Interim Report of the Committee for Dredging on the North and East Coasts of Scotland.

AT the Aberdeen Meeting of the British Association, a Committee was ap pointed for the purpose of carrying on a system of dredging on the Northeastern Coast of Scotland, consisting of Dr. Ogilvie, Dr. Dickie, Professor Nicol, Dr. Dyce, and Mr. Peach; and £25 was granted for that purpose. Of this sum £5 was allotted to Mr. Peach, to enable him to conduct investigations at Wick.

In 1860 the few weeks available for dredging, before the meeting of the Association in July, were so tempestuous and generally unfavourable, that no part of the grant was expended; but in the course of the autumn, a trial was made off the coast of Banffshire. During the past summer, 1861, several dredging expeditions were planned and completed off the Bay of Aberdeen and adjacent coasts, none exceeding a distance of twelve miles from land.

The Committee in Aberdeen considered it advisable to receive the aid of others besides Mr. Peach, and to have trials made at points intermediate between Aberdeen and Wick, in order to render the investigations as complete as possible; and with this view the assistance of the Rev. W. Grigor, of Macduff, was asked, and readily accorded, a part of the grant being allotted to him. They have also secured the cooperation of another zealous naturalist, Mr. Dawson, of Cruden. This gentleman has just put at their disposal a valuable and interesting report on the Mollusca of Cruden Bay; but the others have not yet had sufficient time to allow of any report; and at Aberdeen, the examination of the materials collected being still in progress, the Committee are under the necessity of reserving the details for a further report. The general results, however, have been such as to lead them to hope that the sum of £25 will be granted for one year more, in order that the dredging may be further carried on, and at greater depths and distances from land.

No regular dredging has previously been conducted on this part of the Scottish coast; but the Committee have now the satisfaction of observing that, owing mainly to the admission of parties of students of the University to the dredging excursions, a feeling of interest has been awakened in the pursuit, from which the best results may be anticipated, and there can be no doubt that several ardent young men have thus been thoroughly trained in carrying on such operations in the open sea.

The Committee would urge these as reasons for a renewal of the grant, that they may be thus enabled to procure materials for a complete report at the meeting of the Association in 1862.

August 31, 1861.

GEORGE OGILVIE, for the Committee.

On the Resistance of Iron Plates to Statical Pressure and the Force of Impact by Projectiles at High Velocities. By WILLIAM FAIRBAIRN, ESQ., LL.D., F.R.S., &c., President of the Association. THE discovery of the application of iron plates as a means of defence against ordnance of great power and force are of recent date, and are attributable to His present Majesty the Emperor of the French. Since 1858 numerous experiments have been made to test the quality of the iron, and to determine the thickness of the plates employed for that purpose; but it is only of late years that the value and importance of this description of defence has been

ascertained as a covering for the sides of ships of war. The very powerful resistance of iron to projectiles at high velocities has directed most of the maritime powers of Europe to the advantage of armour-plating ships for the purpose of protecting them from the destructive effects of shot; and it has now been proved that a sheathing of plates 4 inches thick, covering the sides of a ship, extending to a depth of six feet below the water-line, is a sufficient protection against existing guns of the heaviest calibre. It is true that more powerful ordnance may be successfully tried against plates from 5 to 5 inches thick, but they are too heavy for general use on board ship; and as vessels of the present tonnage are not calculated to carry plates of greater thickness than 4 or 5 inches, it is more than probable that the country must be content with such protection as plates of these dimensions can afford.

Much, however, depends on the quality of the material of which they are composed; and the object of this communication is to furnish not only data for the manufacture of them, but to point out their mechanical properties and the best mode of attaching them to the ship.

There are two descriptions of vessels to which armour-plates may be applied, namely, those of iron, and the present existing vessels, composed entirely of wood. In the present state of our knowledge, it is desirable that all vessels of war should be formed of iron; but the transfer is a work of time, and the question now for consideration is, how to make our present wooden ships invulnerable, and how to apply the material to effect a maximum power of resistance to shot. This is the great question for solution, and the Admiralty, fully alive to the importance of the change, has instituted a long and laborious series of experiments to determine these results.

