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 experimentalize further upon the phenomena of caloric, we can operate only upon a minute scale by means of the gas blowpipe, or the 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 form 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 steamengine 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 economized, 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 fire-bars 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 vapor-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 vapor 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 vapor 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 vapor produced from corresponding quantities of different fluids, and the proportion of heat absorbed or rendered latent in each during the process of vaporization. The calculation is sufficiently simple, and the result effectually annihilates all hope of advantage, either potential or economical, from the ethereal 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 vaporization." The same quantity of alcohol will become vapor with 442 degrees, 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; vapor 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, rarefy 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. thus committed to the deep, will never return. It is possible that a buoy composed of a light hollow sphere, filled with alcohol or one of the lighter oils, might be able at once to resist the pressure of the water and retain its levity at every depth. We are not aware that the experiment has been tried; but it appears to offer the means of successfully exploring the most profound abysses. The "strength of materials" is an element that enters into almost every calculation of the mechanist; and it is found to constitute not only an absolute limit to all possibility of advance in certain directions, but also a relative limit universally, when we attempt to reduce, beyond certain proportions, the size, weight, and cost of our mechanical erections. Its variations also are extensive, both in degree and in condition. Some bodies offer strong resistance only to certain modes of attack. Impervious on one surface, they will yield and splinter into laminæ under a slight blow upon another. Some will bear pressure to an enormous extent, but are easily torn asunder; others resist the divellent forces, but crumble under a light weight. A very extensive variety of substances possess a fibrous texture, and are endowed with vast strength to resist a strain in the direction of their length, but are much weaker against a lateral or transverse force. This difference is found to vary to an infinite extent; from that of certain metals where the advantage is only four or five per cent, in favor of the direct resistance, to the vegetable and animal fibres, such as flax or silk, which possess enormous tenacity, combined with most complete flexibility. By an incidental quality, in some measure associated with the specific gravity of bodies, we find that while all substances, without exception, undergo condensation when subjected to pressure, they do not all resume their original condition when the pressure is withdrawn. As might be supposed, the lighter bodies exhibit this peculiarity in the highest degree. Wood, for example, after having been submerged in the sea to a depth of two or three thousand feet, is found to be no longer light enough to float; the hydrostatic pressure, exceeding half a ton on every square inch, having both compressed the fibrous mass and injected the pores with water. By this peculiarity, the usefulness The variations in the natural properties of of an otherwise admirable instrument-the bodies have given infinite scope for the exerSounding Machine-is much restricted. Its cise of human ingenuity. In the erection of apparatus consists of a series of vanes, with engineering works, and in a still higher deattached clock-work, to denote the depth of gree in the contrivance and construction of water through which it has sunk. A buoy moving machinery, the combination of theory or float is fixed on the upper part, and the and practice is perpetually exhibited in surmachine being loaded with a sufficient weight prising perfection. By nice calculation of the descends until it strikes the ground; on this opposing forces, together with great practical the weight becomes detached and the instru-skill in the mechanical details of construction, ment returns to the surface, bringing back a faithful record of the perpendicular distance traversed. For ordinary depths the float consists of a hollow copper sphere; but as the metal must necessarily be thin, it is crush-weight: all irregular strains are skillfully ed in by a comparatively slight pressure. A wooden float is therefore substituted, which is able to command a more extended range of soundings, until the limit is reached at which the pressure already spoken of destroys the buoyancy of the wood; when the machine, if we can now attain a result in which abundant strength is united with the utmost possible economy of space and material. There is no waste; no addition of useless and cumbrous counterbalanced, and the greatest pressure distributed over the points of greatest resistance. Experience has entitled us to place implicit confidence in the scientific precision of our engineers. Every day we trust our lives and fortunes, without misgiving, into situations where a slight error in the calcula- | to sustain itself, is what we have called the tions, or a slight defect in the workmanship, cohesive force, and is due, we must suppose, would inevitably lead to some terrible catas- to some variety of the attractive principle trophe. How little do the crowds who among the corpuscular atoms which causes throng the deck of a Thames or Clyde them to resist a separating or divellent strain. steam-boat, or who allow themselves to be In ordinary bridges and among the usual hurried along at fifty miles an hour in a rail- erections of architects, on the other hand, the way carriage, reflect upon the delicate condi- pressure to be considered is that which crushtions which must have been fulfilled-the es the parts together. To resist this, the complicated mechanical problems which piers of the bridge must have strength suffimust have been solved, in order that they cient to support the loaded arch; and the might accomplish their journey in security! pillars of the cathedral to sustain the fretted A multitude will gather upon a suspension vault that rests upon them. In this case we bridge without fear or danger, although the find that the strength which arises from the rods by which the massive roadway and its cohesion of the atoms between themselves is living freight are sustained appear as mere increased by that due to another quality of threads in comparison with the mass they