126 OF THE GENERATION AND [SECT. in. 



Also the bottom surface multiplied by the side surface = 60 x 59 = 3540 ; 

 which divided by (2 x 146*4) 60 = 232-8 is 



!U 15 feet nearly, 



for one dimension. 

 Consequently, 



-4 feet, 



15 



for the other dimension ; or the boiler should be 15 feet long and 4 feet wide. 



226. If the capacity of the top for steam be the same as that for water, 

 and the form a semi-cylinder, the whole depth of the boiler may be found with 

 sufficient accuracy for practice, by making it twice the depth of the water added to 

 one-tenth of that depth; in the example it will be (2 x 2*44) + -244 = 5-124 feet. 



The proportions given by the rule are different from those commonly used, not 

 much in capacity, but considerably in extent of surface for receiving heat, and in 

 having greater length and less width. Boilers of such proportions are undoubtedly 

 stronger as well as more effective. 



227. Cylindrical Boilers. Cylindrical boilers, with the ends rather flat 

 segments of spheres, should always be used for the production of strong or high 

 pressure steam ; and even for low pressure steam this form seems best. See 

 Plate n. Many schemes have been suggested for using combinations of cylinders 

 or tubes ; but it is extremely questionable whether any plans have been suggested 

 superior to a simple cylinder with convex ends, and applying as many of these as 

 are necessary for the object. 



228. Sometimes the cylinder forming the boiler has the fire wholly within it ; 

 and in consequence of this arrangement it is impossible to get surface for the fire to 

 act on, unless the boiler be of such diameter as to render it extremely dangerous. 

 The immense waste of fuel is shown by the experiments of Mr. Wood, (art. 208.) 

 and yet these boilers have a diameter of 4 feet, with a pressure of 4 atmospheres, 

 tending to separate the parts of the boiler with a force exceeding 140 tons, and 

 only a rude safety valve to limit the steam to this force. 



229. RULE for Cylindrical Boilers. When a fire is applied externally to a 

 cylinder which is to contain both water and steam, let the capacity for water and 

 for steam be added together, and also the quantities of fire surface ; then divide 

 twice the capacity by the quantity of fire surface, and the result will be the 

 diameter. Also 1 '27 times the capacity divided by the square of the diameter will 

 be the length. 



Example I. Let the proportions of a high pressure boiler be determined, so that 



