In the strictest sense of the term the fuel may be denominated the "prime motive power" of an engine, for upon the quantity used the whole economy of steam power depends. It is not the mere price of fuel, it might be ten times dearer or ten times cheaper than it is and yet fail to confer any benefit upon man. Thus for example, if 200 tons of one kind of coal could raise steam enough to drive the Pacific steamship across the Atlantic, it would be cheaper to pay $40 per ton for it than $8 per ton for a kind of coal which would require 1,000 tons to work the engines during one voyage. The quantity of coal used determines the length of steam voyages. The great object in all inventions to improve steam power, or supersede it, should be the develope-ment of force with a saving of fuel ; we want something better than the steam engine if we can get it, and it is all sheer nonsense to say that hot air, as a substitute for steam, will save fuel, as is now said about the hot air ship, and yet that ship not sail as fast as a steamship. If the principle does save fuel it should make a ship sail faster. If the reason is asked, why ? it is easily given. A steamship requiring 300 tons less coal, and equal to another in every respect, must surely sail faster, at least as fast, and have the advantage of carrying 300 tons more of paying cargo. The greatest care has been exercised, and much ingenuity has been expended on marine engines and boilers, in order to save fuelmdash;the quantity of itmdash;for if it required 2,000 tons of coal to navigate a ship ofthat tonnage across the Atlantic, there would be no ocean steam navigation. There are two kinds of steam engines totally distinct in the principles of their operation, the one is the " condensing engine," and the other the " non-condensing "mdash;commonly called the " high pressure." The former allows the steam to escape (after acting on the piston) into a chamber where it meets with a jet of water and is suddenly condensed into its original volume, thus leaving a vacuum for the next jet of steam from the cylinder, and taking away all back pressure from the next stroke of the piston. The non-condensing engine allows the steam to escape into the atmosphere acting against the pressure of the air, which is 15 lbs. on the square inch. The condensing engine economizes fuel because it saves a pressure of 13 lbs. on the square inch (the other 2 lbs. being deducted for the power required to work the air pump,) by forming a vacuum behind the piston by the condensation of the escaping steam in the condenser instead of letting it escape into the atmosphere. For this reason, and owing to the greater safety of low pressure steam, the condensing engine is exclusively employed in steamships. There is one principle, however, in which both engines are alike, we mean the exhaustion of the steam out of, the cylinder into a place where the pressure is below that of the steam. Thus if the pressure of the atmosphere was 45 lbs. instead of 15 lbs. on the square inch, a non-condensing engine with a pressure of steam at three atmospheres, (45 bs.) would not operate at all. If the steam could not be reduced suddenly into- water again, then the condensing engine would be out of the question, so that the success of the high pressure steam engine depends on the pressure (15 lbs.,) of the atmosphere, and that of the condensing engine on the quality of the steam, it being suddenly condensable to its ori ginal volume by a jet of water. The principle, then, whereby every steam engine is rendered operative, depends upon the medium into which the steam escapes after having acted on the piston j it must be a colder medium than the steam. An eagine operated by hot air cannot act upon any other principle ; the hot air must be allowed to escape into a colder medium, or it will not operate. For example, supposing an engine to be operated by hot air at 491 is placed in a room having its atmosphere heated to 491, the hot air engine, if its exhaust ports opened like a high pressure engine into the room, would not operate at all, because the air in the room is of the same tensionmdash;the hot air within the cylinder and the hot air without would be in equilibriummdash; static pressure. How can it be possible, then, for hot air to propel an engine, as has been pretended, and save all the heat of the air. It is a chemical impossibility, and no wonder it baffled Faraday to explain, as was stated in an article copied from a foreign magazine, by a sapient journal in our city. For example, allowing hot air at 491 to be the propelling agent of an engine, and allowing the hot air to have driven the piston to the end of the cylinder, before the said piston can be driven back again, the hot air on one side must be suffered to escape into a condensing, or colder medium, before the hot air applied at the other side of the piston can urge it to the other end of the cylinder to make a full stroke. Well, allowing that the hot air escapes into a series of layers of wire gauzemdash;or a regenerator, as was proposed by Stirling, and mentioned, as he states, in his first patent of 1827, (see London Mechanics Magazine, Vol. 45, for the year 1846, page 563 and 564) it is obvious that just as the wire gauze, takes up the heat of the air, soin proportion as their heat increases, their efficacy as an absorbing mediummdash;condenser, refrigerator, or call it by whatsoever name, is vitiated, and the result of this is, that the back resistance increases, and if the heat of the gauze was allowed to attain to 491, the engine would not act at all, as would be the case with a steam condenser without an air pump. To pretend that the same heated air can be transferred to wire gauze in a regenerator, and used over and over again, the regenerator acting both as a condenser and boiler is an anomaly. Upon the same principle of saving fuel, every engineer should exhaust his steam into his boiler. Not much fuel, to be sure, would be used, but as little power would be developed. If a certain quantity of hot air can be made to act on a piston, exhaust, give out its heat and take it up again, and so keep a round of action, like one jet of steam making a rotary engine run round for ever, then the same thing can be done with steam, for steam is a gas, as well as air, and comes under the same laws in combinaion with heat above 212. The hot air engine cannot act but upon the principle of expansion and contraction, and the steam engine upon the very same principle (evaporation and condensation). The engineer could never make his locomotive fly along the iron track like a whirlwind, but for the absorbing power of the atmosphere, and its cooling effect on the escape of the exhaust steam ; also the cooling property of fluid evaporation. If such a law did not exist the boiler would soon become red hot and be rent to pieces, but that all absorbing property for heat exhibited by water, which renders it, as stated in our last article, so superior to hot air, and which is carried off by the steam at a comparatively low temperature, robs the furnace ef its energy, makes it safer and more economical to use than hot air, and enables a force to be generated with a rapidity for propelling purposes, far surpassing that of the gases.