One of our city dalies, no later than the 26th inst., directed the attention of its readers to the letter of its Paris correspondent, wherein it is stated that very successful experiments had been made in France, with Mons. Trembley's ether engine, in a ship. It was stated that the engine was 75 horse power, and that its superiority was so great over the steam engine, that it saved 75 per cent, of fuel. The same paper very innocently remarks, " were the invention in American hands, and applied to American models, there is no doubt that their speed might be made to exceed greatly the maximum speed here indicated, (16 miles per hour.) " Those who are ignorant ofthe progress of invention—the green ones in engineering—should be very cautious about expressing opinions pro or con about such matters. This Mons. Trembley's ether engine has been in operation in this very city, and could have been seen at the Novelty Works in 1851. If it was a proper substitute for the steam engine, and saved 75 per cent of fuel, does any person suppose that Messrs. Stillman Allen would not have adopted it ? The combined ether engine of Mons. Trem-bley consists ot a common steam engine, with two cylinders and pistons, the one piston acted on by steam, and the other by ether or chloroform, heated by the exhaust steam.— There can be no saving of fuel in this case that we can see; it is a very foolish arrangement, for it would be tar better to use the steam to its utmost limit of expansion, or al-ioffi-ii- tn ....n. a..Ulily, tVcgt-W-trytim "geTsT benefit from its heat by applying it to vaporize chloroform. It there was any benefit to be derived from this ether cylinder, that is in saving fuel, it would surely be more reasonable to apply thg heat ofthe fire at once to the ether or chloroform, and use it as an ether engine entirely. It is well known Jto chemists that neither ether nor alcohol can be used as economical substitutes for steam; how then can ether save any fuel by being combined with a steam .engine ? The saving of 75 per cent of fuel is a grand idea, but how this can be done is a most perplexing question to answer; no logician would have made such a statement. It is like making a statement of this kind, "the real effect of the steam engine is only equal to 25 per cent, of the fuel; but the exhaust steam oi the same engine applied to heat chloroform produces a mechanical effect equal to 75 per cent of the fuel ; in other words, 75 per cent, of the tuel is lost in the exhaust steam ofthe steam engine." A little learning is not a dangerous thing; it is the absence of the little which makes pretenders to it dangerous.