it may be repaired at a trifling cost, and with but little delay. In examining the effects of this engine, we cannot to a certain extent withhold our approbation; for the patentee has undoubtedly effected and applied a vacuum, produced by ignition, in a manner different and more manageable than any attempts that have hitherto come to our knowledge. The probability of its entering efficiently into competition with the steam-power, is a question that requires the data of experience, which, in this early state of the invention, cannot be procured. We understand it is the intention of the inventor to apply the effects of the vacuum thus produced to the movement of a piston in a cylinder, which object will, when attained, afford a much greater scope for the application of its powers, and render it peculiarly applicable to locomotion. The obstacle which at present suggests itself to the attainment of this end is, the difficulty of procuring a rapid condensation without allowing cold water to enter the cylinders at each stroke, which in the present form of construction is allowed, and which greatly aids the operation by keeping the chambers entirely cool. Without, however, seeking for obstacles, we wish the ingenious inventor success in surmounting them. ON THE STRENGTH OF MATERIALS AN accurate knowledge of the following experiments made by Mr. George Rennie, Jun., and communicated by him in a letter to Thomas Young, M. D. For. Sec. R. S., is of so much importance in the construction of machines, that we have extracted it from the Transactions of the Royal Society; to which we have annexed some useful notes by Mr. T. Tredgold. "In presenting the result of the following experiments," says Mr. Rennie, "I trust I shall not be considered as deviating from my subject, in taking a cursory view of the labours of others. The knowledge of the properties of bodies which come more immediately under our observation, is so instrumental to the progress of science, that any approximation to it deserves our serious attention. The Royal Society appears to have instituted. at an early period, some experiments on this subject, but they have recorded little to aid us. Emerson, in his Mechanics, has laid down a number of rules and approximations. Professor Robison in his excellent treatise in the Encyclopædia Britannica, Banks on the Power of Machines, Dr. Anderson of Glasgow, Colonel Beaufoy, &c. are those, amongst our countrymen, who have given the result of their experiments on wood and iron. The subject, however, appears to have excited considerable attention on the continent. A theory was published in the year 1638, by Galileo, on the resistance of solids, and subsequently by many other philosophers. But however plausible these investigations appeared, they were more theoretical than practical, as will be seen in the sequel. It is only by deriving a theory from careful and well-directed experiments, that practical results can be obtained. It would be useless to enumerate the labours of those philosophers, who, in following, or varying from the steps of Galileo, have merely tended to obscure a subject respecting which they had no data to proceed upon. It is sufficient to enumerate the names of those who, in conjunction with our own countrymen, have added their labours to the little knowledge we possess. The experiments of Buffon, recorded in the Annals of the Academy of Sciences at Paris, in the years 1740 and 1741, were on a scale sufficiently large to justify every conclusion, had he not omitted to ascer tain the direct and absolute strength of the timber employed. It however appeared from his experiments, that the strength of the ligneous fibre is nearly in proportion to the specific gravity. Muschenbroeck, whose accuracy (it is said) entitled him to confidence, made a number of experiments on wood and iron, which, by being tried on various specimens of the same materials, afforded a mean result considerably higher than other previous authorities. Experiments have also been made by Mariotte, Varignon, Perronet, Ramus, Rondelet, Gauthey, Navier, Aubry and Texier de Norbeck, as also at the Ecole Polytechnique, under the direction of M. Prony. With such authorities before us, it might be deemed presumption in me, to offer you a communication on a subject which had been previously treated of by so many able men.* But It is true that the subject has been considered by many able philosophers, from Galileo down to the present period, but it is only lately that the proper object of attention has been ascertained; or at least the results of their inquiries had not been brought forward in a practicable form. For when Dr. T. Young published his Lectures, there was little on the subject besides the intricate, and I may add unsatisfactory, investigations of Fuler and Lagrange. As to the resistance to fracture, which with the greater part whoever has had occasion to investigate the principles upon which any edifice is constructed, where the combination of its parts are more the result of uncertain rules than sound principle, will soon find how scanty is our knowledge on a subject so highly important. The desire of obtaining some approximation, which could only be accomplished by repeated trials on the substances themselves, induced me to undertake the following experiments. A bar of the best English iron, about ten feet long, was selected and formed into a lever, whose fulcrum is denoted by f, fig. 209. The hole was accurately bored, and the pin turned, which suffered it to move freely. The standard A was firmly secured by the nut c to a strong bed plate of castiron, made firm to the ground. The lever was accurately divided in its lower edge, which was made straight in a line with the fulcrum. A point, or division D, was selected, at five inches from the fulcrum, at which place was let