Sir Benjamin Baker says:--“Many engineers ignore the fact that a bar of iron may be broken in two ways--by a single application of a heavy stress, or by the repeated application of a comparatively light stress. An athlete’s muscles have often been likened to a bar of iron, but if ‘fatigue’ be in question, the simile is very wide of the truth. Intermittent action, the alternative pull and thrust of the rower, or of the laborer turning a winch, is what the muscle likes and the bar abhors. A long time ago Braithwaite correctly attributed the failure of girders, carrying a large brewery vat, to the vessel being sometimes full and sometimes empty, the repeated deflection, although imperceptibly slow and free from vibration, deteriorating the metal, until in the course of years it broke. These girders were of cast iron, but it was equally well known that wrought iron was similarly affected, for Nasmyth afterward called attention to the fact that the alternate strain in axles rendered them weak and brittle, and suggested annealing as a remedy, having found that an axle which would snap with one blow when worn, would bear eighteen blows when new or just after annealing. We know that the toughest wire can be broken if bent backward and forward at a sharp angle; perhaps only to locomotive and marine engineers does it appear that the same result will follow in time even when the bending is so slight as to be unseen by the eye. A locomotive crank-axle bends but 1/34 inch, and a straight driving axle but 1/64, under the heaviest bending stresses to which they are exposed, and yet their life is limited. Experience proves that a very moderate stress alternating from tension to compression, if repeated about a hundred million times, will cause fracture as surely as bending to a sharp angle repeated a few hundred times.” Hence an axle, or other structure, should be tested by just such stresses as it is to withstand in practice. A steel bar may satisfactorily pass a tensile test applied in one direction, only to break down disastrously under alternating stresses each less severe. Magnetization. That matter virtually remembers its impressions is plain when we study magnetism. Steel when magnetized for the first time does not behave as when magnetized afterward. It is as if magnetism at its first onset threw aside barriers which never again stood in its way. If the steel is to be brought to its original state it must be melted and recast, or raised to a white heat for a long time. In quite other fields of channeled motion we remark that violins take on a richer sonority with age; their fibres, under the player’s hand, seem to fall into such lines as better lend themselves to musical expression.