lanthorn by Messrs. Wilkins: the whole is illustrated by a series of drawings, which fully describe this useful construction, which has hitherto withstood the most violent attacks of the sea to which it is exposed.

In answer to questions from the President, Mr. Wilkins stated, that he had been in the Eddystone and the Malpin Sand lighthouses, during severe gales of wind; that, as might be conceived, from the nature of the construction, the latter building was more affected than the former by the striking of heavy seas: the motion appeared to be more like torsion than simple vibration, which he attributed to the waves striking the ladder and its projecting stage, and thus tending to twist the upper part. Still the motion was not such as would cause injury to the building.

The President pointed out two diagonal braces, extending downward from the end of the ladder-stage, to the piles on either side, which had been introduced in order to counteract the twisting described by Mr. Wilkins. In constructions of this nature, it was of importance to oppose as little resistance as possible to the seas, especially in the upper part of the building; a system of bracing had therefore been adopted, which consisted principally of two series of continuous circular horizontal ties between the piles, at the several heights of 6 feet and 15 feet above low-water mark of spring tides. From the external ring of piles, two sets of diagonal stays, extended to the centre pillar, forming strong triangular trusses, in the direction of each pile, and two sets of horizontal stays, stretched between the piles and the centre pillar, at the levels of the circular bands. The amount of direct vibration was very small, and he did not conceive that the twisting motions, which had been described, was sufficient to warrant the introduction of diagonal braces, which would materially augment the surface upon which the waves would act.

Mr. Vignoles directed the attention of the meeting to the system of diagonal bracing between the piles, which had been adopted at the Port Fleetwood lighthouse; he apprehended that as the principal force of the waves would be exerted against that part of the structure which was above the high-water level, the diagonal braces, extending between the upper part of the piles and the level of low-water, were preferable to the horizontal continuous bands of the Maplin Sand lighthouse, although assisted

by the system of radiating central truss-braces which it possessed: he conceived, that both buildings were strong enough for the purposes for which they were constructed, but he preferred the mode of bracing adopted in the Port Fleetwood house, the vibration of which he knew to be very small, although situated in an exposed position, where the rise of tide is 30 feet.

Mr. Donkin observed, that there could not exist a doubt of the introduction of diagonal braces rendering the building stronger; how far they were necessary, or might be prejudicial, in offering additional resistance to the passage of the waves, should be well examined before adopting them. He considered the position of the suspended ladder decidedly objectionable, as any torsion, caused by the waves striking it, must tend to dislocate the fibre of the material of the piles, and to fracture them.

Mr. Farey believed the construction of the Maplin Sand house to be better adapted than the Fleetwood house for resisting the direct action of waves, but the diagonal bracing of the latter enabled it to withstand torsion better than the hoop-bracing of the former. He inquired, why the lower part of the lightkeeper's house was made conical? as he apprehended that it would receive a heavier blow from a wave than if it had been flat.

The President replied, that the main body of the waves seldom or never rose so high as the bottom of the house, and that the conical form allowed the air and spray to rise up and be guided off, without affecting the building, as it would do if the bottom was flat.

With regard to the torsion, that had only been felt at first when the ladder extended too low down and received a constant succession of blows from every wave, which naturally communicated a vibration to the whole structure; the ladder was now shortened and nothing of the kind was felt; the waves scarcely, even in the roughest weather, struck the suspension stage or the boat. He preferred the continuous horizontal bracing, which bound all the piles firmly together, like the staves of a barrel; and, from observations he had made, he believed the amount of vibration to be greater in the Port Fleetwood lighthouse than in that at the Maplin Sand.

In answer to a question from Mr. G. H. Palmer, the President said, that at present there was not any indication of a change in the condition of the cast-iron from its contact with the salt water.

Professor Brande was unable to give any additional evidence

on the observed facts connected with the change suffered by castiron exposed to the action of salt water, or in mines and in various other positions; from experiments which he had made, he was led to believe, that many of the appearances observed in the changes of cast-iron, arose rather from a peculiar mechanical combination of the molecules, than from a difference in the chemical constitution of the metal: no difference could be detected by analysis in the metal which had undergone change and that which had not.

It should be remarked, that the contact of two metals was not essential to cause galvanic action; a film of oxide upon the surface of the body of metal, formed a very active galvanic pile; hence arose the necessity for preventing oxidation, by proper paints or varnish, before using pieces of cast-iron in exposed situations.

Mr. Farey observed, that in the early engines, constructed by Woolf, in Cornwall, in which the packing segments were of gunmetal, and the body of the piston was of cast-iron, wherever the two metals were in contact, the iron was turned to plumbago: this had been particularly observed where high-pressure steam was used it might be a question, whether the temperature of the steam, and the quantity of mineral water carried over with the steam, by the large amount of priming of the engines in that day, had not materially contributed to produce the effect.

