While speaking of Savery's apparatus it may be adviseable to notice the very ingenious adaptation of the same principle to the construction of a _gas engine_, by Mr. Brown. In the latter case a vacuum is formed by the introduction of an inflamed jet of carburetted hydrogen gas, which consumes the oxygen, and rarefies the nitrogen, by the increase of temperature which ensues. The vacuum thus produced is much more perfect than would at first view have been supposed, from the nature of the process resorted to by the patentee; but the economy of employing carburetted hydrogen gas as a substitute for condensible vapour is still somewhat problematic.[10] [10] Since writing the above, the Editor has seen a report on Mr. Brown's engine by Professor Millington, in which it is distinctly stated that the apparatus is fully adapted to the purpose for which it is intended. To more fully understand the nature of Mr. Brown's engine, it may be better to revert to a diagram, which will sufficiently explain its general principles. [Illustration] In the above view, the cylinders _c_ and _d_, are the vessels in which a vacuum is alternately effected; _g i g_ and _h j h_ are two pipes, leading into the lower cylinders _x_ _x_, shewn in the next page, from which the water rises along those pipes to fill the vacuum cylinders alternately. The water thus supplied is discharged through the pipes B into the tank or trough _z_, where it falls upon the overshot water-wheel, and, by the rotatory motion thus produced, gives power to such machinery as may be connected to it. The water runs from the wheel along a case surrounding the lower half, into a reservoir _v_, from which the lower cylinders _x_ _x_, are alternately supplied. The gas is supplied to the cylinders by the pipes _k_ _k_ _k_, which must be, of course, attached to a gasometer, or some other reservoir of gas. The gas also passes along the small pipe _l_ _l_ (which communicates also with the gasometer), and being lighted at both ends of that pipe, is kept constantly burning in order to ignite the gas within the cylinders. The gas being admitted along the pipe _k_, the flame from the pipe _l_ is now freely communicated to the gas in the cylinder, through the orifice, by the opening of the sliding valve _s_, which is raised by the arm _r_, lifted by the rod _o_ by means of the beam. The water in the reservoir _v_ passing down one of the pipes _w_, into one of the lower cylinders _x_, causes the float _y_ in that cylinder to rise, and, pushing up the rod _o_, raises the end _b_ of the beam, which, of course, draws up with it the cap _f_, and forces down the cap _e_ of the other cylinder _c_. [Illustration]