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MODERN STEAM PRACTICE.

any method whereby the boiler could be lightened, and yet keptefficient, would be of great value for mercantile purposes. It isshown that Prof. Rankine and others estimate that a factor of safetyof eight is necessary for such a live load as steam. The authorhowever, States, “ Now experiments show conclusively that up to atemperature considerably exceeding that at which it is practicableto use steam, wrought-iron does not lose strength; and as no partof a properly designed boiler is subjected to a temperature muchgreater than that of the steam within it, without being speciallystrengthened, there does not appear to be any reason for this greatdifference of factor of safety. The Manchester Steam Users’ Asso-ciation, founded by Fairbairn for the prevention of boiler explosions,consider that where boilers are well built and carefully examinedperiodically a factor of safety of four is sufficient, and the correctnessof these views is shown by the freedom from accidents in boilersguaranteed by them; but of course we are not warranted in con-cluding from this that the same factor would be sufficient for marineboilers, which often cannot be subject to the same careful andsystematic examinations as land boilers. The old-fashioned boxboiler working at from IO to 30 pounds had only a factor of aboutfour, and yet the accidents which have happened with this low factorof safety were quite as few in proportion to the number of boilersin use as with the higher factor of six, which is about the presentpractice of the country, although at the same time improved appli-ances have enabled boiler-makers to make better and more reliablework than formerly. But although the present factor of safety isnominally six in many boilers which are at present at work, thereare parts which, either from oversight or want of knowledge on thepart of their designers, are very much weaker than the other parts,and which considerably reduce the actual factor of safety. Yet wefind that these specially weak parts are often quite strong enoughfor their work, for even after many years’ Service they do not showany signs of weakness. If these parts are strong enough, then un-doubtedly the extra strength of the remainder of the boiler has beenso much useless weight.”

On the question of proportioning the strength of boiler, the effectof expansion is pointed out as an important agent in the tear andwear of a boiler. “ There are certain strains which boilers are sub-ject to which are, under certain conditions, much greater than anywhich the working pressure can bring upon them, and which are