18 ANNUAL OF SCIENTIFIC DISCOVERY. 



to this source of the power, by the action of the expansive force 

 of the gaseous products of the combustion of the fuel, which here- 

 tofore have been permitted to escape into the chimney without 

 being in any way utilized in the production of power. This result 

 is accomplished by placing the fuel in a grate which can be her- 

 metically closed, and forcing the air required for combustion into 

 it by means of an air-pump worked by the engine itself, so that no 

 part of the heated air of 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 reg- 

 ularity, 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 form of engine yet introduced. 



/ ^J V 



The heated air, together with the gases produced by the com- 

 bustion of the fuel, passes from the fire-box directly into the cylin- 

 der, so that every unit of heat produced is converted into force. 

 The piston consists 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, 

 a 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, etc. ; while 

 another portion of the air in excess takes up heat, and is there- 

 by 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 common 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 required. 



The chief advantages of the hot-air engine will be found in tho 

 very important fact, that there is not the most remote danger in 

 its use. The furnace is perfectly insulated, so that 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. The great difficulty in the way of using the hot 

 gases from the fire directly in the cylinder has been the apparent 



