In the annexed diagram, the cylinder A is furnished with a steam-tight piston, the rod of which is supposed to be connected with the working beam. B represents the pipe which admits the steam from the boiler, the quantity being regulated by the throttle valve _c_, and the elastic vapour is now passing through the box _d_ _d_, so that it enters beneath the piston. At the same instant of time, a communication is formed through the aperture _m_ _n_ to the pipe _p_, which leads to the condenser. When the piston reaches the top of the cylinder, the sliding bridge or valve has its direction changed, so that the pipe _r_, and consequently the bottom of the cylinder, is connected with the condenser, while a passage is opened from the pipe _m_ _n_ to the steam box. Thus a communication is alternately made between the top and bottom of the piston. The slide-valve represented above is not invariably employed in the double-acting engines, and we frequently find the annexed contrivance resorted to, in some of the best engines. [Illustration] The pipe 14 represents the passage to the cylinder, and a communication is now opened with the steam chamber _g_. The raised valve is perforated and a similar valve beneath closed by the rod which passes through it. On closing the valve _g_, the lower valve _h_ is opened, and a free passage between the condensing pipe beneath and the upper part of the cylinder is the result. If we now suppose a similar double valve placed at the bottom of the cylinder, it will easily be seen that an effect similar to that described in the sliding valve will be produced. The speed of the engine is regulated by a very ingenious contrivance introduced by Mr. Watt, called the _governor_, and represented beneath. [Illustration] The balls _i_ _i_ are supported by the bent levers _h_ _f_, and as they are made to revolve with the fly wheel axis, by means of a band passing round the pulley _c_, any increase in the speed of the engine will cause the balls to diverge. The moment this takes place, the shorter arm of the lever _n_ is depressed, and as the extremity _l_ is connected with the steam-pipe by the throttle valve, the supply of steam must of necessity be diminished, and the speed of the engine reduced. As the working power of the engine depends very materially on the accurate fitting of the piston, it may be adviseable to examine some of the modes of effecting this important object.