inventor would regard the opinion of any men competent to judge of the matter referred to them. A writer in the Patent Journal expresses upon this point only the prevailing sense of the public when he observes:

'Hogarth said that he would allow all the world to be judges of his paintings, except members of his own profession: and, in general, scientific men would submit their ideas to the approval of all, with the exception of men of their own pursuits. No man is a prophet in his own country, and men of science are too often the least qualified to form an estimate of an invention in their own branch of knowledge. To submit a novelty for the approval of men accustomed to the routine and forms in present use, is oftentimes to ensure its rejection."

The writer then proceeds, according to the invariable rule, to invoke the overworked shades of Harvey and Galileo as illustrations of his statement. A more popular suggestion has been made, that every patentee should be required to deposit in some public museum an accurate model or specimen of his invention; which would thus prove highly useful as an object of interest and instruction to others, as well as by rendering more easy of determination any litigated question of priority. We should anticipate this further advantage from the plan, the attempt to construct his model would often leave the inventor selfconvicted of the inutility of his scheme and save him much disappointment. Even the preparation of an accurate drawing often has a salutary effect. Mr. Babbage relates that in the construction of his calculating machine, not one single portion of the works, although these were of extraordinary complication, required any alteration after it was once made, owing to the admirable care which had been bestowed upon the drawings.

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It is not, however, solely with the view of saving a few inventors the pain of disappointment, that we would have the conditions and limits of practical attainment accurately traced out. Still less is it in the spirit of the ancient geographers, who drew the lines that marked the boundaries of their known world upon their maps, and then wrote nil ultra' outside them. For to us, who have learnt that the universe is inexhaustible, the time will never come when we shall believe, of any field of research, that there is nothing more to be discovered in it. But we conceive that to ascertain the precise nature and place of the obstacles which at present retard our advance, is the surest preliminary to any attempt at their removal. To know where the barrier lies, will instruct us also where lie the domains of richest promise, not yet rifled by discoverers. To know what it is, will guide us to the selection of those aids and appliances by which it is to be broken or overleapt. Dr. Hooke has remarked, that whenever in

his researches he found himself stopped by an apparently insurmountable difficulty, he was sure to be on the brink of a valuable discovery. In his day the world was so little explored, that its richest prizes might still be stumbled upon by mere chance. The philosopher upon his voyage of discovery, like Genseric upon his voyages of conquest, might abandon the helm and let his bark sail whithersoever the winds might carry her;' trusting that fortune would lead him within sight of some region wealthy and unknown, of which he could claim possession by the prior right of occupancy. But such happy casualties are now barely possible; the harvest has been too well gleaned for mere adventurers. Within the limits of the nearer horizon, science has left, in the words of the old feudal law, Nulle terre sans 'seigneur; but it must not be forgotten that she has at the same time afforded aid and means to furnish us forth for more distant enterprises. And we are enabled also to save ourselves the trouble of many a profitless voyage; for we have, by her help in several instances accomplished that most difficult task, whether in Law or Physics, of proving a negative. We may feel sure that nothing more is to be done at least in certain directions - with our present means and instruments; as their range has been already ascertained and their powers tasked to the uttermost. On another side, we can determine, without the necessity for costly experiments, and indeed often by the application of theory alone, which of two or more possible arrangements of mechanism will prove most efficacious for the accomplishment of the desired purpose.

In fact, the votary of Science is now able to proceed towards discovery with sure and certain steps. He knows whither he is going; and he allows nothing to escape him unnoticed on the road. Every new phænomenon as it comes within his ken is duly compared with his previous experience, and is not admitted to assume its title until it has been examined and tested with the most minute accuracy. In the same manner, every deduction to which he arrives is scrutinised with jealous care, and not until it has undergone every trial that ingenuity can devise, is it permitted to take rank among the links destined to compose the great chain of his theory. The end of all his researches is indeed always kept in sight; but he never jumps at a conclusion; nor suffers his impatience for a result to hurry him into a neglect of those precautions which can alone secure for that result the certainty and precision on which its value depends. By no meteor of the marsh must the traveller be guided, who would penetrate the trackless expanses of the Unknown!

The subject we have here traced out is far too extensive for

us to attempt, within our allotted limits, to fill up its outline at every point. We can but endeavour to indicate, by a few precepts and examples, the peculiar nature of the problems which every inventor will have to work out for himself, whenever he wishes to determine the limits between the possible and the impossible.

Diffi

The limitary principles (by which term we purpose to specify everything, whether quality or accident, which tends to limit our progress towards perfection) may be divided into two great categories, including, first, those derived from the natural properties of matter; and secondly, those arising from the construction or arrangement of the mechanism necessarily employed. The higher importance of the former class is at once manifest. culties which arise from construction may be overcome or eluded: but the task is very different where we find that nature herself raises the barrier in our path. Man has succeeded in rendering almost every quality of every various form of material substance available for some purpose of utility. On certain occasions only, and for certain purposes, some one or other of those qualities will be found to stand in the way of his success.