matter, namely, its incompressibility. When have to support: while, if any one reflects at any solid body yields to a crushing weight, all upon the matter, it is to assure himself the consequent effect must be, either that its that every possible amount of pressure has particles are actually pressed into a smaller been theoretically provided for; and that, space; or that, being made to exert a wedgepractically, every separate bar and joint has like action upon one another, the exterior been severely tested, so that no single flaw in layers are forced out laterally. The addition the material, or defect in the workmanship, of a band or hoop will then bring the incomcan have passed without detection. Fribourg, pressibility of the atoms more fully into before the civil war of the Sonderbund had play; and bodies that are endowed with given it a political notoriety, was celebrated slight powers of cohesion may thus be renchiefly for its wire bridge, hung at an alti- dered enormously strong. Indeed we find tude of nearly one hundred feet between two that fluids, in which the cohesive force is summits. "It looks," says a recent travel- practically at zero, cannot be crushed by any ler, "like a spider's web flung across a pressure we can exert, provided the hoop or chasm, its delicate tracery showing clear and tube that surrounds them can be secured. distinct against the sky." Diligences and Now the interior atoms of every substance heavy wagons loomed dangerously as they under pressure are more or less thus hooped passed along the gossamer fabric. in and strengthened by the exterior. To the strength from cohesion is added that from incompressibility; and this effect is produced in a rapidly increasing ratio as the sectional area of the body is enlarged. A cube of lead suspended from its upper surface and held together only by cohesion, will break down if larger than 180 feet to a side. If standing upon one side as a base, it might be made of infinite size without danger of fracture from its own weight. In works of similar construction to the Fribourg bridge, the limit of magnitude is of course found in that proportion, where the erected mass is only just able to sustain its own unloaded weight without fracture. Practically testing the strength of the various metals, we find that a regularly shaped bar or column of steel, if suspended perpendicularly by its upper extremity, will be torn asunder by its own weight at a length of 44,350 feet: iron would break at about 25,000; copper, at 9500; gold at 2880; and lead at only 180 feet. The processes of annealing and wiredrawing will modify to a considerable extent the tenacity of all metals; but the above proportions may be taken as a general average. Hence we arrive at an absolute limit of possibility; which no ingenuity of construction can enable us to evade, and which is to be conquered only in the most improbable contingency, of our discovering some new material of still greater strength among the stores of nature. The force that enables a suspension bridge We may conclude, therefore, that the total force of resistance is amply sufficient to answer any call we are likely to make upon it. It is certain, at all events, that we have not, as yet, built up to the strength of our actual materials. Our marble and granite columns will sustain ten times the weight of any edifice the present generation can wish to erect. Or if not, they will use iron. The theoretical limit to the span of our bridges is that only at which the voissures of stone or iron would crumble under the intensity of pressure. The cost and inutility of even approaching to such a limit, will always assign them much narrower dimensions; though large enough, nevertheless, to admit of the accomplishment of that magnificent project-of which the first design is due to the genius of Telford-for spanning the Thames at Westminster by a single arch. Such a work would be worthy alike of the age and the site; and we see no reason why it should not be undertaken, and completed at least as soon as (supposing promises to be kept in future only as heretofore) the last stone is laid upon the Victoria Tower. The tubular bridges now in course of erection by Mr. Stephenson, upon the Chester and Holyhead line of railway, will probably remain for years unsurpassed, as specimens of science and engineering skill. While we write, the success of the experiment is verified only in the smaller of the two, known as the Conway Bridge. But the result is even now sufficient to guaranty the success of its larger companion, to be thrown across the Menai Straits. In Telford's celebrated suspension bridge over these straits, the problem was already solved of constructing a safe pathway for the transit of heavy burdens. But the new fabrics were required to have something more than strength; perfect rigidity was in this case necessary, both as regards the lateral oscillations produced by the passage of the enormous trains at high velocities, and the perpendicular undulations so perceptible in ordinary bridges built upon the suspension principle. This requisite is obtained by forming the massive iron beam into a hollow rectangular chamber, 25 1-2 feet high, 15 feet wide, and (in the Conway tube) 412 feet in length, in the inside of which the trains are to travel along the rails. It forms, in fact, a long gallery, whose sides are composed of iron plates half an inch thick, and its ceiling and floor are formed of compound plates, consisting each of two laminae of metal kept apart at a distance of about 21 inches, by a series of plates of that breadth extending the whole length of the tube, dividing the top and bottom strata into a series of longitudinal cells, and aiding greatly in the resistance offered to the weight of the passing trains. The whole mass of iron employed is sufficient to form a solid beam 412 feet long from pier to pier, and 46 inches or nearly 4 feet square. Employed in this form, the beam would possess ample strength; but it would have been drawn down by its own weight into a catenary curve, dipping several feet in the centre, and altering in shape upon the passage of a few tons along its surface; while even the action of a high wind would | have impressed on it a considerable lateral or horizontal vibration. The same metallic mass distributed into the compound parts of the gallery we have described, was fashioned into a curve rising only 7 inches in the centre, which the action of its own weight (1,300 tons) drew, as was intended, into perfect horizontality; and which has been proved to sink not more than a single inch by the added pressure of 100 tons. A number of ingenious contrivances were brought into use during the process of construction. The compound tube consists of many thousand separate pieces, with every joint secured by covering plates, and T angle irons, fastened together with rivets, all driven