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For loading and unloading carts and waggons, the convenience of the old outside sling is well known; it is also a well-attested fact that accidents rarely happen with sling hoists, although they appear to be less safe than running platforms or lifts. As a general rule, the most dangerous machinery for handling or raising material is that which pretends to dispense with the care and vigilance of attendants, and the safest machinery that which enforces such attention. The condition which leads to danger in hoisting machinery is, that the power employed is opposed to the force of gravity, and as the force of gravity is acting continually, it is always ready to take advantage of the least cessation in the opposing force employed, and thus drag away the weight for which the two forces are contending; as a weight when under the influence of gravity is moved  at an accelerated velocity, if gravity becomes the master, the result is generally a serious accident. Lifting may be considered a case wherein the contrivances of man are brought to bear in overcoming or opposing a natural force; the imperfect force of the machinery is liable to accident or interruption, but gravity never fails to act. Acting on every piece of matter in proportion to its weight must be some force opposing and equal to that of gravity; for example, a piece of iron lying on a bench is opposed by the bench and held in resistance to gravity, and to move this piece of iron we have to substitute some opposing force, like that of the hands or lifting mechanism, to overcome gravity.