MESSRS. EDITORSI have read with grei pleasure your criticism on the Hot Air Er gine, and greatly admire your frank and he nest course about this inventionyour cours with every thing. You look the naked facts i the lace, and speak out what you think, with out fear or favor. By this course your pape has become the real guardian of invention and inventors. I have looked back over a] your articles on the Caloric Engine, and in case can I see that you opposed this inven tion, but that in every case (it appears to me you have been actuated solely by a desire seeing and exhibiting what the thing really k Vet I cannot agree with you in your conclu sions, for I think you have left the relativ specific heats of air and water out of your cal culation. So far as your dissertation relates to the va pors of fluids, you are right: you handle Prof Apjohn coirectly, excepting that he is right ii saying "that equal volumes of the vapors different liquids will have the same elastii force at their respective boiling points," fo the boiling point is that temperature at whicl the elastic force of the vapor becomes equa to the atmospheric pressure. But equal bulk: of liquids converted into vapor exert a forc( inversely as the densities ol these vapors hence the vapor of alcohol, ether, &c, canno do the work of steam. But no comparisor can be made between the elastic force produ ced by that expansion of vapor due to increased temperature, and that due to the making of vapor; unless we take the specifii heats in consideration along with the boiling points and latent heats, when the result wil be largely in favor of the permanent gas oi vapor, or in favor of expansion and againsi vaporization. By the doctrine of specific heats, different substances have a capacity or appetite foi heat, which is inversely as their atomic weights ; a pound of hydrogen will hold as much heat at the same temperature as 100 pounds of gold or quicksilver, 14 pounds oi air, or 3 pounds of water: hence a pound of water will require 33 times as much heat to raise its temperature one degree, as a pound of mercury ; or the same quantity of heat which will raise the temperature of one pound of water one degree will raise a pound o! mercury 33 degrees. The specific heat of water is nearly four times that of air, consequently the heat or caloric which will elevate a po und of water one degree will heat a pound of air four degrees, or four pounds one degree. Now the latent and boiling heat (not the latent and specific, as Mr. Apjohn has it) of water, as steam, combined, are 1150, or if a cubic foot of water, at 32, were confined and heated 1150 or to 1182, then, when released, it would all become 1728 cubic feet of steam at atmospheric pressure, with a sensible heat or temperature of 212 ; and the available force would be 1728 feet. But this same heat which raised the water 1150, and produced a force of 172S, will raise an equal weight ol air 864 cubic feet, 4 times 1150 or 4600, which will expand it Si times its bulk, equal to 8200 cubic feet, which is the measure ofite available forceequal to 4 j times the force gotten from water. The heat which produces a given volume by expansion is always less than that required to make the same volume by vaporization, and this is the case even with steam and waters which have nearly the same specific heat, for if 624 peunds or 1728 cubic feet of steam at 212 deg. are heated apart from water to 1182 deg., or raised 980 deg., then it will expand to three times 1728 ; if water at 212 deg. is then let in, the 980 deg. will become latent in producing one volume of steam from the water, and we shall have two times 1728 at 212 deg. instead of 3 times 1728 at 1180 deg. If we have this odds in favor of hot drj steam, what will it make with air which hat four times the advantage in specific heat. Let us take one more view ot the question. One cubic foot of water at 32 deg. will give 1728 feet of steam of atmospheric pressure and 212 deg. temperature, by the application of 1182 deg. more of heat. If the cubic foot of water were resolved into its component gases they would occupy 2000 feet. Now i the water and the gases had the same capaci ' for heat, then the 1182 deg. which produ- ced 1728 feet by vaporizing the water, would l' make the 2000 feet of the gases increase 4800 ' feet more for each degree, would expand it '" 1-491 of its bulk at 32 deg., which will give e 2f times advantage in favor ol the gases over n steam ; but the difference of the specific heats l" will make the advantage about double this ; ir for the specific heat of the steam is so much lS greater than the ga'ses that, taken with the II specific gravity, it is double the gases ; for 0 steam, being composed of one volume of oxy- gen, with two volumes of hydrogen condensed ) into one volume, makes its specific gravity at 'f 212 deg., and atmospheric pressure, compared 1 with its gases at the same heat and pressure - as 24 is to 16, and its specific heat double an e equal bulk of the gas: (I use round numbers - only for these points about the