fire for 181 years. For about 40 years after its erection by the great engineer John Smeaton, the Eddystone showed no better light than a few miserable tallow candles.

So lately as 1801, the light at Harwich, in addition to the coal fire, had a flat plate of rough brass on the landward side, to serve as a reflector. Such methods were most imperfect in every respect.

In recent times, improvements have rapidly succeeded each other. First, the sources of light were nearly perfected by the adoption of Argand's oil lamps. Next, the true principles of reflecting, or catoptric lights, were applied to Argand's lamps. By this natural but important combination very efficient sea-lights were put into many lighthouses. The progress was by gigantic strides, under the guidance of Fresnel, and Robert Stevenson of Edinburgh.

The oil, the wicks, the lamps, the reflectors, successively received the attention of men of skill and science, with the best results in the efficiency and economy of the lights.

Some idea of the perfection to which the shaping and: polishing of parabolic reflectors have been brought, may be formed from the parabolic reflector exhibited by W. C. Wilkins of Long Acre.

The arrangement of lights on the catoptric system has been even recently improved, as we shall have another opportunity of describing; but the dioptric and catadioptric systems of preventing loss of light, for directing the greatest possible amount of light towards the horizon, have for some years been adopted for the more important sea-lights of great maritime nations.

The two great lighthouses exhibited are examples of these improved. They are termed of the "first order,” that is to say, they are of the largest dimensions employed in lighthouses.

The object to be obtained by the use of lenses in a lighthouse is the same as that obtained by the use of reflectors. Each apparatus effects the same result by different means, collecting the rays which diverge from a point, called

the focus, and projecting them forward in a beam, the axis of which coincides with the produced axis of the instrument.

In the case of reflection, this result is obtained by the light being thrown back from a surface formed into a parabolic shape, which causes all the rays to proceed in one and the same direction. In the case of refraction, on the other hand, the rays pass through the refracting medium, and are bent or refracted from their natural course into that desired.

The names of Buffon, Condorcet, and Brewster are con spicuous in the history of the dioptric system; but to Auguste Fresnel is due the merit of having first constructed this apparatus on true principles, and of finally applying it to the practical purposes of a lighthouse.

The Dutch were the first to follow the French, and the Northern Lights Commissioners were the next, and then the Trinity House, and now all maritime nations are adopting this important improvement.

The manufacture of such masses of pure glass as are required is a matter of great difficulty. The difficulties are of two kinds-first, to get the glass free from stric or flaws, and, secondly, to get a colourless glass. The facilities for making crown-glass free from striæ determined Fresnel in the adoption of the material of the lenses of his first dioptric apparatus, notwithstanding the greenish colour which it invariably has, and which is very marked when the light passes through a great thickness, as happens when we see the the lenses of these apparatus sideways. In both the arrangements exhibited, crown-glass has been employed.

The catadioptric part of the apparatus-the combination of reflection and refraction-is in the upper part or dome of the apparatus. This important improvement is due to Mr. Alan Stevenson of Edinburgh. The illuminating effect of the Cupola of Jones is to that of the mirror formerly used as 140 to 87.

Astronomical Telescope (254), with its object-glass of 114 inches diameter, with equatorial movements and other adjustments, well displays the magnificent kind of instruments which are required for penetrating into space. The difficulty of obtaining large lenses is very great. Under the immediate inspection of Fraunhoffer of Munich they were brought to a high degree of perfection; but in this country until the present time they have not been satisfactorily produced. The first difficulty is that of obtaining glass sufficiently free from striæ to give a welldefined magnified image of a point of light; and the second is that of grinding and polishing, which require the utmost mathematical precision. The grinding is effected by brass tools of various kinds, corresponding with the required curvature; and about six sizes of washed emery are employed for grinding the lens to the true figure, which is called tracing the lens, the grinding with every size emery being continued until all the marks made by the previous size are removed; the polishing being completed with putty powder shaken uniformly over cloth and moistened with a little water, which is then worked into the cloth with a brass convex tool. This operation is so important, that it is often a task of from two to three hours to make up a polisher of from eight or nine inches diameter. Mr Ross's other apparatus spread around this will receive notice in their proper groups.

