His specification[42] described how to drive a paddle-wheel by converting a reciprocating rectilinear motion into a continuous rotary one. But though Hulls’ mode of obtaining a rotary motion was new and ingenious, and would, perhaps, enable a steam-boat in a calm to be moved through the water, moreover is the first _steam-boat_ authentically recorded, it was probably not such as could be made practically useful for the general purposes of commerce, and I have been unable to find any record of this or of any such vessel having been so used. At the same time, it must be added that boats not unlike Hulls’ may now be seen trading in parts of the world remote from each other, as, for instance, on the Murray in South Australia, where various vessels, of which the following is an illustration, are employed, and on the upper Thames where one, at least, to my knowledge is now worked, which does not seem to be any very marked improvement on the boat of Jonathan Hulls.[43] [Illustration] [Sidenote: James Watt’s engine.] In 1756, Gauthier, a French mathematician, wrote a treatise on “Navigation by Fire,” which attracted the attention of the Venetian Republic.[44] But whatever merit some of these ingenious discoveries may have possessed, it was not till the 5th January, 1769, when James Watt obtained his patent, that any steam-engine could be effectually adopted in marine propulsion. Among various other improvements in the steam-engine patented by him, the most important was one for causing the steam to act above the piston, as well as below it, described as the “double impulse,” or, now more commonly called, the double acting engine.[45] On the old principle, when the weight of the atmosphere had pressed down the piston, a valve opened in the bottom of the cylinder whereby a fresh supply of hot steam rushed in from the boiler, which, acting as a pressure in excess of that of the atmosphere above the piston, combined with the weight of the pump rods at the other end of the lever, carried that end down, and of course raised the piston in the steam cylinder. The orifice for the emission of the steam having been then shut, and the cock opened for injecting the cold water into the cylinder, condensation took place, and another vacuum was made below the piston, which was again forced down by the weight of the atmosphere: thus the work was continued as long as water and fuel were supplied, and the steam-engine rendered capable of successful application for pumping purposes, a contrivance used even at the present day.[46] But the method contrived by Watt rendered the power of the engine much more effective by the use of a detached condenser, whereby the cooling of the cylinders by the injection of water was prevented and considerable economy obtained.[47] [Sidenote: Matthew Wasborough.]