mode of fixing the cast-iron screws and piles, made by Messrs. Rennie -the iron-work, by Messrs. Gordon of Deptford-the wood-work by Messrs. Gates and Horne of Poplar, and the lantern, by Messrs. Wilkins.

NEW EGYPTIAN LIGHTHOUSE.

MEHEMET ALI has caused a new Lighthouse to be erected on Point Eunootos, near his palace at Alexandria. The tower is of stone, and 180 feet high; the lantern, (supplied from England, by Messrs. Wilkins and Son,) consists of thirteen lamps, with parabolic reflectors. The light can be seen from a distance of twenty miles at sea. The structure was designed by, and has been executed under the superintendence of, our countrymen, the Messrs. Galloway.

THE LIGHT FOR ALL NATIONS."

We regret not to have to record the execution of the bold design of fixing a Light upon the Goodwin Sands, by its enterprising projector, Mr. Bush, who has not succeeded in sinking the caisson, or base. Meanwhile, the details of this portion of the Light may be acceptable, in addition to the brief notice of the design in our last Year-book, p. 55.

The caisson may, in one point of view, be described as an enormous diving-bell, as far as affording the means of working under water to an extent hitherto unattempted. But, it is something more. It is a diving-bell which, by excavation, will enable these operations to be pursued, to any depth, through the water, and even through the semi-liquid sand. Unlike the diving-bell, when brought to its situation and permanently sunk, and the period of its office over, this caisson becomes part and parcel of the very foundation which, in the first instance, it was intended to accomplish; for its void will then be filled in with solid masonry, upon which the superstructure will hereafter be erected.

The caisson is composed of cast-iron plates, of a conical shape, thirty feet diameter at the base, the upper diameter being twenty-five feet, and thirty feet in height. These plates are arranged in courses or tiers, each six feet high, and twenty-four plates in each tier. The whole of the horizontal and vertical joints are connected together with flanches and bolts; and afterwards with iron-cement, through the joints, to render the machine perfectly air and water tight. The section is divided into three chambers. In the lower chamber, the work of excavation will be carried on. This chamber has a domed top, with a covered aperture or air-tight valve, four feet in diameter in the centre, communicating with the chamber above. The second chamber has an upward and external communication by means of a cylinder, four feet in diameter, also covered by a valve, to be opened as required. This chamber is fitted with air-pumps, valves, and air-gauges, to obtain and regulate the necessary supply of air for the workmen during the progress of the works. For the supply of air, another difference will be apparent from the process employed in the common diving-bell: for, instead of the air being forced down from above according to the

usual method, the pumps stationed below draw the air from above, with all the pressure of the atmosphere in favour of their action, instead of forcing against it. As the process of excavation is carried on in the lower chamber through this cylinder, the sand and soil removed will be discharged over the top of the aperture into the sea, and the gradual sinking of the caisson effected by its removal.

The third, or upper chamber, is covered air and water tight, excepting by means of the valves, through which air is supplied, and is fitted as a residence for the workmen during the progress of the works. The caisson being sunk, the work of excavation will be commenced. This will be carried on in a similar manner to that pursued with the kit of a well, or rather as were the shafts at the entrance to the Thames Tunnel, with the only difference that these are on land. The workmen, however, in each case, are stationed within the cylinder, where they excavate; and, by their operations, and the removal of the soil, the gradual sinking of either shaft or caisson is effected, proceeding downwards until a solid foundation is obtained. The surface will then be levelled, and the lower flanch of the caisson brought to a permanent and solid bearing. After this process has been completed, the masonry will coinmence; and the whole centrical contents of the caisson will be filled in with solid masonry, which will be further protected by the outer coating of cast-iron. The conical form of the caisson thus embedded in the sand, which will silt in upon it, it is concluded, will secure the whole body firmly.

IMPROVEMENT OF LIGHTHOUSES.-LIGHTS IN RAPID MOTION.

