MECHANICS AND USEFUL ARTS. 19 



impossibility of preventing the surfaces from cutting, by the com- 

 bined action of the heat and dirt. Mr. Shaw seems to have obvi- 

 ated this by attaching to his pistons a drum of a diameter very 

 slightly say one-eighth of an inch less than that of the cylin- 

 der, and a length equal to the stroke, and providing an arrange- 

 ment by which, before each stroke, the annular space between the 

 drum and the cylinder is filled with cold and clean air of the same 

 pressure as that which comes upon it afterward. Having no way 

 of escape, this air remains in the space during the stroke, and pro- 

 tects the working faces from injury by preventing the hot and 

 dirty gases from impinging at ail upon them. As a further and 

 very important means of economy, regenerating surfaces of iron 

 and brick are provided, for abstracting as much as possible of the 

 heat remaining in the exhaust air, and imparting it to the incom- 

 ing cold air. A trial was had lately with this engine, which 

 showed an economy of coal superior to what has been obtained 



with the steam engine. 



GAS ENGINE. 



A new gas engine; by Mr. Hugon, substitutes a constant gas 

 light for the electric spark, to explode the mixture of gas and air 

 in the cylinder. The gas jets which fire the mixture, and are of 

 course extinguished by the explosion, flow by flexible pipes through 

 orifices in the ends of the slide valve, and as often as extinguished 

 are relighted by constantly burning jets which they meet in their 

 oscillations. A very small jet of water is also introduced into the 

 cylinder at each explosion, which by its instant vaporization ex- 

 tracts heat and serves moreover as a partial lubricator. 



HORIZONTAL SUBMERGED PADDLE-WHEEL. 



At a recent meeting of the Massachusetts Institute of Technol- 

 ogy, Mr. Peter Lear exhibited and explained a model of a sub- 

 merged horizontal paddle-wheel, modified by himself. Some 

 years ago Lieut. Hunter, U.S.N., invented a submerged hori- 

 zontal paddle-wheel, which was placed in the middle of the ves- 

 sel ; but it was found that, when the wheels revolved rapidly, and 

 the more according to the rapidity of the revolution, a vacuum 

 was formed at the centre, causing a flow of water to fill it, and 

 requiring great additional power to overcome this weight of back- 

 water. Mr. Lear found by experiment that the simple admission 

 of air to the centre of the wheels remedied the difficulty, and 

 caused the flow of water to be uniform in the desired direction, 

 saving power and increasing speed. He also placed the wheels 

 in the run of the vessel, one of the chief objections to Hunter's 

 wheels being the space the machinery occupied in the middle of 

 the vessel. His wheels externally do not project beyond the line 

 of the vessel, n.::d internally are encased in boxes. Being always 

 submerged, they possess advantages over the side-wheels and 

 propellers, which in a heavy sea are more or less out of water. 

 The wheels and machinery, being below the water-line, are sate 

 tVom an enemy's shot, and, acting independently, may, in case of 