It is well known that all substances of a brittle nature are easily broken by impact, and the best kind for resisting blows is a tenacious, tough, and ductile material. To secure all these properties is a desideratum in the manufacture of iron plates, and one which never ought to be neglected. In submitting the following results obtained from the experiments, it may be interesting to show the chemical compositions of some of the best irons experimented upon, and those marked with the letters A, B, C, and D, when carefully analysed, were found to contain the following ingredients :—

Comparing the chemical analysis with the mechanical properties of the irons experimented upon, we find that the presence of '023 per cent. of carbon causes brittleness in the iron; and this was found to be the case in the homogeneous iron plates marked C*; and although it was found equal to A plates in its resistance to tension and compression, it was very inferior to the others in resisting concussion or the force of impact. It therefore follows, that toughness combined with tenacity is the description of iron plate best adapted to resist shot at high velocities. It is also found that wrought iron, which exhibits a fibrous fracture when broken by bending, presents a widely

*Homogeneous iron is that description of iron or steel which is not rolled or manufactured from piled bars, but obtained by the boiling process from the furnace, where the amalgamation is complete; or, in other words, it is obtained from cast ingots according to the Bessemer process, or direct from the bloom as it leaves the puddling furnace.

different aspect when suddenly snapped asunder by vibration or a sharp blow from a shot. In the former case the fibre is elongated by bending, and becomes developed in the shape of threads as fine as silk, whilst in the latter the fibres are broken short and exhibit a decidedly crystalline fracture. But, in fact, every description of iron is crystalline in the first instance; and these crystals, by every succeeding process of hammering, rolling, &c., become elongated, and resolve themselves into fibres. There is, therefore, a wide difference in the appearance of the fracture of iron when broken by tearing and bending, and when broken by impact, where time is not an element in the force producing rupture.

The mechanical properties of iron best calculated to resist the penetration of shot at high velocities are enumerated as follows.

The plates were subjected to statical tensile strain, to compression, and to punching, with the following results.

1. Specific Gravity.

The mean specific gravity of the 12, 2, 24, and 3-inch plates of each series were as follows:

The order of merit is therefore C, A, B, D. These results coincide with the following tests.

2. Tensile Strength.

The statical resistance to tensile strain was as follows:

Tensile strain per square inch in tons.

Thicker plates.

24.644

23.354

27.032

24.171

The general order of merit in this case is C, A, B, D. The homogeneous metal plates have the highest tenacity, but decrease in strength progressively as the plates increase in thickness.

3. Ductility of the Plates.

A measure of the ductility of the plates is afforded by the ultimate elongation under tensile strain.

Here the order of merit is nearly the same as that for density and tenacity. On the whole the elongations increase progressively with the thickness fo iron plates, and decrease for homogeneous metal plates. But with iron th› ductility is nearly the same for 2, 2, and 3-inch plates.

4. Resistance to Impact.

Mr. Mallet has pointed out that the product of the tensile breaking weight

and the ultimate elongation of iron indicates its resistance; or, in other words, the product of the tenacity and ductility of iron affords a measure of the dynamic resistance of the material, or its resistance to impact. The following numbers give this coefficient of rupture:

To ascertain this coefficient with accuracy, rather longer specimens should have been tested; but, bearing in mind this source of inaccuracy, the numbers strikingly correspond with the results obtained by impact. It is not of much use to compare directly the resistances obtained with those given above, because the former were made with such large intervals (half-inch) in the thickness of the plates that they afford no criterion of the relative values of the different descriptions of iron. But we may compare the iron and steel plates, where the difference of resistance, being greater, is to some extent indicated in the experiments with ordnance.

Thickness of plates.
Half inch

Dynamic resistance.

Iron plates.

Steel plates.

With these results obtained by simple pressure, we compare those obtained by ordnance. The resistance of the iron plates being again taken as unity, the resistance of the steel plates was as follows:

From the above it will be seen that there is quite as close an approximation in the ratios in these two tables, for corresponding thicknesses of plate, as could be expected from the nature of the experiments. Both the series of experiments (viz., that with dead pressure and that with ordnance) indicate the same increasing resistance of the iron plates, and decreasing resistance of the steel plates; and the ratios of their relative resistances are nearly the same. In making the comparison, the resistance to ordnance is assumed to be as the square of the thickness of the plates—a law which will hereafter be demonstrated.

The relative values of the plates in resisting shot are, according to the experiments with dead pressure, as follows:

These numbers are deduced from the results on the 14, 2, 2, and 3-inch plates. With 3-inch plates the iron is much stronger than the steel.