Mr. ALLAN STEVENSON has experimented with the apparatus by which Captain Basil Hall proposed to increase the intensity and power of fixed Lights for Lighthouses, to such an extent as to render their constant effect little inferior to that of the bright flashes which alternate with the dark intervals in revolving lights.*

Mr. Stevenson has repeated Capt. Hall's experiments, and has also made some others, which appeared to him to bear on the subject. Captain Hall, however, made all his comparisons at the short distance of 100 yards; whereas, Mr. Stevenson arrived at his conclusions by comparing the lights from a distance of 14 miles; and he describes them briefly in the following order :

"1. The flash of the lens revolving slowly was very much larger than that of the rapidly revolving series; and this decrease of size in the luminous object presented to the eye, became more marked as the rate of revolution was accelerated, so that, at the velocity of 8 or 10 flashes in a second, the naked eye could hardly detect it, and only a few of the observers saw it; while the steady light from the refractor was distinctly visible.

2. There was also a marked falling off in the brilliancy of the rapid flashes as compared with that of the slow ones; but this effect was by no means so striking as the decrease of the volume.

*For the details of Captain Hall's apparatus, see Year-Book of Facts 1841, p. 93.

3. Continuity of impression was not attained at the rate of 5 flashes in a second, but each flash appeared to be distinctly separated by an interval of darkness; and when the nearest approach to continuity was made, by the recurrence of 8 or 10 flashes in a second, the light still presented a twinkling appearance, which was well contrasted with the steady and unchanging effect of the cylindric refractor.

4. The light of the cylindric refractor was, as already stated, steady and unchanging, and of much larger volume than the rapidly revolving flashes. It was not, however, so brilliant as the flashes of the quickly revolving lenses, more especially at the lower rate of 5 flashes in a second.

5. When viewed through a telescope, the difference of volume between the light of the cylindric refractor and that produced by the lenses, at their greatest velocity, was very striking. The former presented a large diffuse object of inferior brilliancy, while the latter exhibited a sharp pin-point of brilliant light.

Upon a careful consideration of these facts, it appears warrantable to draw the following general conclusions:

1. That our expectations as to the effects of light, when distributed according to the law of its natural horizontal divergence, are supported by observed facts as to the visibility of such lights, contrasted with those whose continuity of effect is produced by collecting the whole light into bright pencils, and causing them to revolve with great velocity.

2. It appears that this deficiency of visibility seems to be chiefly due to a want of volume in the luminous object; and also, although in a less degree, to a loss of intensity; both of which defects appear to increase as the motion of the luminous object is accelerated.

3. That this deficiency of volume is the most remarkable optical phenomenon connected with the rapid motion of luminous bodies, and that it appears to be directly proportional to the velocity of their passage over the eye.

4. That there is reason to suspect that the visibility of distant lights depends on the volume of the impression, in a greater degree than has, perhaps, been generally imagined.

5. That as the size and intensity of the radiants causing these various impressions to a distant observer, are the same, the volume of the light, and, consequently, cæteris paribus, its visibility, are, within certain limits, proportionate to the time during which the object is present to the eye.

Such appear to be the general conclusions which these experiments warrant us in drawing; and the practical results, in so far as lighthouses are concerned, seems sufficient to discourage us from attempting to improve the visibility of fixed lights in the manner proposed by Captain Hall, even supposing the practical difficulties connected with the great centrifugal force generated by the rapid revolution of the lenses to be less than they really are.

Mr. Stevenson adds, in conclusion, that this decrease in the volume of the luminous object caused by the rapid motion of the lights, is an interesting effect, from its apparent connexion with the curious pheno

