So long as wood is cheaper than iron in our ountry, it will be used in prefence to it for lie sake of economy in the first cost. Every ear, however, tends to increase the scarcity nd price of timber, and iron is every day ex-ending in use, and it will yet be as common o see iron houses of all kinds as it now is to ee those of wood. We shall not live to see his result, but we can see it afar off. In our ity, iron pillars are universally taking the ilace of those made of wood and stone, and in Iritain, five iron ships are now built for one f wood. The employment of iron in marine tructures forms an important era in respect o its use for lighthouses. The great expense nd difficultly heretofore experienced in form-ng foundations of stone for lighthouses in and banks, and in yielding soft places, have >een overcome by Mitchell's iron screw piles, nd Potts' iron cylinders, and then raising the uperstructure on these. We have a letter efore us from Mr. C. Pontez, stating that he 3 progressing with his foundations of iron cy-inders tor a lighthouse in the course ot erec-ion fifteen miles below Baltimore. A num-ler of these cylinders are now sunk, and when ill completed, they will form two concentric ircles, the outer one twenty-three feet in di-imeter, composed of cylinders 26 inches in liameter each, and one inch thick; the inner :ircle will be seventeen feet) in diameter with ylinders of 17 inch diameter. These cylin-lers will be filled with concrete, capped with ron plates, and all the caps connected togeth-ir by wrought-iron ties, thus forming a con-inuous circuit. Around and within, the cir-:les will be filled with large masses of granite ;o the level of low water, and on the top of ihe iron circuits the regular courses of masonry will be laid. The site is two miles from ;he shore, in water 10 feet deep, and thus a itrong and permanent lighthouse will be built jy the employment of iron foundations at an jxpense of less than one-half ot what a stone foundation could be laid ; indeed, the employment of iron, enables our marine engineers te build lighthouses in situations where it would be utterly impossible to build stone towers. At the exhibition of the Franklin Institute now open in Philadelphia, there is the model af an iron lighthouse by Merrick & Son, to be built on screw auger piles bored 12 feet into the coral reef, at Sand Key, Florida, it has a base of 50 square feet will be 132 feet high, and weigh four hundred and fifty tons.— The lighthouse on Carysfort Reef, Florida, completed by the Topographical Bureau this year, is a wonderful iron structure, and was made by Merrick & Towne, of Philadelphia. It is built on piles arranged upon the angles and centre of an octagon; the heads of these piles are united by iron ties, and on this arise courses of iron pillars and a strong central column from the centre foundation to a level with the top of the upper series of pillars— from this central column, there radiate, at proper levels, iron girders of great strength, which, added to the horizontal ties extending from one pillar to another, form a combination so compact and stiff that no force of the wind, it is supposed, will ever disturb it. For the residence of the keepers of the light, a cast-iron dwelling of a circular and conical form is fitted to the above described frame-work of pillars, ties, &c, at a point 35 feet above the level of the reef, and 20 feet above the highest tides. This dwelling consists of two stories. The lower one being about 8 feet in height, and 40 feet in diameter, is designed for the depo-posit of stores, the kitchen, etc. It is fitted with 8 windows and 16 bull's eyes—the former for air, the latter for light. It contains six iron tanks for water and oil. The upper story is divided into six roomSj with a hall in the centre to allow a free ventilation in all the apartments. There is a door at each end of the hall, and a large window in each room. Surrounding this story is a gallery, exterior to the house, 5 feet in width, where the keepers may exercise. From the centre of the hall rises a spira staircase to the top of the structure. This staircase is enclosed by an iron cylinder, the whole weight of which rests upon the roof of the dwelling house. On the top of the structure is placed the watch room, and lantern, or light room, fitted to contain a Fresnel apparatus of the largest size, that will produce a light of the highest power. The diameter of the structure at the base is 50 feet, and 20 feet at the level of the watch-room floor. The height of the entire work above the surface of the reef is 127 feet, and the height of the centre of the light 115 feet. It was' tor this lighthouse that the Fresnel Light was intended, which was sold in the New York Custom House for old iron and glass, when in charge of the Topographical Engineers, and not under that of the Lighthouse Board, as we have since been informed. The benefit of iron in marine structures, such as lighthouses, was first displayed by A. Gordon, C. E., of London, who, in 1841, erected one on Morant point, in the Island of Jamaica, on a position difficult of access, and wherei from the frequency of earthquakes, no stone lighthouses above two stories high could stand. This lighthouse is made of cast-iron, and has stood several severe shocks of earthquake. A cast-iron lighthouse was erected by the same engineer on the Island of Bermuda, in 1845; it is 105 teet high, and is provided with a Fresnel light, which can be seen at 30 miles distance. Owing to the great expense, or total inability of erecting stone structures in many exposed situations, we cannot but feel grateful that iron meets and surmounts all such difficulties. The iron lighthouse in Bermuda has been the means of greatly reforming the habits of a large number of the inhabitants who formerly gained their livelihood as wreckers, an occupation not very favorable to the developement of the best qualities of humanity. The iron lighthouses on the dangerous Florida Reefs will also be the means of doing a vast amount of good in this respect. Within view ot a first-class light on Carysfort Reef, there was wrecked in three years and foui months, property to the amount of $1,147,500. The wrecking fleet on the Florida Reefs amounts to 47 vessels with a tonnage of 1,200 tons, and crews amounting to 350 men. At Key West, Florida, the amount ot salvage de-ereejLta, the wreckers, in 1848, amounted to $199,140, and the wrecked vessels and cargoes amounted ta$l,282,000. The iron lighthouses on the coast of Florida, if they do not prevent all this great amount of wreckage, will no doubt prevent nearly the whole of it; success then, we say, to our Iron Lighthouses