Polyt. J. ccxii., by Dr. Ferd. Fischer, confirmatory of these remarks, and showing the influence of elevated temperature and pressure on the decomposition of various salts in water.

Corrosion-Causes of corrosion of exterior of boilers are briefly glanced at, including damp settling, accumulation of damp ashes and of soot, accompanied by careless firing, which causes sulphur, acids, and other corrosives to combine with

the soot.

With regard to corrosion of the interior of boilers, investigations on various corroding forces are first quoted. Prof. Crace Calvert's experiments on the action of sea-water and of various gases on metals are alluded to, to prove that sea-water exerts such an action upon steel and iron, that carbonic acid, in presence of water, acts energetically, and that distilled water, free from gases, has no action.

The application of these researches by W. Kent (of the Stevens U.S. Institute of Technology) to the examination of the corrosion of iron railway-bridges in the United States is then referred to; and the investigations of A. Wagner (from Dingl. Polyt. J. ccxviii.), on the influence of various solutions on the rusting of iron, are quoted. This author corroborates Calvert's report of the action of carbonic dioxide, and notes the fact that the presence of chlorides of magnesium, ammonium, sodium, potassium, barium, and calcium in water largely increases the production of rust, the action of chloride of magnesium alone being increased by heat.

These facts correspond with that observed by J. Gamgee, that lime solutions used as media of congelation in ice-making corrode the pipes or channels which convey them.

Stingl's valuable contribution to this subject, viz., his paper on the effects of condensed water containing grease on boilers fed with it (Dingler, Polyt. J. ccxv.), is quoted at some length.

This author proves that grease, with a small quantity of salts of lime and magnesia, at a temperature not exceeding 60° to 70° Cent., forms lime-soap, which, under the influence of a higher temperature, partially decomposes into free fat acid and a basic lime-soap, which adheres to the boiler-surfaces, the free acid, which is usually oxalic acid, attacking and dissolving the iron. In the crust the fat is recognized by the addition of hydrochloric acid, the separated organic mass being afterwards shaken with ether.

Even with lime and magnesia salts present in very insignificant proportion, the presence of grease is injurious, as, with saponification, under considerable pressure, a small quantity of lime suffices to occasion the splitting up of a neutral fat into free fat acid and glycerine. With low pressure the same action proceeds more gradually.

Various cases of corrosion from greasy water are noticed by this author, and in particular that of a steam-boiler of Cornish design, into which the condensed steam from two engines (of 300 and 100 horse-power) was fed. This boiler was constructed of steel; and after only three weeks firing was leaking in the fire-tubes. A deposit was found adhering to the upper part of the tubes, of which the analysis is given. The water in the boiler had a milky appearance, which was at once removed by ether. Ether is recommended as a good qualitative test for the presence of grease in water.

The analysis of the condensed feed-water is given, and the various operations in testing the deposit from it also recorded.

Means were adopted to purify this water by precipitation of the calcium carbonate and part of the magnesium carbonate along with the grease, which was carried down with the precipitate, and by subsequent filtering; and the analysis of the purified water is given. The boiler afterwards worked for three months with this water without any bad results, a pure deposit, consisting principally of magnesium hydrate and calcium carbonate and sulphate, being found to a small extent on the surfaces of the boiler.

Finally a letter addressed by the author to 'Engineering' (Oct. 1874) is referred to, in order to call attention to the difference between pure natural waters and genuine distilled water, i. e. distilled water free from air. The difference consists in the presence of gases in all natural waters. The distilled water from sur

face condensers of steamers necessarily contains some air, and it is therefore not "genuine distilled water."

Examples of boilers subject to corrosion are classed under the heads :

1. Land boilers using natural fresh water; and

2. Marine boilers.

1. Loch-Katrine water, from its great purity, affords the best opportunity of studying the effect of pure natural water on boilers. The former water-supply of Glasgow having been calcareous, the boilers using it became coated with lime, and did not suffer in consequence when afterwards supplied from Loch Katrine. In cases where the lime coating was removed corrosion quickly set in, and new boilers working with Loch-Katrine water from the first were rapidly destroyed. Several examples illustrating these points are quoted, and the remedy adopted is described. This was the formation of an artificial coating of lime by feeding a whitewash for some time into the boilers.

Analysis (by Dr. Mills) of Loch-Katrine water is given; and by reference to the investigations of Calvert and Wagner its action on iron is explained.