Model of Britannia Bridge.—It may be stated that the tubes of the Britannia and Conway Bridges are formed of iron plates riveted together. The whole of the riveting was done by hand, and 1,095,050 rivets were employed in each tube.

The entire bridge, including both lines, contains nearly a million and a half cubic feet, or 105,000 tons of masonry, 44,200 cubic feet, or 9,480 tons of cast iron. The two tubes in their complete state contain 9,360 tons of wrought iron, 1,015 tons of cast iron, and 165 tons of permanent way. They are composed of about 186,000 separate pieces

of iron, pierced by seven millions of holes, and united by upwards of two millions of rivets. They contain 435,700 feet, or 83 miles of angle iron; and their total weight is 10,540 tons.

This enormous mass has been raised 103 feet, where it rests upon its towers, the span between them being 460 feet. When we know that the span of the centre arch of Southwark Bridge is but 240 feet, the enormous space over which this hollow iron beam is extended, is faithfully conveyed to the mind.

The other models of the wrought-iron bridges over the Wye, at Chepstow, by Mr. Brunel, which is the first of the kind constructed, and of the bar-chain suspension-bridge at Kieff, across the river Dnieper, in Russia, by Mr. Vignoles, are well deserving inspection, as illustrations of engineering skill.

Model of 3,000 miles of Northern England (90) very clearly shows the peculiar features of that section of our island, extending over portions of Leicestershire, Yorkshire, Nottinghamshire, Derbyshire, Cheshire, and Lancashire.

Trigonometrical Model of the Undercliffe, Isle of Wight. -In this model, Captain Ibbetson has not merely shown the topographical features of the part of the island represented, but has also exhibited the geology of the line of coast. In the sections of the cliffs the succession of strata is beautifully displayed; various divisions of the lower greensand are seen capping the Wealden strata, which are the lowest beds exposed. The lower greensand is capped in turn by the gault; and then succeed the upper greensand, the chalk marl and the chalk forming the highest portion of the ground modelled. As there are many considerable disturbances displacing and dislocating the strata within the area selected, very curious geological phenomena are exposed in a highly-instructive manner. The influence of the mineral character of the different strata in determining the outline of the land is finely seen.

Limestone Model of Plymouth Breakwater.-This model conveys a good idea of this great national work.

The Breakwater is an insulated mole, or vast heap of stones, stretching across the entrance to the Sound, so far as to leave a passage for vessels at either end, and opposing a barrier to the heavy swell rolling in from the Atlantic. Its length is 1,760 yards, the eastern extremity being about 60 fathoms to the eastward of St. Carlos's Rocks, and the western 300 west of the Shovel Rock. The middle part is continued in a straight line 1,000 yards, and the two extremities incline towards the northern side of the straight part in an angle of about 120 degrees. This great work was begun August 12, 1812. During its progress convincing proofs of its efficacy and utility were afforded. The expense of erecting the Breakwater is estimated at 1,171,100%.

Near this is another model showing the manner in which the lighthouse built at one end of the Breakwater has been constructed. It will be seen that the granite blocks of which it is built are so accurately dovetailed as to render it impossible to move one stone without the whole; in fact, the building is as secure as if it had been constructed from a solid rock. These models are made from the same limestone as is used in the Breakwater itself.

The models, &c., which occur at the western end of the Nave, are in themselves sufficiently explanatory, or they will be noticed under the Class to which they especially belong.

OUTSIDE THE BUILDING, WEST END, NORTH.

Within this space are gathered a collection of such things as could not be well placed in the Building. These are, however, in many respects of the utmost importance, and attention should be directed to some of their characteristics.

As the groups to which many of these things belong