Fig, 17.

opposite sides, as at bb; or, to describe the case more accurately, the drift is forced rather into an oblique position, as shown He then inserts the by the lines c c. rivet, which is hammered up in the usual way. If the rivet be very hot, and the hole not very irregular, by dint of hard blows the hole is well filled, and the work sound, though

about this there is no certainty. But suppose the hole is very much distorted, the countersink half destroyed, and an abrupt angle at the centre caused by one plate partly overlapping the hole of the other, the effect is then such as is described in fig. 17: the holes are not filled, and the rivet when severely tested becomes loose. There are, of course, all degrees of distortion; some so great that the head of the rivet will scarcely conceal the vacancy. I have frequently seen cases

Fig. 18.

similar to that shown in fig. 18. It is evident that wherever this vacancy occurs, however small it may be, the efficiency of the rivet is greatly reduced; and, supposing further (a case which occurs every day in our very best ships) that in the same seam both

the rivets and joints have the defects above described, how greatly must the general strength of the ship be diminished! To meet these defects large additions to the quantity of the iron have been made.'

* On this question of imperfect punching it seldom occurs that the hammered rivets are tight or made to fill the hole. Machine riveting or those inserted by compression almost invariably fill the hole, as the head of the rivet is not formed until the vacancy in the hole is effectually filled.

upon the chain principle with the rivets in the line of the strain, as shown in fig. 19, are the strongest and best application, and approximate closer to the strength of the solid plate, taken through the line of the rivet-holes, than any of the others experimented upon. This has already been proved by previous experiments on the tensile strength of the joints of horizontal girders and iron bridges.

III. RIVETS.

47. On this subject we have to consider the diameter, pitch, and length necessary to be observed in forming sound steam and water-tight joints, without injury to the plates beyond the amount of metal punched out for the reception of the rivets, and in this we are assisted by the investigation to which we have referred, and the remainder we embody in the text for the sake of convenience, as under:

In the pursuit of the foregoing inquiry I was naturally led to the consideration of the best proportions and best forms of riveting plates together. I investigated this subject with great care; and, from my own personal knowledge and that of others, have collected a number of practical facts, such as long experience alone could furnish. From these data I have been enabled to complete the following table, which for practical use will be found highly valuable in proportioning the distances and strength of rivets in joints requiring to be steam or water-tight.

TABLE X.

TABLE EXHIBITING THE STRONGEST FORMS AND BEST PROPORTIONS OF RIVETED JOINTS, AS DEDUCED FROM THE EXPERIMENTS AND ACTUAL PRACTICE.

The figures 2, 1·5, 4·5, 6, 5, &c., in the preceding table are multipliers for the diameter, length, and distance of rivets, also for the quantity of lap allowed for the single and double joints. These multipliers may be considered as proportionals of the thicknesses of the plates to the diameter, length, distance of rivets, &c. For example, suppose we take plates, and require the proportionate parts of the strongest form of joint, it will be— = 750 diameter of rivet, inch.

.375 × 2

375 × 4 ·375 × 5

375 x 5

375 × 5+= 3.438 quantity of lap for double-riveted joint,

3 inches.

75, 168, 1.87, 2:06, and 3-43 are therefore the proportionate quantities necessary to form the strongest steam or water-tight joints on plates three-eighths of an inch thick.

47

CHAPTER IV.

ON THE PROPERTIES OF IRON-(continued).

THE RESISTANCE OF PLATES AS APPLIED IN DIFFERENT FORMS TO THE FORCE OF COMPRESSION.

1. We have already noticed, when treating of the tensile strain to which a ship is subjected, that another equally important force is in operation in the movements of the vessel-that of crushing or compression. This is more apparent in iron than in wooden structures, as thin plates are liable to distortion when forcibly compressed in the direction of their lengths, and in ship building, as in tubular girder bridges, this tendency to 'pucker' requires to be carefully guarded against. ducting the experiments for the Conway and Britannia Bridges this weakness was carefully considered, and as the strains in that of a ship and a monstrous tubular girder are analogous, it is requisite in both structures that the forces should be clearly understood, and the tendency to buckle prevented.

In con

To enable the practical ship builder to acquire this knowledge, and become acquainted with the laws which govern iron structures of different forms and conditions, it will be necessary to investigate this question attentively, and endeavour to establish sound principles of construction in the minds of those who are entrusted with designs of such importance.

2. To construct a perfectly secure iron ship, every one of the transverse joints should be planed, in order that the ends of the plates may butt and form a solid joint. This connection is the more important, as the action of a vessel pitching at sea (as before explained) is a continued series of alternate strains of tension and compression at midships. By extending the weights or cargo in the direction of the bows and stern these strains

are increased, and this, as a general rule, should be avoided by concentrating the cargo as much as possible at the centre of the ship.

FLEXURE, CRUSHING, AND COMPRESSION.

3. The immediate effect of the transverse strains to which vessels of great length are exposed is alternately to tear and compress those portions of the vessel above and below the neutral axis. It is therefore important that the resistances of these opposing forces should be clearly defined, and in order to simplify the investigation it will be necessary to consider the measure of the forces required to crush a cubic inch of different kinds of material. The table following is compiled from the results of recent experiments.

4. In the experiments by the late Professor Hodgkinson on cast iron, it is interesting to observe that the resistance of cast