compared the results with experiments of a similar character on timber, it may be useful to offer a few general observations on the question now under consideration.

Dr. Robinson, in his article on the strength of materials,* when discussing the nature of a stretching force applied to materials, observes, that in pulling a body asunder the force. of cohesion is directly opposed with very little modification of its action; that all parts are equally stretched, and the strain in very transverse section is the same in every part of that section.' From this it would appear, that a body of a homogeneous texture will have the cohesion of its parts equal; and since every part is equally stretched, it follows that the particles will be drawn to equal distances, and the forces thus exerted must be equal. Now if this were true, the application of an external force to a body might be increased to such an extent as not only to separate the parts furthest asunder, but ultimately to destroy a cohesion of all the particles at once, a circumstance under which instantaneous rupture would follow as a result. These views are, however, not borne out by facts, as the experiments of Mr. Hodgkinson on iron wire show that the same iron may be torn asunder many times in succession without impairing its strength:† and some recent experiments at the Royal Dockyard, Woolwich, clearly show, that an iron bar may be stretched until its transverse section is considerably reduced and ultimately broken, without injury to its tensile strength. Nay, more, the same iron (so elongated), when again submitted to experiment, exhibited increased strength, and continued to increase, under certain limitations, beyond the bearing powers of the same bar in its original form.‡ That all the parts of a body subjected to a tensile strain are equally stretched' is therefore questionable. Bodies vary considerably in their powers of resistance, and exhibit peculiar properties of cohesion under the influence of forces calculated to tear them asunder. Fibrous substances, for instance, such as

Encyclopædia Britannica.

Manchester Memoirs, vol. v.

I am indebted to Mr. Thomas Loyd, of the Admiralty, for a series of interesting results on this subject.

ropes, and some kinds of timber, having their fibres twisted, are enabled to resist tension under the influence of considerable elongation without impairing their ultimate strength. Many of the fibres are stretched, but only to the extent of bringing the others to bear upon the load, which done, their united force constitutes the maximum of resistance to a tensile strain.

Other bodies of less ductility and more of a crystalline structure, such as cast-iron, stone, glass, &c., seem to be subject to the same law. In these cases it seldom happens that the whole of the particles are brought into action at once, as much depends upon the conditions of the body, the unequal state of tension of its parts, and the strain which some of the particles must sustain before the others receive their due portion of the load. Should the non-resisting particles be within the limits of elongation of the other particles, the body will then have attained its maximum power of resistance; but in the event of rupture to any of the resisting particles, the cohesive force of the body is thereby reduced, and that to the extent of the injury sustained by the fractured parts.

'There are, however,' as Dr. Robinson truly observes, ‘immense varieties in the structure and composition of bodies, which lead to important facts, and prove that the absolute cohesion of all bodies, whatever be their texture, is proportional to the areas of their sections.' Undoubtedly this is the case in bodies having an uniform texture with straight fibres, and hence it follows that the absolute strength of a body, resisting a tensile strain, will be as the area of its section.

The peculiar nature of the material combining a crystalline as well as a fibrous structure has led to these observations. In some instances the specimens experimented upon exhibited an almost distinct fibrous texture, and in others a clearly developed crystalline structure.* At other times some of the specimens were of a mixed kind, with the crystalline and fibrous forms united; the fracture having a laminated appearance, with the crystalline parts closely bound on each side by layers of the fibrous structure. These varieties are probably produced in the manufacture, and may be easily affected either by the mode of

* See the fractured parts of the different specimens, Plate II.

'piling' the layers of bars which form the plate, or from the unequal temperature of the parts as they pass through the rolls, But in whatever way they are produced, it is evident, from the experiments, that the fractures gave, in most cases, indications of an unequal and varied texture.

In the foregoing experiments, and also in those which follow, great attention was paid to the appearance of the fracture, in order to ascertain the structure of the plate, and to determine how far it could be depended upon in its application to the varied purposes for which it was intended.

These appearances are all shown in the drawings appended to the experiments, and to which I beg to refer.

PART II.

On the Strength of Iron Plates united by Rivets, and the best Mode of Riveting.

