The Mining Journal, of London, makes the following criticisms upon the proposition of Capt. Ericsson to utilize the direct heat of the sun as a motive power, which we copy on account of their suggestive character, rather than the justness of its views in regard to Capt. Ericssons invention. Knowing j confessedly nothing of the means employed, the Mining Jour-1 Mil has put itself into the position deprecated by Solomon,: that of one who answereth a matter before he heareth it, and it may be that the developments promised us by Capt. Ericsson in the coming spring will prove that it is a folly and a shame unto it. The Journal says that at the outset of liis proposition, Mr. Ericsson appears to have fallen into the I error of considering concentration and condensation of the suns rays as convertible terms. This, however, is not so. Every schoolboy who has blistered his hand with a burning glass knows very well the practical meaning of concentrating the suns rays. The condensation of them that is, the fixing of their calorific forces in some latent and portable form, whence they can afterward be liberated and utilized—is a very different matter indeed. The one is simply mechanical, the other is chemical, and the only agency with which we are at present acquainted capable of perform-I ing this latter very important operation is the leaf of the i living tree or plant, which, under the influence of the suns j rays, separates the constituents of the carbonic acid floating j in the atmosphere, absorbing the elemental carbon and lib I erating the oxygen. j This operation is of a refrigerating nature, but the caloric of the suns rays, instead of being dissipated in effecting the I separation above described, becomes fixed or latent (that is, hidden or concealed), in connection with the elemental carbon, and the resultant accretion is what we commonly term carbon—that is, elemental carbon plus the latent heat of the j suns rays, which has been expended ipon it in separating it j from the oxygen with which it was previously combined in the carbonic acid, and which latent heat has taken the place of that gas, through the agency of that undefinable something which we call the vital energy of the plant, but about which we know very little. Thus, in solid carbon we have the calorific forces of the rays truly condensed. When I this substance is first formed it is, of course, mixed with ] various volatile juices and earthy matters, formed in the growing plant, and which it has derived from other sources. But once formed, this solid carbon is capable of a great many transformations without parting with the latent heat it has received, and which it still retains, stored up in connection j with it. Thus it is changed into peat, coal, diamond, and, by assimilation from the carbonaceous food we take, it forms a large portion of the flesh and bones of animals. This solid carbon can only be made to give up the latent heat stored in connection with it by its being brought into contact with oxygen at a certain temperature and under certain conditions. We effect this operation by placing the solid carbonaceous matters in some suitable receptacle, such as a common fire-grate, or boiler furnace, and having raised the initial temperature to a sufficient point by some extraneous means, we then allow a stream of atmospheric air to pass through the materials. The oxygen from the air then uniting again with the elemental carbon, liberates the stored-up latent heat, which we utilize for the production of force in various ways, the elemental carbon passing away with its original companion, the oxygen, as carbonic acid. This hypothesis, which we admit is open to discussion, is the history and meaning of the condensation of the calorific influences of the suns rays, and of the means by which, I after they have been stored up for unknown ages, we may again derive use and comfort from them. But now the question comes—Has Mr. Ericsson indicated a cheaper or more portable plan for the storing the calorific forces of the suns rays than that worked out for us in the natural way? We venture to think not; and that our coal owners need not at present fear him as a rival, while those who are annually expending large sums in utilizing the forces of heat must wait for a long time before they find a cheaper method of doing so than by comsuming our ordinary fuel. Mr. Ericsson does not tell us that he has discovered some other cheap and portable vehicle by which the suns rays can be absorbed, and stored in a latent shape, and thence by simple and cheap means liberated again at will for use. It is evident that he has no idea of chemical condensation, but only of some mechanical concentration, the economics of which it may be worth while to consider a little. He states that in weather suitable for the action of sun machines, the action of the sun, in a super-fices of 100 square feet, can evaporate 489 cubic inches of water in an hour, which is equal to the action of a motive force capable of raising 29,750 lb. one foot high. This quantity will be more or less as the weather is more or less suitable. The word suitable, as descriptive of the evaporative power of the suns rays, not being fixed by any thermometric scale, it is impossible to follow Mr. Ericssons calculation with the exactness we should desire, but, taking his basis, we have as our starting point the statement that 100 square feet of concentrators will produce nearly 1-horse power when the weather permits—that is, when the sun shines suitably , whence it follows, under the same conditions, that 10-horse power will require 1,000 square feet of the same instruments. But as, unfortunately, the power will only be produced while the sun shines suitably, and as that luminary is only above the horizon on the average of twelve out of twenty-four hours, j and as in the winter time his presence is much less propitious, and as, in addition, there are many days together when he does not shine at all, then, in order to produce anything like a uniform power, we must have an area of concentrators at least five times that which would be necessary if the sun shone suitably all the twenty-four hours round, and there were no such things as clouds. We shall thus find that nearly an acre of concentrators will be necessary to pro-1 duce, with any regularity, 100-horse power. But even this could not be guaranteed in the November Fogs and winter months generally, when it might require an 1 area equal to a large estate to produce 100-horse power. I fancy for a moment the blank despair of some enthusiastic : manufacturer, living in a crowded town, who desired to drive his cotton mill by concentrated suns rays, being told that to do so he must purchase an acre of concentators, which we presume must be some sort of reflector, of metal or glass; I that he must then purchase an acre of ground on which to erect them; that he must cover that acre with a network of steampipes, fittings, and boilers; that each concentrator would require expensive machinery always to keep it facing the sun at the proper angle; that all this machinery would have to be worked prior to any power passing into his accumulators, which would be massive and extraordinary ma- chines, at least five times as large as an ordinary 100-horse power engine; that, further, lie would have to keep a small army of men, with cotton-waste and wash-leather, constantly cleaning his acre of concentrators, to have them in readiness to reflect the rays of the sun; and that into it all, in the event of a succession of dark or cloudy days supervening, he could not even then be guaranteed the continuous demand of his 100-horse power—and then we may form some slight idea of what Mr. Ericsson thinks he can offer in competition with our ordinary fuel. In further proof, however, that the time of the sun machine is not yet, we should note that our ordinary fuel has driven the windmill out of the market, the motive power of which is as cheap as sunlight; and as, horse power to horse power, the outlay to establish a combination of windmills will be less than to establish a combination of sun machines, it necessarily follows that these latter will not be brought into practical use until not only our coal is exhausted, but the wind ceases to blow.