Model Steam Locomotives
neglect of the induced draught of the exhaust,
was the cause of all the early failures,
(7) Reduction in the number of joints, and, in the case of copper loco-fype boilers especially, good fitting ; and
(8) Equal strength in all paris, with a factor of safety varying from 6 to 10.
Types of Boilers.—Mode! locomotive boilers may be divided into four types: (a) ihe plain cylindrical pot-boiler, (b) the water-tube boiler which was invented by Mr. F. Smithies in 1900 and with the early development of which the writer had much to do, (c) the loco-type boiler, and (ad) the flash steam generator.
Outside-fired Boilers. — With regard to the first lype, supplementing the remarks already made as to the necessity for extreme lightness, it is essential that such boilers should be heated over their whole length, and that the barrel should be pitched as high as possible above the level of the rails, otherwise there will not be sufficient height for the flame of the lamp. The vaporising lamp {this lamp does not work well under a fierce induced draught, therefore difficulties may occur if it is applied to internally fired engines) is perhaps the best of all spirit lamps for this purpose. This question of height is the one difficulty in making seale working models in No. 0 (14 in.) gauge. Abnormal types of locomotives are therefore usually chosen as protelypes in this gauge.
One of the most satisfactory arrangements of plain boiler is that shown in Fig. 54. A tank engine with long tanks is adopted, and the latter are used to conceal the flame of fhe lamp, which, of course, burns under natural draught. One or two water tubes may be added, but to allow this material fo be used all joints should be silver soldered.50