The extensive and almost innumerable uses to which iron is applied, constitute one of the most important features in the improvements of civilised life. It contributes to the domestic comforts and commercial greatness of the country, and from its cheapness, strength, and power of being moulded, rolled, and forged into almost every shape, it is not only the strongest, but in many respects the most eligible material for the construction of vessels exposed to severe strain. Large vessels composed of iron plates, such as steam-boilers, cisterns, ships, &c., cannot however, be formed upon the anvil or the rolling-mill. They are constructed of many pieces, and these pieces have to be joined together in such a manner as to ensure the requisite strength and effect all the requirements of sound construction. This operation is called riveting, and although practically understood, it has not, to my knowledge, on any previous occasion, received that attention which the importance of the subject demands.

Up to the present time nothing of consequence has been done to improve or enhance the value of this process. We possess no facts or experiments calculated to establish principles sufficient to guide our operations in effecting constructions of this kind,

*1838, when these observations were written.

on which the lives of the public as well as the property of individuals depend. In fact, such has been our ignorance of the relative strength of plates and their riveted joints, that until the commencement of the present inquiry the subject was considered of scarcely sufficient importance to merit attention. Even now, it is by many assumed that a well riveted joint is stronger than the plate itself, and a number of persons, judging from appearances alone, concur in that opinion. Now this is a great mistake; and although the double thickness of the joint indicates increased strength, it is nevertheless much weaker than the solid plate, a circumstance of some importance, as we hope to show in the following experiments.

It would probably be superfluous to offer any lengthened description of the principle upon which wrought iron plates are united together; riveting is so familiar to every person in this country, that it might appear a work of supererogation to attempt it; and assuming the usual method of riveting by hammers to be generally known, we shall proceed to describe another method by machinery which effects the same object in considerably less time and at less cost, and completes the union of the plates with much greater perfection than could possibly be done by the hand. In hand-riveting it will be observed, that the tightness of the joint and the soundness of the work depend upon the skill and also upon the will of the workman, or those who undertake to form the joint and close the rivets. In the machine-riveting neither the will nor the hand of man has anything to do with it; the machine closes the joint and forms the rivet with an unerring precision, and in no instance can imperfect work be accomplished so long as the rivets are heated to the extent compressible by the machine. This property of unvarying soundness in the work constitutes the superiority of the machine over the hand-riveting. The machine produces much sounder work, as the time occupied in the hand process allows the rivet to cool; and thus by destroying its ductility, the rivet is imperfectly closed, and hence follow the defects of leaky rivets and imperfect joints. It is evident that an instrument, such as the riveting-machine, having sufficient force to compress the rivet at once, or within an almost infinitely short period of time, must obviate, if not entirely remedy, these evils, as the force of

compression being nearly instantaneous, the heads on both sides cannot be formed until the body of the rivet is squeezed tight into the hole; and in every case (even where the holes are not exactly straight) the compressed rivets are never loose, but fill the holes with the same degree of tightness as if placed directly opposite to each other. If, for example, we take a circular boiler, such as represented at A,* Plate II., and having all the perforations made, and the plates attached to each other by temporary bolts and suspended over the machine in the position as shown at A, and having brought the holes in a line with the dies marked i, k, the machine is then set to work, and by means of the cam or eccentric raising the pulley of the elbow joint C, the die k is advanced against the fixed die i in the wrought-iron stem, and the rivet is compressed into the required form with an increasing force as the die advances which gives the 'nip,' or greatest pressure, at the required time, namely, at the closing of the rivet.

From this description it will appear that a very limited portion of time is occupied in the process, and as twelve rivets can be inserted and finished by the machine in a minute, it follows, from the rapidity of the operation and the absence of hammering, that the ductility of the rivets is retained, and the subsequent contraction upon the plate renders the joint perfectly tight and the rivets sound in every respect. Under all the circumstances the machine-riveting is preferable to that executed by the hammer; it saves much time and labour, and that in proportion of 12 to 1, when compared to a long series of impacts applied by the hammer.

Having described the process of uniting wrought-iron plates by rivets, it may be of some importance to know the value of joints thus formed as regards their strength when compared with the plates themselves. To attain this object, and satisfactorily to determine their powers of resistance to a tensile strain, a great variety of joints were made, and having prepared the different specimens with the utmost care and attention, they were submitted to the test of experiment, as follows:

:

*The plan represents the machine in the act of riveting the corners of a square cistern or a locomotive fire-box.