red-hot. In drilling the rivet holes, more than a million in number, a curious machine was used, imitated from that employed in making the perforated cards for Jacquard looms, by which the work was done with beautiful regularity. The foundations of the supporting piers are laid upon piles driven by Nasmyth's steam piledriver-an engine which seems to have been invented just in time-as by the old-fashioned "monkey," the same task would have occupied many months' additional labor. huge structure was floated from the temporary stage whereon it was built, upon caissons which the tide lifted; and was elevated to its destined place by hydraulic pressure. So extreme is the accuracy of this wonderful work, that the thermometric change of shape produced by an hour's sunshine upon one side, or on the top, becomes readily perceptible; and one end of the tube is left loose upon the abutment to allow for this expansion. The The hypothesis that the force of cohesion is proportional to the area of section, leads us to the ordinary rule of practice-that as the magnitude is increased, the strength increases as the square, and the strain as the cube of the dimensions. The proportions consequently which offer abundant strength in a model, must be materially altered when the design is executed at full size. When any of the parts are intended for motion a new element is introduced, from the inertia of the moving masses; and thus both the size and the velocity of our machinery are confined within definite limits. To extend these limits, it is often necessary to solve the most complicated problems of dynamics, and to follow the train of motion through an intricate series of action and reaction. We must simplify and reduce the number of moving parts, and so adjust the momentum of the inertia, that the resulting strain shall be neutralized, or reduced to a minimum; and where it is necessary that the direction of motion should be reversed, we must accomplish this object with no such sudden or violent shock as would dislocate the machinery. The difficulty of this attempt in many instances is proved by the heavy motions and hideous noises that accompany the working of almost all newly-invented mechanism, and of the simplest machines found among nations less skilled than we are in the arts of construction. The approach of a Mexican wagon is announced at a distance of three miles, by the creaking of its wheels. It is only after repeated trials and improvements, that we reach the perfection of which so many striking examples are presented in our various manufactories and ateliers. When the first steam-printing machine was "working off" the impression of the "Times" newspaper at the rate of 2500 copies per hour, the noise could be heard through the silence of early morning nearly across Blackfriars bridge. At present, conversation proceeds in the very room where the type-loaded frame, of far larger dimensions than heretofore, is travelling to and fro beneath the cylinders, and perfecting between 5 and 6000 double sheets in the same time. Dr. Cartwright describes his first power-loom as requiring the strength of two men to work it slowly, laboriously, and only for a short period. We may now enter a single apartment in a Lancashire mill, and see 250 looms at full work, each throwing 150 threads a minute; while a single shaft carried along the ceiling communicates motion to the whole, and with a noise by no means overpowering. In the manufacture of needles, the slender bars of steel are forged out by a succession of hammers, each one less in weight and quicker in stroke than its predecessor. As the motion of the hammer is necessarily alternating, the dislocating effects of its momentum when thrown into rapid vibration would be enormous, but for the contrivance of giving the hammer a double face, and causing it to strike every time it rises against While these sheets are passing through the press, Mr. Applegarth has succeeded in effecting a new improvement in the steam-printing machine. The "chase," or type-frame, no longer travels to and fro, but is curved into the segment of a circle, and the whole "form" is placed round a cylinder, and works off the sheets by a circular and uninterrupted motion. This machine already completes 9,600 double sheets per hour; and, with additional steam-power, which is in preparation, is expected to accomplish at least 12,000. a block of steel placed above, from which it is thrown back upon the anvil. The vibration is thus produced by a series of rebounds, between two opposing surfaces; five hundred strokes can be made in a minute, while the power is materially economized, and the strain upon the stalk and axle nearly annihilated. But it is needless to multiply examples. It is equally unscientific, and almost equally dangerous, to give too much strength to our constructions as too little. No machine can be stronger than its weakest part; and therefore to encumber it with the weight of a superfluous mass, is not only to occasion a costly waste of material, but seriously to diminish the strength of the whole fabric, by the unnecessary strain thus produced upon the parts least able to bear it. This fault is one which is most frequently discoverable in new machinery; and which, when once adopted in practice, retains its hold with the greatest inveteracy. It requires no common powers of calculation, and not a little faith, for men to trust to the safety of structures which have apparently been deprived of half their former strength. There can be no better proof of the difficulties which oppose the adoption in practice of any new principle of construction or configuration, than that exhibited in the history of ship-building. In no creation of human labor was it more necessary to secure the greatest possible strength from the minimum of material; as none were required to possess such vast bulk in proportion to their mass of resistance, or were exposed to more violent varieties of strain and shock, in the natural course of their service. The men who superintended the public dock-yards were often well versed in mathematical science; and were certainly acquainted theoretically with the common axiom, that, among right-lined figures, the triangle alone will preserve its form invariable by the rigidity of the sides, without depending upon the stiffness of the joints. Yet none, until a recent period, worked out the axiom into its very obvious practical development. For centuries were our ships constructed on principles which caused the whole frame-work to be divided into a succession of parallelograms. Every series of the timbers, as they were built up from the keel to the decks, formed right angles with their predecessors and with their successors; so that the whole fabric would have been as pliable as a parallel ruler, but for the adventitious firmness given by the mortices, bolts, and knee-pieces. At least |