The acting medium employed in the Caloric-Engine is atmospheric air; and the leading peculiarity of the machine, as originally designed by Ericsson, is, that by means of an apparatus styled the Regenerator the heat contained in the air which escapes from the working cylinder is taken up by the air which enters it at each stroke of the piston and used over and over again. The machine constructed by Ericsson in London was a working engine of five-horse power, the performance of which was witnessed by many gentlemen of scientific pretensions in that metropolis. Among others, the popular author, Sir Richard Phillips, examined it; and in his "Dictionary of the Arts of Life and of Civilization," he thus notices the result of this experiment:--"The author has, with inexpressible delight, seen the first model machine of five-horse power at work. With a handful of fuel, applied to the very sensible medium of atmospheric air, and a most ingenious disposition of its differential powers, he beheld a resulting action in narrow compass, capable of extension to as great forces as ever can be wielded or used by man." Dr. Andrew Ure went so far as to say that the invention would "throw the name of his great countryman, James Watt, into the shade." Professor Faraday gave it an earnest approval. But, with these and some other eminent exceptions, the scientific men of the day condemned the principle on which the invention was based as unsound and untenable. The interest which the subject excited did not escape the British Government. Before many days had elapsed, the Secretary of the Home Department, accompanied by Mr. Brunel, the constructor of the Thames Tunnel, made his appearance in the engine-room where the new motive power was in operation. Mr. Brunel, who was at that time somewhat advanced in years, conceived at the outset an erroneous notion of the nature of the new power, which he would not suffer to be corrected by explanations. A discussion sprang up between him and the inventor, which was followed by a long correspondence. The result was, that an unfavorable impression of the invention was communicated to the British Government.