gravity, and specific heats of the gases cannot be nicely de- termined). Hence we see that steam and water will actually hold one-third more heat than the f very gases which compose them. Water is a - fire-eater, and lor this was it made by Infi-r nite Wisdom. How wondrous, then, may be its mechanism; probably it does not consist 1 of two little balls, one of hydrogen and the s other of oxygen sticking side by side ? " This superior power of expansion over va-, porization was first noticed, I believe, by Mr. Frost, who so clearly showed through your 1 paper, that it was the cause of the boiler explosions ; and that dry steam (his stame) might greatly economise fuel or increase the 1 power of the engine ; and you gave, in the - last volume, a letter from a person who says ' that he saved 25 per cent, by heating the ' steam (expanding it dry) after it left the boil- er. I thought that this was a settled matter-t of-fact, by Mr. Frost's experiments and the other things you published in favor of it. I never shall forget the sorrow I telt when I read in your paper that that truly scientific man had breathed his last moments in com-1 parative poverty. How often is this the reward ofthat friend of manthe Inventor. I trust that you will receive this in the spirit in which it is given. I doubt if the * Ericsson " would have a greater speed with larger engines; for the rate of working will be the rate ot heating the air, and a larger fire surtace will heat no fas-Cerit must be made hotter; or else the motor cylinder must condense into a receiver, and this supply the cylinder which propels the boat. Georgk Mathioi. Washington,' D. C, 1853. Messrs. EditorsIn the " Scientific American " of Jan. 8th, in giving the reasons why hot air must continue to lail in competing with steam as a force to move machinery, I think you have fallen into an error in not taking into account the difference in the specific and relative heat of water and air. The specific heat of air, or the actual quantity of heat required to raise the same weight of air and water, each the same number of degrees, is in the proportion of water 1, air 0'2669, and as air is less dense in the proportion of 830 to 1, the quantity of heat for an equal volume, or, as it is called, the relative heat is as 1 to 0 0003215, or as 1 to 13110, that is, the same amount of heat that will raise 1 cubic foot of water 1 deg. is sufficient to raise the same volume of air 3110 deg. ; or, what raise 1 cubic foot of water 1184 degrees, converting it into steam, increasing its volume 1728 times, will raise 1 cubic foot of air 3110X1184=3682210 degrees, which, divided by 479 (the number of degrees by the books necessary to double the volume of air) gives 76S7 as the number of times its volume is increased by the same amount of heat which changes the same volume of water into steam. Divide 76S7 by 1728, it gives 4'45 as the ratio of increase in volume by the same amount of heat in iavor of air over water. 7687X2160 gives 16603920 lbs., raised one foot high by the air, against 3,732,480 lbs., by the water, or otherwise the aeat that will raise one volume of water into steamwill raise76&7 volumes of air 479 decrees, doubling its bulk and coming to the iame result. You say that it requires S64 in. f air (it should be 1728 in.) raited 491 deg. to equal 1 cubic inch of water raised into steam. Let us see what proportion of heat it wil take. What will raise 1 cubic inch of water one degree, will laise one cubic inch of air Ji, 187 3110 deg, or 1728 cubic inches 1*8 deg. ; 273 times that amount will double the volume of the air, while it will take 1184 times the same absolute amount of heat to change the water into steam, giving the proportion of 434 in favor of air. The difference from the propor tion 4 45 is made by using 91 deg. in place ot 479, which I think is more correct. It also requires more than 50 per cent. of heat to raise one cubic foot of steam at 212 deg., a given number of degrees, more than it does to raise one cubic toot of air the same number of decrees ; that is, the relative heat is, by the books as 1 53 to 1, 'veing as 3 to 2 in favor of applying heat to air rather than steam, and about as 3 to 1 in favor of applying heat to steam rather than to water, to change it into steam. Why. then, has air not been used 1 I 8Upp)Se one reason is, that it takes half or more of the power to do the necessary pump. ing. Mr. Ericsson uses twothirds nearly, the remaining difference is balanced by the power gained by condensation of steam and the application of the expansion principle ap plied to high pressure steam, leaving them, perhaps, not lar from equal. But when Mr. Ericsson saves fivesixths ot the heat, and conseqDent]y the same proportion of fuel, that is a different metter, and it becomes evident that if necessary, human ingenuity and pow er will be taxed to their utmost capacity to insure the success of his experiment. I think you will have to give it cp at last. Akron, Ohio, 1853. S. H. Bass. [For editorial remarks on the above two letters, see page 189.1