M. Girard on the Resistance of Cast-Iron. 273 



the same time at the two extremities of its diameter. The re- 

 sultant of the forces producing rupture is in the direction of a 

 i-adius perpendicular to that diameter, precisely as if the pipe 

 were fixed parallel to its axis, and loaded at the lower part 

 with a weight sufficient to overcome the cohesion which retains 

 the one half to the other at the plane of the horizontal dia- 

 meter. 



To calculate the thickness of metal, we ought to give cast- 

 iron pipes, so that they may be sufficient to resist a given strain, 

 the cohesive force of cast-iron; for an unit of surface should be 

 known. This datum Mr. Girard takes from the Experiments 

 of Mr. G. Rennie, after remarking that Captain S. Brown's 

 and Mr. Rennie's are the only ones made on the direct cohe- 

 sion. 



From this datum Mr. Girard calculates that a pipe of one 

 metre (39*371 inches) diameter should be about half a milli- 

 metre (0*01968 mches) thick to sustain a pressure of a column 

 equal to the weight of the atmosphere. And the thickness be- 

 ing proportional to the force, the pressure of a column equiva- 

 lent to 100 atmospheres would require a thickness of 50 milli- 

 metres (1 '968 inches), the diameter being the same. Applying 

 these calculations to an experiment made by Mariotte, he in- 

 fers that solder has only about 1-1 5th of the cohesive force 

 of iron. 



He also infers that the cast-iron pipes which serve to raise, 

 in one jet, the water fi-om the Seine to the top of the mountain 

 of Marly, nearly 500 pieds m height, — consequently the strain at 

 the lower partis equal to about 15 atmospheres, — have a super- 

 abundant degree of strength, their diameter being 8 pouces, 

 and thickness 6 or 7 lines*. 



In applying these principles of theory and data of experiencef 

 to determine tlie resistance of a cylindrical boiler of cast-iron, 

 of one metre diameter (39*371 inches) and 15 lines (1^ inches) 

 in thickness, we find, says Mr. Girard, that resistance equiva- 

 lent to the pressure of 67 atmospheres; that is to say, a pressure 



* If pipes were ex|)ose(l only to the pressure of the head of water, their 

 strength might be adjusted l)y Mr. Girard's rule, provided the strain was li- 

 mited by tlie force which the material would resist without permanent al- 

 teration of its structure. But the greatest strain generally arises frojn the 

 recoil of the column when the pioving force ceases to act on it, which even 

 an air vessel is not siifiicient to reduce in any considerable degree. And ho- 

 rizontal pipes are exposed to other strains which it is as necessary to provide 

 for as the pressure of water. 



f The data necessary for this iiKpiiry have not been directly obtained 

 from cx|)eriment; for it is the combined etf'ect of heat and pressure which 

 overcomes the cohesive power of the metal of a boiler. Mr. Girard has 

 not noticed the eflbct of heat ; an approximatt! idea of this effect has been 

 yiven by Dr. Young in his Lectures, Vol. i. p. (J4.'J. 



Vol.61. No. 300. y//^/// Ib'^.'i. Mm 10 



