Our readers will bear with us for so frequently referring to the Ericsson, So many men from different motives have assailed us, because from the first we have given good reasons why hot air can never supersede steam,—that it is altogether inferior to it as a motive agents—we have to notice some of these attacks, lest our silence might be construed into an argument in favor of those who propagate falsehoods. We have stood decidedly alone upon this subject, and have done our duty conscientiously with a regard to truth only. Without any reason for so doing, many of our editorial brethren of the press, in various parts, have used language towards us which at present we will treat with silent contempt ; we have laid up their words, and " will bide our time." We learn by the " Louisville, (Ky.) Journal" of the 24th ult, that a certain Prof. Rainey delivered, on the evening of the 18th ult., a lecture in that place, on the Ericsson. The " Journal " says, " the lecture was delivered to one of the most intellectual houses to be obtained in Louisville." So far as it regards the capacity ot judging of the comparative merits of steam and air engines, we suppose that many of those who heard the professor, might be considered intellectually far beneath a less imposing audience.. We are sorry at least that any person in Louisville should have been imposed upon with any of the falsehoods represented to have been uttered by the lecturer, as represented in the " Journal." In the course of his lecture, this professor said (we quote from the " Journal ") ' while Mr. Ericsson's thoughts were turned on this subject (hot air as a motive power) many persons in Europe and even America, were experimenting on air engines, or such only as heated a new supply of air at every stroke. Mr. E. in the meantime discovered the regenerator, and proposed it to the Savans in London where he was residing. Prof. Faraday, among others, eagerly grasped at the theory, and was so pleased with it that he lectured on the new discovery in the Royal Institution, in London, DJU, Ute alsenftuftpasteiJii—Aixc- time a Mr. Stirling, of Scotland, heard ot the idea of a regenerator, which he saw at once was indispensable to an air engine, and attempted its construction, although he had no idea how Mr. Ericsson's regenerator was formed, as he had not permitted his develope-ment to go to the public. Mr. Stirling constructed a regenerator which was a tube from one half to two and a half miles long, and supposed that the air in passing through it would be heated, or that the- hot air, in escaping, would deposit all its heat. It took the air too long to pass through, and produced too few strokes. When the air was heated in a tube, its expansion was lost to a great extent on its sides. This is the caloric engine that the Scientific American has so industriously paraded before the mechanical and scientific world, and which, as that journal knew nothing of the structure of Mr. Ericsson's engine, must be the only engine, and consequently it has deceived the people, by what the slightest observer will see is palpably misrepresented." All that we have quoted above from the "Louisville Journal," respecting Stirling's air engine, and what is asserted respecting our conduct, we pronounce to be falsehoods, uttered for the purpose of deceiving his audience regarding the real merits of the case.— We never published an illustration of Stirling's air engine in the Scientific American, but we did that of C pt. Ericsson;—the one he first patented in 1833—in No. 20, on the 29th of last January. We copied it from Sir. Richard Phillip's " Arts of Life," as we stated, and any one who has read that work knows this to be true. The said air engine oi Capt. Ericsson, had a tubular regenerator, but whether the tube was half a mile or two miles and a half long we cannot tell, we can only say it was not so long as the falsehood uttered in the face of that intellectual audience in Louisville by this itinerant lecturer. The falsehood consists in this, that he said we put forth the air engine of Stirling having a tubular regenerator tor Capt. Ericsson's, and thus deceived the people, whereas we never published Stirling's engine, the one we published with the tubular regenerator was Ericsson's. If any of that audience in Louisville had read the work referred to, he could easily see that the professor was uttering what was not true. Let any one examine the said work, and dare to say that we deceived the public by misrepresentation, as this professor charges us. Our columns are open to Capt. Ericsson or any of those interested in his engine to contradict us in these statements if they can. We have resorted to no subterfuges nor misrepresentations in speaking of the Ericsson ; we have had no personal interests to subserve, and no personal feelings to gratify in, uttering our opinions respecting it. We have spoken of those connected with the enterprize as honorable men, and would be glad if it could prove successful. Our language has always been respectful, and no wilful untruths have we ever uttered about it. Who this Prof. Rainey is we do not know. The "Louisville Journal" speaks of him as somebody ot consequence. Only let a man have Professor before his name, and go among strangers, and then be he hungry or henpecked at home, h? at once becomes a hero, and that too among no class of people so readily as among those who are so olten called in snobbish language " intellectual people," not engineers and machinists mind you. The "Journal" states, that "the professor intends going to London early in May, and. will lecture in but three or four other cities." He had better go to school for a while longer ; the most of his lecture reported in the " Journal" is derived from an old lecture delivered in Boston about Capt. Ericsson in 1843, by John O. Sargent. We can show any person the printed lecture. We hope the professor while on his way to London will visit New York and make the same statements here that he made in Louisville, as reported in the " Journal." As the "Louisville Journal" receives the Scientific American regularly, the editor can turn back to No. 20, and see whether we are honest in the matter or not, by comparing the statements we have made with the work we have referred to, which is surely in some li-tittTfTimtSmk, By our last number h " would also see that the Ericsson was already getting new crowns to its furnaces, although, as reported in the " Journal," the professor stated they could not burn out sooner than boilers. All we have said will come out straight before twelve months pass away. In connection with this subject we would state that we have read with great satisfaction an editorial article in the " Albany (N. Y.) Evening Transcript " of the 31st ult., on hot air as a motive agent. The article was a reply to a correspondent, and exhibits a great amount of knowledge on the subject. One remark shows the editor to have looked into it far beneath the surface ; it is this : " there are mechanical difficulties in the direct application of heat as a motive agent that cannot be overcome." This shows to us that he sees deeply into the difficulties of hot air.— Neither dry steam (stame) nor hot air can be profitably employed as motive agents, for steam is not only a motive agent but a lubricator also, and thus has qualities which hot air does not possess. Hot air engenders great friction, and renders valves and pistons so difficult of working tight, that it never can be employed with profit in comparison with "team as a motive agent,