-1800, its practical introduction in this century and improvements therein of others. After the great improvements in the steam engine made by Watt, water, steam and air pressure joined their forces on the threshold of this century to lift and move the world, as it had never been moved before. The strong hands of hydraulics are pumps. They are divided into classes by names indicating their purpose and mode of operation, such as single, double-acting, lift or force, reciprocating or rotary, etc. Knight, in his celebrated _Mechanical Dictionary_, enumerates 100 differently constructed pumps connected with the various arts. In a broader enumeration, under the head of _Hydraulic Engineering and Engineering Devices_, he gives a list of over 600 species. The number has since increased. About nine-tenths of these contrivances have been invented during the 19th century, although the philosophical principles of the operation of most of them had been previously discovered. The important epochs in the invention of pumps, ending with the 18th century, were thus the single-acting pump of Ctesibius, 225 B. C., the double-acting of La Hire in 1718, the hydraulic ram of Whitehurst, 1772, and the hydraulic press of Bramah of 1795-1802. Bramah’s press illustrates how the theories of one age often lie dormant, but if true become the practices of a succeeding age. Pascal, 150 years before Bramah’s time, had written this seeming hydraulic paradox: “If a vessel closed on all sides has two openings, the one a hundred times as large as the other, and if each be supplied with a piston which fits it exactly, then a man pushing the small piston will equilibrate that of 100 men pushing the piston which is 100 times as large, and will overcome the other 99.” This is the law of the hydraulic press, that intensity of pressure is everywhere the same.