210 APPENDIX. 



SAFETY VALVE. 

 7. To find the aperture of a safety valve, suited to any given pressure. 



RULE. Divide the area of the fire surface by the excess of the pressure above the atmo- 

 sphere, expressed in tbs. per square inch, and the quotient will be the square of the diameter 

 of the narrowest part of the valve in inches. (142.) 



Example. Required the aperture of a safety valve for a boiler with 60 feet of fire surface, 



the pressure being 5 tbs. per square inch above the pressure of the atmosphere. 

 The feet of fire surface (60) divided by the tbs. per square inch (5) gives 12 for the square 

 of the diameter. Hence, by finding the square root, the diameter of the narrowest aperture 

 of the valve should be 3*46, or about 3 \ inches. The aperture should be made rather in excess 

 than defect. 



STEAM PASSAGES. 







8. To find the diameter of the steam pipe. 



RULE. Multiply the length of the stroke in feet by the number of strokes per minute, and 

 divide the product by 6 ; take the square root of the quotient, multiply it by the diameter of 

 the cylinder, and then divide by 20. The result will be the diameter of the pipe in inches. 

 (95.) 



Example. To find the diameter for the steam pipe of an engine, of which the diameter of 

 the cylinder is 2 feet, the length of stroke 2'5 feet, and the number of strokes per minute 38. 



Length of stroke 2'5 

 Number per minute 38 



200 

 75 



Divide by 6) 95-0 



16, nearly 



Square root 4 



Diameter cylinder 24 inches 



Divide by 20) 96 



4'8 inches. 



The diameter of the pipe should therefore be about 5 inches. 

 The same rule applies to the steam passages, and the passages to the condenser. 



FIRE GRATING. 



9. To find the area of the grating. 



RULE. Divide twice the square root of the height, in feet, from the bottom of the ash pit 

 to the bottom of the chimney by the said height, and multiply the quotient by the number of 

 horse power. The result will be the area of the grating expressed in square feet. (116.) 