in a piece of hardened steel. The lever was balanced by a weight, and in this state it was ready for operation. But in order to keep it as level as possible, a hole was drilled through a projection on the bed plate, large enough to admit a stout bolt easily through it, which again was prevented from turning in the hole by means of a tongue t fitting into a corresponding groove in the hole. So that, in order to preserve the level, we had only to move the nut to elevate or depress the bolt, according to the size of the specimen. But as an inequality of pressure would still arise from the nature of the apparatus, the body to be examined was placed between two pieces of steel, the pressure being communicated through the medium of two pieces of thick leather above and below the steel pieces, by which means a more equal contact of surfaces was attained. The scale was hung on a loop of iron, touching the lever in an edge only. I at first used a rope for the balance weight, which indicated a friction of four pounds, but à chain diminished the friction one half. Every movable centre was well oiled. Of the resistances opposed to the simple strains which may disturb the quiescent state of a body, the principal are the repulsive force, whereby it resists compression, and the force of cohesion, whereby it resists extension. On the former, with the exception of the experiments of Gauthey and Rondelet, on stones, and a few others, on soft substances, there is scarcely any thing on record. In the memoir of M. Lagrange, on the force of springs, published in the year 1760, the moment of elasticity is represented by a constant quantity, without indicating the relation of this value to the size of the spring: but in the memoir of the year 1770, on the forms of columns, where he considers a body whose dimensions and thickness are variable, he makes the moment of elasticity proportional to the fourth power of the radius, in observing the relations of theory and practice to accord with each other. This a of mechanical writers is the only object attended to, it is of very inferior importance. The laws of flexure constitute the chief guide in the construction of buildings; and the intention of these notes is to call the attention of experimentalists to this part of the subject; and as it is probable the ingenious author of the experiments now before me may be tempted to resume his Jabours, I feel certain that he will not feel displeased to have his attention called to some interesting points of inquiry, which he has either neglected to notice, or has not given to the public.-T. T. was admitted by Euler in his memoir of 1780, in his elaborate investigation of the forms of columns. Mr. Coulomb had however shown before that time, how inapplicable all these calculations were to columns under common circumstances; and you, sir, have repeated the observation in your lectures on natural philosophy. The results of experiments have also been equally discordant; since it is deduced from those of Reynolds, that the power required to crush a cubic quarter of an inch of cast-iron is 448000 pounds avoirdupoise, or 200 tons; whereas by the average of thirteen experiments made by me on cubes of the same size, the amount never exceeded 10392-53 lbs. not quite five tons. This may be seen by referring to the tables. There were four kinds of iron used, viz. 1. Iron taken from the centre of a large block, whose crystals were similar in appearance and magnitude to those evinced in the fracture of what is usually termed gunmetal. 2. Iron taken from a small casting, close grained, and of a dull grey colour. 3. Iron cast horizontally in bars of th inch square, 8 inches long. 4. Iron cast vertically, same size as last. These castings were reduced equally on every side to of an inch square: thus removing the hard external coat usually surrounding metal castings. They were all subjected to a gauge. The bars were then presumed to be tolerably uniform. The weights used were of the best kind that could be procured, and as the experiment advanced, smaller weights were used. Averages. Experiments on cast-iron in cubes of } of an inch, &c. lbs. avoirdupoise. .... 1439-66 1454 On specimens of different lengths. Specific gravity of iron 6·977. April 23, 1817. Experiments on cubes of of an inch taken from the block. .... Averages. A prism, having a logarithmic curve for its limits, resembling a column; it was of an inch diameter by one inch long, broke with.... lbs. avoirdupo se. 6954 The anomaly between the three first experiments on cubes, and the two second of a different length, can only be accounted for, on the difficulty of reducing such small specimens to an equality. The experiments on inch prisms of different lengths give no ratio. The experiments on inch cubes, taking an average of the three first in each, give a proportion between them and the three oncubes, as 1 : 6.096 in the block castings In several cases the proportion is as the cubes. The vertical cube castings are stronger than the horizontal cube castings. The prisms usually assumed a curve similar to a curve of the third order, previous to breaking. The experiments on the different metals give no satisfactory results. The difficulty consists in assigning a value to the different degrees of diminution. When compressed beyond a certain thickness, the resistance becomes enormous. Experiments on the suspension of bars. The lever was used as in the former case, but the metals were held by nippers. They were made of wrought-iron, and their ends adapted to receive the bars, which, by being tapered at both extremities, and increasing in diameter from the actual section, (if I may so express it,) and the jaws of the nippers being confined by a hoop, confined both. The bars, which were six inches long, and square, were thus fairly and firmly grasped. |