Mr. P. Taylor believed, that the temperature of the steam had not any connection with the subject. In the metallic packing of steam-pistons of low-pressure marine engines, which he had constantly under repair at Marseilles, wherever the wedge-pieces were of gun-metal, the backs of the cast-iron segments were converted into plumbago, whilst those surfaces of cast-iron, which were ground together and worked against each other, remained unchanged; the same might be said of the rubbing surfaces of cast-iron against gun-metal; it appeared, therefore, that the formation of an oxide was necessary to commence the change. He repudiated the use of cast-iron in situations where these changes were to be apprehended; he would employ wrought-iron, as although that did become oxydized, it retained its relative strength to the last, whereas cast-iron, when changed into plumbago, retained its bulk, but lost nearly all power of cohesion.

Mr. John Taylor said, that in Cornwall the cast-iron puinptrees, exposed to the action of mine water, were very speedily destroyed; and even although 1 inch thick, they could be cut

to pieces with a knife when first taken out of the pit. The airpump buckets of steam-engines, in which the body was of castiron and the valves of gun-metal, formed the most perfect kind of galvanic apparatus; they should be made entirely of gun-metal. In manufactories of vinegar and pyroligneous acid, the decay of cast-iron was very rapid.

Mr. Glynn attributed, in a great degree, the rapid decay of cast-iron, in coal mines, to the presence of sulphuric acid evolved from the pyrites.

Mr. Philip Taylor agreed with Mr. Glynn; even copper pipes were rapidly destroyed in the bilge-water of vessels, which always contained much sulphuretted hydrogen, he recommended the use of stout lead pipes in such situations; they would be found much more durable.

Mr. Davidson had found it necessary to substitute gun-metal gratings for the cast-iron ones at Messrs. Hanbury's brewery, as although they were 3-inch thick, they had been entirely destroyed in four years.

The President gave a short account of the construction of a lighthouse now making by Messrs. Gordon and Co. at Deptford, under his directions, for the Point of Air. The lanthorn for it would be cast from a gun which had been raised from the wreck of the Royal George.

Mr. C. W. Williams exhibited and explained the sight-tubes which he now used for the marine boilers of the City of Dublin Steam Packet Company's vessels.

The instrument consists of a wrought-iron welded tube, 2 inches diameter, with a screw-thread cut upon the exterior; it is inserted across the water spaces of the boilers, and secured by means of nuts in such positions behind and opposite the furnace, as enables the engineer to see all that goes on interiorly, particularly the degree of perfection or imperfection in which the gaseous matter enters into combustion, and the effect of admitting or excluding the air.

The instrument had been found very useful, not only in experiments but in practice, on the large scale, and he deposited it in the Gallery of the Institution in order that it might serve as a model for those who were inclined to adopt it in marine boilers.

May 31, 1842.

The PRESIDENT in the Chair.

"On the construction of Model Maps, as a better mode than Sectioplanography, for delineating the Drainage and Agricultural Improvements of a Country, or projected lines of Railways, Canals, &c."-By John Bailey Denton, Assoc. Inst. C. E.

This communication was accompanied by a map in relief of an estate, as a specimen of the method which the author recommends. The subject of mapping in relief is not new, and the author had previously published a treatise on the subject, but having made extensive experiments, he was enabled to bring the subject before the Institution in a more defined form, showing that the construction of the models had been reduced to a simple and cheap method. These models are peculiarly recommended for pointing out the capabilities of district for drainage, either for agricultural purposes or for collecting waters together for manufacturing power. They are superior to maps, as they show at a glance the relative heights of the various points, display the geological phenomena, and may be made to delineate the state of cultivation of the districts. The lines of railways, of roads, or of canals, can be more clearly defined upon them, and they are stated to be peculiarly adapted for parish surveys.

The expense of making a model of an estate, of compact form, is stated to be from two shillings and sixpence to three shillings and sixpence per acre.

"Observations on the Periodical Drainage and Replenishment of the Subterraneous Reservoir in the Chalk Basin of London." -By the Reverend James Clutterbuck, &c. &c.

This paper, which formed the substance of a letter to the Reverend Dr. Buckland, and was by him communicated to the Institution, consists of a series of observations on the periodical drainage and replenishment of the subterranean reservoir of the chalk basin of London, especially that part of it which lies in a N. W. direction between London and the Chiltern Hills.

The author divides the district into two portions, that to the north and that to the south of the river Colne.

The northen portion is mostly covered only with a bed of gravel,