Chemistry has gone far towards establishing the hypothesis that all natural bodies are susceptible of assuming three forms -the solid, fluid, and gaseous according to the degree of HEAT by which they are affected. At all events it is certain that heat exercises, in various proportions, such an influence on the constituent atoms as to destroy or diminish their mutual attraction; and even when the mass does not subside into fluidity, it loses its strength and cohesive properties, and becomes disintegrated. The uses to which this property of matter has been applied are infinite. Let us see how it may become a limitary principle.

It is supposed that the possible heat of a burning atom (in which of course we shall find the theoretical limit) is very far above the highest known temperature attained in our furnaces; and it would consequently follow that we might more nearly approach that limit by varying the arrangement of the fuel and the supply of air for combustion. This has been accordingly done, until we have found our progress stopped by the impossibility of discovering any substance, whereof to build our furnaces, which will bear the heat. Porcelain, firebrick, and plumbago, in various combinations are adopted: but they either crumble, or sink down into a pasty mass, as the fire is urged. The qualities of matter itself here act as a complete estoppel :' and if we would experimentalise further upon the phænomena of caloric, we can operate only upon a minute scale by means of the gas blowpipe, or the

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heated arch evolved from charcoal points interposed in a galvanic circuit. But for this limit, many useful purposes might be accomplished, by the mutual actions or changed forms of material bodies when subjected to the intense action of heat. For instance, in the case of platinum,-we might then separate it from its ores by the ordinary methods of smelting and fusion; in place of being compelled to adopt the laborious and costly process of solution in acids. The steam-engine offers an example nearly parallel. The power of a steam engine depends primarily upon the area of surface in the boiler exposed to the action of the fire, and the intensity of the fire itself. In marine and locomotive engines, where space must be economised, the practical limit is fixed only by the degree of heat; and this of course must be kept below the utmost limit which the material of the boiler furnace will endure. As yet, there has not been discovered any material better fitted for this purpose than iron; and we have made our fires as fierce as the melting point of iron will permit even now, the firebars are destroyed sometimes upon their first journey.

Farther than this we obviously cannot go, so long as we use water for the power-producing agent. Attempts have however been made to conquer the difficulty by taking advantage of some other properties of matter in its relation to heat; based upon the fact that the evaporating point' —that is, the degree of heat at which fluids expand into vapour-is found to differ considerably in different liquids, just as does the melting point of solid bodies. It would, therefore, appear probable that, by filling the boiler with alcohol, which boils at 173°, or with ether boiling at 96° Fahrenheit, the tension of the vapour and consequent power of the engine could be increased without increasing the heat of the furnace. As both of the above-mentioned fluids are expensive, it was first requisite so to contrive the machine that no loss should be experienced, but the whole vapour be recondensed and returned to the boiler. For this purpose a variety of ingenious contrivances have been suggested, the earliest of which, and one perhaps as effectual as any other, was patented by Dr. Cartwright in 1797; while new forms of mechanism, with the same object in view, are even still appearing on the patent rolls from time to time. Whatever the ingenuity of man could do, has probably therefore been done: but the practical utility of all these contrivances was destroyed by the influence of other properties of matter altogether overlooked, although of necessity involved in the question. These regard the relative bulk of the vapour produced from corresponding quantities of different fluids, and the proportion of

heat absorbed or rendered latent in each during the process of vaporisation. The calculation is sufficiently simple; and the result effectually annihilates all hope of advantage, either potential or economical, from the etherial or alcoholic engines. Thus, to convert a given weight of water into steam, 997 degrees of heat are required as what is called caloric of vaporisation.' The same quantity of alcohol will become vapour with 442 degress, and sulphuric ether with only 302°. But to set against this apparent gain, we find that the specific gravity of steam (air being 1) is 6235; vapour of alcohol 1.603; ether 2.586; and the result may be thus tabulated.

The disadvantage of the latter fluids will be farther enhanced by the circumstance that, being lighter than water, a larger boiler will be required to hold the same weight of vaporific fluid i.e. a pound of water, when evaporated, will form about 21 cubic feet of steam; while a pound of ether will require a larger boiler to hold it, and will only form 5 cubic feet.

WEIGHT is one of the properties of matter which in practice we encounter chiefly as an obstacle or inconvenience, tending to increase friction, to resist motion, and generally to crush and destroy. Meanwhile, the limits of its range are comparatively narrow -that is to say, on one side. We can, indeed, rarify a gas until its weight disappears in infinite tenuity; but we very soon find ourselves at the extreme verge of any possible increase of specific gravity. The most ponderous substance known is not quite 22 times heavier than water. And yet there are many purposes for which bodies of greater weight might be made useful. If, for example, closer or deeper search amid the stores of the mineral kingdom should lead to the discovery of some substance bearing the same proportionate gravity to platinum that platinum does to cork, how many possibilities of improvement would be placed within our power! A thin sheet of such a substance, interposed among the keel timbers of a ship, would give stability and other sailing qualities at present unattainable. Blocks of it would afford sure foundations for piers, bridges, and all marine works. It might then be found no longer impossible to establish a lighthouse on the Goodwins. As a regulator, or reservoir, of power