Fig. 38.
Fig. 39.
Fig. 40.
Fig. 41.
The desire to economise fuel led to placing the fire inside the boiler, in a tube running from end to end, as shown in [Fig. 38], and the great number of boilers of this form used in Cornwall gave it the name of the Cornish boiler. The exceedingly good duty performed by these boilers led many to believe them the most perfect for economy and durability; but the great number of explosions, or more properly of collapsed flues, that have happened, have altered this opinion, and led to the double-flue boiler shown in [Fig. 39], in which not only is the heating surface increased but the strength also, by having two tubes of smaller diameter in the same shell. There are a great many varieties of the two-tube boiler, which have been made for the purpose of obtaining various particular results. In some cases the two tubes have been made to unite into a single tube immediately behind the fires, forming what is known as the Breeches-tube boiler, as shown in [Fig. 40], and in other instances the outside shell of the boiler has been made oval, as shown in [Fig. 41], with the two tubes continued through from end to end. The heating surface has also been increased, and the strength of the main tubes, by placing smaller transverse tubes across them at right angles; but these advantages are gained by increased complication, leading of course to greater difficulty in examination and repair.
