1867.] Physics. 435 



part of the heated air or the gases produced by the combustion of 

 the fuel can escape without passing through the cylinder, and there 

 doing duty in the production of force. It is obvious that by such 

 an arrangement the employment of separate iron generators for the 

 purpose of heating the air is dispensed with, and that thereby one 

 of the chief difficulties of the old caloric engine is avoided ; for in 

 the hot-air engine the fuel is contained in a fire-clay furnace sur- 

 rounded by an air-tight iron casing, which in this way is entirely 

 protected from injury. The fuel, which may be anthracite, smoke- 

 less coal, or coke, is thus burned under pressure with great 

 regularity and with the production of a uniform temperature, and 

 at a rate exactly proportionate to the duty the engine is called 

 upon to perform, thus avoiding all waste of fuel — a result which 

 has not been attained with any other form of engine yet introduced. 

 The heated air, together with the gases produced by the combustion of 

 the fuel, passes from the fire-box directly into the cylinder, so that 

 every unit of heat produced is converted into force. The piston con- 

 sists of a hollow plunger, to which the piston-rod is attached ; the 

 packing is placed around its upper circumference, where the heat is 

 so moderate as to permit of efficient packing and lubrication. By 

 means of an air-pump worked by the piston, the supply of air is 

 forced into the grate. It here comes in contact with the fire, and a 

 portion of it, in maintaining combustion, combines with the carbon, 

 producing carbonic acid, &c, while another portion of the air in excess 

 takes up heat, and is thereby expanded. The mixed heated air and 

 gaseous products of combustion speedily accumulate such an amount 

 of expansive force as to set the engine in motion, by pressing on the 

 piston. At the end of the stroke the expanded gases escape by the 

 waste-pipe, which may be connected by a stove-pipe with an 

 ordinary chimney. Each upward stroke of the piston produces a 

 downward corresponding stroke of the air-pump, and forces a fresh 

 charge of cold air into the grate to maintain the combustion of the 

 fuel, thus keeping up a continual supply of heated air and gaseous 

 products. The power is increased or diminished by dampers, which 

 pass the air through or over the fire according to the amount re- 

 quired. The chief advantages of the hot-air engine will be found 

 in the very important facts, that there is not the most remote 

 danger in its use ; the furnace is perfectly insulated, all risk of fire 

 is entirely avoided, and the presence of water, whether in large or 

 small quantity, is dispensed with ; so that this engine can be em- 

 ployed under circumstances where it would be impossible to use a 

 steam-engine. 



Father Secchi, a scientific experimentalist who is not usually 

 mistaken in his statements, has lately made the somewhat startling 

 announcement that red-hot iron is transparent. The reverend 



