232 REPORT— 1876. 



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 tbe heads : — 



1 . Land boilers using natm-al fresh water ; and 



2. Marine boilers. 



1. Loch-Katrine water, from its gi'eat pmity, 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 fi-om 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-Katiine water is given ; and by reference to the 

 investigations of Calvert and Wsgn^r its action on iron is explained. 



2. Marine Boilers. — Those using exclusively fresh water ai'e cited, viz. Rowan 

 and Horton's and Perkins's, to illustrate the kind of corrosive action known imder 

 these circumstances. The author's letter to ' Engineering' gives the remedies used 

 in the case of Kowan 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 com- 

 municated 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 ii'ou 

 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 hydi-ochloric 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 circum- 

 stances 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 de- 

 composition of the magnesium chloride of the sea-water, and to the liberation of 

 the carbonic acid held in solution by repeated boUiug. 



The popular error that corrosion is due to some change produced in the constitu- 

 tion 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 efiects 

 might be examined. 



The author proposes as a remedy the coating of all new boilers with calcium 

 sulphate and magnesium hydrate artificiallj', 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., Kirlccaldy. 



The sand is tipped from wheelbarrows into a box, in the under part of which 

 there is a diaphi-agm pierced with many small holes, through which a supply of 

 water under pressure is introduced. The sand is agitated by the cm-rent, and the 

 mud and water flow over the top of the box. When the water Hows 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 

 mag-nitude 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 con- 

 ditions 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 iuclies in height. This apparatus, as used in the Kirkcaldy and Dysart Water- 

 v/orks, had been found, in respect to tlioroughness and in economy, to be very 

 greatly superior to the former machines. It is able speedily to wash fresh pit- 