2. Marine Boilers.-Those using exclusively fresh water are cited, viz. Rowan and Horton's and Perkins's, to illustrate the kind of corrosive action known under these circumstances. The author's letter to Engineering' gives the remedies used in the case of Rowan and Horton's boilers.

·

Another instance of a coasting steamer using nearly all fresh water in her boilers, which, however, were destroyed by corrosion, is quoted. This instance was communicated to the Graduate section of the Institute of Engineers in Scotland by Mr. Jas. Gilchrist. It was found by two chemists that the decomposition of iron in her boilers was caused by the use of tallow. The author points out that the chemists did not make allowance for the presence of a small quantity of sea-water in the boilers, and its decomposition setting free hydrochloric acid.

The description of corrosion given by Mr. Miller in his paper communicated to the Cleveland Iron-Trade Foremen's Association is quoted, as this author enters fully into the matter, and describes two examples which well illustrate the general practice of the day in marine engineering. His deductions from the circumstances of these two examples are combated; and the author proceeds to show that corrosion in marine boilers, where a proportion of sea-water is used, is due to decomposition of the magnesium chloride of the sea-water, and to the liberation of the carbonic acid held in solution by repeated boiling.

The popular error that corrosion is due to some change produced in the constitution of water by redistillation is pointed out, as is the fact that in no case of marine practice has distilled water, pure and simple, ever been present so that its effects might be examined.

The author proposes as a remedy the coating of all new boilers with calcium sulphate and magnesium hydrate artificially, and thereafter the exclusive use of fresh water, which does not dissolve such a coating.

On an Apparatus for cleaning Filtering-Sand.
By JOHN LANG, C.E., Kirkcaldy.

The sand is tipped from wheelbarrows into a box, in the under part of which there is a diaphragm pierced with many small holes, through which a supply of water under pressure is introduced. The sand is agitated by the current, and the mud and water flow over the top of the box. When the water flows over clear, a door in the side is opened, the clean water is discharged into wheelbarrows below and is conveyed to the filter. The size of the apparatus depends altogether on the magnitude of the supply of water, and its success depends on the size being adapted to the supply. From very many experiments with various sands, the best conditions were found to be that the water should pass through the box with a velocity of from 3 feet 9 inches to 4 feet per minute, and that the box should be 27 inches in height. This apparatus, as used in the Kirkcaldy and Dysart Waterworks, had been found, in respect to thoroughness and in economy, to be very greatly superior to the former machines. It is able speedily to wash fresh pit

sand, or to rewash the sand forming the body of the filter; but it was explained that it was unable to wash the impurities from the filter-scrapings. Neither the old machines nor any mechanical means even in the laboratory are able to do this. By careful experiments, samples of the mud on the surface of the Kirkcaldy filters were obtained separate from the underlying sand; and it was found that 100 parts of the mud consisted of about 95 parts of diatoms, 4 parts of animalcules, and 1 part of inorganic matter, beside the sarcoid matter of the diatoms from which the offensive smell of the mud is derived. The only way to recover the sand from these scrapings is to allow them to lie exposed to the air for some years, until the sarcoid matter is decomposed. The flinty valves of the diatoms may then be removed by washing. A portion of the mud passes below the surface into the body of the filtering-sand, and in course of years is spread through its interstices and reaches even to the bottom. This mud consists almost wholly of the frustules of two minute kinds of diatoms, Orthosira and Cymbella; and by means of the microscope, used sand may be at once distinguished from fresh sand by the presence of these. They are easily removed by washing.

On a Pneumatic Tramway Car. By W. D. SCOTT-MONCREIFF.

On an Elevating Steam Ferry. By WM. SIMONS.

The object of this vessel is to supersede the present inclined approaches or slips to ferry stations, and therefore lessen the wear and tear in horses and haulage, to enable a greater traffic to be conducted with greater dispatch and economy and on the same level as the adjoining quays. The valuable ground required for slips is unnecessary, and the ferry-steamer is not confined to a special berth or locality. To effect the above objects, it is proposed to construct a steamer with a centre platform of sufficient capacity for the traffic, and capable of being elevated and lowered to suit the rise and fall of the tide, and thus enable the vessel to receive (level with the adjoining quays) waggons, goods, horses, carriages, and passengers.

On the Brake Problem. By JAMES STEEL.

On Communications between Passengers and Guards in Railway Trains. By W. STROUDley.

On Naval Signalling. By Sir W. THOMSON, F.R.S.

On Steam-Ship Resistance. By J. EVELYN WILLIAMS.

INDEX I.

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REPORTS ON THE STATE OF SCIENCE.