MANUFACTURE OF BOILERS.

89

cotters to the T-irons. With boilers having hemispherical ends it isquite evident that no stays are required. Flat fire-boxes (see page 88)for general purposes, and similar in construction to the locomotivetype, should be well stayed; the inside fire-box can be made circularat the top, and for moderate pressure the flat sides are only stayedto the inside fire-box, with inch screwed stays, rivetted on the Out-side, the distance between the squares being 6 j 4 inches for 50 lbs.per square inch, to 4^ inches for 100 lbs. steam pressure per squareinch. The plates are all flanged at the corners, but the fire-box issometimes united to the outer casing, with angle-iron at the bottom.The short flange pipe, to form the furnace-door case, strongly staysthe fire-box at that part.

As we consider that this subject closely afifects the interests ofsteam users, we append the following from a report presented tothe National Boiler Insurance Company:—“A large number ofthe boilers proposed for insurance are so weak in constructionthat some general remarks, based on the extensive experience ofthe construction and working of all kinds of steam boilers, willdoubtless be found useful to many owners and makers. Of thenumerous varieties none are more generally used than the Lanca-shire, or the cylindrical two-flued, and the Cornish one-flued boilers,and where these are well constructed, properly fitted up, and care-fully attended to, their performance is generally satisfactory. Thereare various modifications of these forms, some of which are valuable.In designing such boilers excessive length, as compared with the dia-meter, should be avoided. Long boilers strain considerably, andfrequently give great trouble by leakage at the rivetted seams. A fairProportion is when the length is about three-and-a-half times the dia-meter. The staying of the end-plates, and the attachment of the flue-tubes to the ends, should be so arranged that the tube may expandfreely, unless there be some special arrangement in the form of theflue-tubes to attain the same object. Many boilers, otherwise wellmade, have given considerable trouble by leakage and fracture, owingto the severe strains of unequal expansion to which their rigid con-struction exposed them. In some of the boilers inspected the endswere so heavily stayed, and so rigid, that considerable leakage andoccasional fracture at the ring seams of the lower part resulted. Inothers the staying was so slight that the ends were bulged outwards,and serious risk of explosion thus occurred. Flue-tubes should neverbe stayed to the shell, but be attached at the ends only. Many boilers