exception that some makers adopt girders, having otherwise a low factor of safety, it is disregarded in the rules for determining the safe load. There are various formulæ by which the number and proportions of girder stays may be determined, but they are for the most part stated in such a manner as to be of little service to many of those interested in the construction and safety of steam boilers. With a view to simplifying these, the following rules have been deduced, and they may be relied upon to give results that will compare favourably with the best practice. Referring to Figs. 36, 37, 38, and 39 it will be seen that the letters used to denote the various dimensions are as here given-Let Then P = Working pressure in lbs. per square inch. L= Length of girder in inches = length of firebox. H = Half length B = Breadth of girder in inches, or if formed of two plates, take the sum of the two. D= Depth of girder in inches at centre. S = Distance between centres of girders. C = Constant, 7,500 for iron, and 10,000 for steel. C x B x D2 P= HX LX S D = HxLxSxP and Bone-fourth to one-fifth of the depth. The diameters and pitch of the bolts which transmit the load to the girders are determined by the rules already given for screwed stays. Taking the dimensions as given, and using constant for iron The disadvantages referred to as attending the use of girder stays, have induced several locomotive builders to adopt a mode of staying between the crown plates of firebox and outer casing, somewhat similar to that employed in staying the flat sides and end plates. In some instances bolt stays have been screwed through both plates with the heads at firebox side, the ends through casing being fitted with nuts or rivetted over. With casing crown plates, however, of the usual circular form, this method of staying is very rigid, and offers great resistance to the expansion of the firebox; it is also defective, inasmuch as the stays do not take the leads direct, and are thus exposed to bending stresses which must lead to rapid wear and tear. Figs. 40 and 41 show a modification of this form of staying, designed to overcome these objections. A series of channel irons are rivetted to the casing roof, and to these swing brackets suitable for receiving the stay bolts are jointed. The stays are screwed through the firebox plate in the usual manner, and the ends through the brackets are fitted with nuts. Provision is made for the upward expansion of the firebox, and the stays are admirably adapted for supporting the crown plate under pressure; the slight deflection that will occur owing to the difference of expansion between the firebox and the casing being immaterial. The casing roof is doubtless very much stiffened by the channel irons to which the stays are attached, and the altered conditions may in time develop "wear and tear" at parts hitherto comparatively free from it. The first cost and the difficulty of renewing or repairing the stays are also points which require consideration, but the improved circulation and facilities for cleaning which the arrangement possesses over girder staying will probably make its adoption very general, particularly in the case of large boilers. BURSTING PRESSURES OF CYLINDRICAL BOILERS. 81 Figs. 42 and 43 represent a mode of constructing locomotive fireboxes, which admits of crown plates being stayed directly to outer casing without disadvantage. It will be seen that the crown plates of both outer and inner boxes are quite flat and of about equal area; the side plates of casing being connected to the roof by bends of large radii. Unequal expansion between the boxes is accommodated by the springing of the plates in such a manner as to prevent serious straining. The easy bends at corners of casing will also be serviceable in this respect, and under ordinarily favourable conditions they are not likely to be affected by grooving. This design for the many small boilers of "loco" type possesses great advantage over girder staying, particularly when the feed-water is bad; it is also much simpler and less expensive than other good forms of direct staying. ་ (4.) BURSTING PRESSURES OF CYLINDRICAL BOILERS. The cylindrical, next to the spherical, is the form that offers the greatest resistance to rupture, and as cylindrical boilers are not only simpler to construct, but are better adapted in other respects to suit the requirements of steam generators, this form has naturally been most generally adopted. The resistance of a cylindrical shell to rupture is equal to the tensile strength of the material multiplied by twice its thickness, and the force sufficient to overcome the resistance is the product of the pressure in pounds per square inch multiplied by the diameter in inches. Thus, a boiler 60 inches in diameter, constructed of plates inch thick, having a tensile strength of 40,000 pounds per square inch of |