menon of irradiation. When luminous bodies, such as the lights of distant lamps, are seen by night, they appear much larger than they would do by day; and this effect is said to be produced by irradiation. M. Plateau, in his elaborate essay on this subject, after a careful examination of all the theories of irradiation, states it to be his opinion, that the most probable mode of accounting for the various observed phenomena of irradiation is to suppose, that, in the case of a nightview, the excitement caused by light is propagated over the retina beyond the limits of the day-image of the object, owing to the increased stimulus produced by the contrast of light and darkness; and he also lays it down as a law, confirmed by numerous experiments, that irradiation increases with the duration of the observation. It appears, therefore, not unreasonable to conjecture, that the deficiency of volume observed during the rapid revolution of the lenses may have been caused by the light being present to the eye so short a time, that the retina was not stimulated in a degree sufficient to produce the amount of irradiation required for causing a large visual object. When, indeed, the statement of M. Plateau, that irradiation is proportional to the duration of the observation, is taken in connexion with the observed fact, that the volume of the light decreased as the motion of the lenses was accelerated, it seems almost impossible to avoid connecting together the two phenomena as cause and effect.-Abridged from Jameson's Journal, No. 64.

EMBANKMENT OF THE NILE.

DR. LABAT has transmitted from Egypt the following details of the plan adopted by Mehemet Ali for carrying into execution the Improvement of the Nile, first conceived by the Emperor Napoleon. The first great work is to be the establishment of a bridge of eightythree arches, running from the point of the Delta to each of the opposite banks of both branches, similar to the Pont-Neuf, at Paris. On each side of the spur, a sluice is to be formed for the purpose of navigation. All the eighty-three arches are to be also furnished with flood-gates of iron or wood, to be opened or shut, according to the wants of traffic and navigation. A tunnel is to be cut through the spur of the Delta, forming a communication between the two branches of the river. Canals are also to be cut from each branches running to the east and the west, with various minor channels, with sluices for the commerce and irrigation of the country. Above the bridge, the Nile is to be embanked on each side, so as to keep the water always within a certain level; and all these embankments will be faced with masonry. Concrete will be used for all the submarine works, and the rest will be done with square stone, rubble, and bricks. These materials are found in abundance in Egypt, and even in the immediate neighbourhood of the works. Artificial pozzolane is in general use, being obtainable in all parts of the country from pulverized bricks. This matter, which is analogous to that produced by volcanos, being mixed with lime and rubble, form what is called béton or

concrete. Before the discovery of this pozzolane, which costs 5f. the cubic-metre on the spot, it was brought from Italy, at the expense of 45f. or 50f. the cubic-metre. The cost of the bridge has been estimated at 7,000,000f., and cannot exceed 10,000,000f. When once the materials are collected, it will require no more than three years for 5,000 men to complete this colossal undertaking, which will vie in grandeur with the celebrated monuments of ancient Egypt. According to a calculation recently made by a Parisian engineer, it appears that the present irrigation of Egypt, though very limited in comparison to what will hereafter be accomplished, costs the labour of 200,000 oxen and 100,000 men.

REID'S FLOATING BREAKWATER.

FLOATING Breakwaters are, at present, occupying much of the public attention; which has led the inventor of the present one to communicate his design to the Civil Engineer and Architect's Journal, No. 56.

A is an arched frame of timber in thicknesses, 6 feet high in the centre ; B, frame of timber bolted together, 2 feet square, and 20 feet long on the chord line; C, sloping frame of timber, 1 foot square, secured with iron straps, bolts, and stays, and protected at the point by iron shoes; D, inclined plane or shutter, 24 feet 6 inches long, laid to an angle of 35°, with planking diagonally bolted to a framework of timber, at 3 or 4 inches between the planks; E, iron cable; F, bridle, and G, chain for lifting shutter; H, bit or head, to which the cables are secured.

If the depth of the wave be 9 feet below the chord bar of the arch, there will be 6 feet (perpendicular) of the shutter below that; the inclined plane will underrun the wave, and the arched framework above will offer a gentle resistance for the wave to fall upon, and distribute itself harmless. There will be no strain upon the hinges, the cables being secured to bits in the centre of the raft. When the sea strikes the front of the inclined plane, the framework will yield, and the hinges will prevent that sudden check which the cable tightening would give to the work. Were they not there, the frame would almost instantly resume its place: the buoyancy of the shutters takes all the weight from the hinges. The principal use of the hinges to the shutters is for the more easy recovery of the moorings. A small chain