Abstract
WHEN a careful engineer sets about designing a structure, he first determines the strain which the several parts of it will have to withstand; he then selects his material and proportions it so that it will be able to carry the strains safely; in determining the margin which should be allowed he uses judgment based upon his own experience or that of others who have designed similar structures; and if the strains be difficult to determine, or if they be of the nature of severe and sudden shocks and complicated cross strains he increases his margin in proportion. Experience has shown, for example, that wrought iron in the form of a railway bridge may be worked safely at a load equal to one-fourth of that which will break it down, a cast-iron bridge to one-sixth. The builder of a steam-engine rarely loads those parts of his machinery which have to endure sudden and reverse strains to more than one-eighth or one-tenth of their ultimate strength. If his structure fail, the first thing the engineer does is to re-calculate his strains and the dimensions he has given to the various parts, and if these should prove correct he seeks for the cause of failure in unlooked-for defects in his material; and if failure in the same class of structure, of various sizes, recur repeatedly in the same place, he comes to the conclusion, either that he has under-estimated the strains, or that the margin of safety which he has allowed is not sufficient. An engineer accustomed to act in this manner must look with dismay upon the report of the Committee appointed to examine into the cause of the failure of the 12-inch gun of the Callingwood, and of other guns of similar construction. It is possible, of course, that the Committee have been able to calculate the strains which tend to destroy the guns, and have satisfied themselves that sufficient metal has been provided for the purpose; but, if so, it is much to be regretted that they have not seen fit to make the results of their investigations public, because it would have been instructive to know how the stresses are arrived at and what margin of safety is considered sufficient for a gun. When Colonel Maitland read his paper on our new guns at the United Service Institution in the middle of 1884, the strains certainly were not known to the Ordnance Committee, because the pressure curves, purporting to represent one-fourth of the bursting pressure of the guns, and which were given on the official drawings, were incompetent to account even for the muzzle energy which the shot was supposed to possess, and consequently provided nothing whatever for other important work which has to be performed during the discharge. This fact was pointed out by the Engineer early in 1885, in an article commenting on one of the Howard Lectures which had just been delivered at the Society of Arts, and the weak point in our guns was actually indicated before any failures had taken place!
Article PDF
Rights and permissions
About this article
Cite this article
Our Guns . Nature 34, 517–518 (1886). https://doi.org/10.1038/034517a0
Issue Date:
DOI: https://doi.org/10.1038/034517a0