ON THE STRENGTH OF LOCOMOTIVE BOILERS. 63 



The mathematical theory would lead us to expect that the strength of the 

 plates would be inversely as the surfaces between the stays ; but a comparison 

 of the results of these experiments shows that the strength decreases in a 

 higher ratio than the increase'of space between the stays. Thus, according 

 to the mathematical theory, we should have — 



Ult. strength 2nd plate per sq. in. = strength 1st plate Xff 



=815xf|=1273 lbs. 



Now this plate sustained 1 625 lbs. per square inch, showing an excess of 

 about one-fourth above that indicated by the law. 



This is in excess of the force required to strip the screw of a stay -j-^ths 

 of an inch in diameter, such as those which formed the support of the flat sur- 

 faces in the exploded boiler. 



It will be found that a close analogy exists throughout the whole experi- 

 ments, as respects the strengths of the stays when screwed into the plates, 

 whether of copper or iron ; and that the riveting of the ends of the stays 

 adds to their retaining powers an increased strength of nearly 14 per cent, to 

 that which the simple screw affords. The difference between a fire-box stay 

 when simply screwed into the plate and when riveted at the ends is there- 

 fore in the ratio of 100 : 76, nearly the same as shown by experiment in 

 the Appendix. 



It is desirable, therefore, that we should ascer- Fig. 6. 



tain the strain exerted on each stay or bolt of 

 the fire-box. 



Let A, B, C, D, E, F represent the ends of the i d C 



bolts or stays ; Oj, Og, O3, O4 the centres of the O O O 



squares formed by the bolts. Suppose a pressure o^ °q 



to be applied at each of the points Oj, O^, O3, O4 ^ * r^ 



equal to the whole pressure on each of the ^^ ^ ^ 



squares, then the central bolt A will sustain one- '^ "^ 



fourth of the pressure applied at Op also one- ^ ^ ^ „ 



fourth of the pressure applied at O^, and so on ; ^ j, j, 



so that the whole pressure on A will be equal to 

 the pressure applied to one of the square sur- 

 faces. Hence we have — 



Strain on the stav of Table III.= ^^^^^^ =9 tons. 



2240 



Strain on the stay of Table IV. = ^625x16 __^^^ ^^^^ nearly. 

 •' 2240 '' 



The stay in the latter case was yl^'^^ ^^ ^" '^^^^ ''° diameter ; hence the 

 strain upon one square of section would be about 13 tons, which is con- 

 siderably within the limits of rupture of wrought iron under a tensile force. 



In the experiments here referred to, it must be borne in mind that they 

 were made on plates and stays at a temperature not exceeding 50° of Fahr- 

 enheit ; and the question naturally occurs, as to what would be the differ- 

 ence of strength under the influence of a greatly increased temperature in 

 the water surrounding the fire-box, and that of the incandescent fuel acting 

 upon the opposite surface of the plates. 



This is a question not easily answered, as we have no experimental facts 

 sufficiently accurate to refer to ; and the difference of temperature of the 

 furnace on one side, as compared with that of the water on the other, in- 

 creases the difficulty, and renders any investigation exceedingly unsatisfactory. 



