[Sidenote: Depreciation of manufacturing appliances] There is a great difference in the length of life of manufacturing appliances. The building is fairly durable; yet an average depreciation-rate of one and one half per cent. a year must be allowed to offset a reduction in its value of over fifty per cent, in thirty years. Machinery differs greatly in durability; well-made, substantial machinery depreciates about five per cent. yearly. The engines and boilers depreciate more rapidly than the running gear; the loose tools have to be replaced every second to fourth year; while the materials consumed in the industry must be repaired and replaced at every repetition of the process of manufacture. If a factory is to be maintained in its efficiency in accordance with the terms of the renting contract, and is to continue its renting power, everything about it must be from time to time repaired and replaced. [Sidenote: Neglect of repairs often has evil effects] 4. _Neglect or postponement of repairs must cause a falling off of the rent-earning power._ The neglect of repairs may have different results in the factory. The neglect of one kind simply reduces present rental while not preventing the future restoration of the plant to its full efficiency. If certain necessary tools wear out and are not replaced, the factory as a whole will be less efficient. Each part of the entire outfit being needed in due proportion, the loss in rental will correspond not merely to the lost efficiency of the missing tools, but to the crippled efficiency of the remaining appliances. Failure to apply seed to the land causes the land as a whole to be useless for that year's crop. In other cases, neglect of repairs increases the expenses of repairs and cuts off future rental. The adages, "A stitch in time saves nine," and "An ounce of prevention is worth a pound of cure," must be acted upon in every industry. The neglect to repair a roof causes damage to an amount many times the cost of a new roof. Failure to replace a bolt costing five cents may result in the rack and ruin of a machine worth many dollars. A handful of earth on a dike may save a whole country from destruction. [Sidenote: But sometimes is economical] Neglect of repairs may be economical, however, when outer conditions have first reduced the demand for the agent and consequently the rental. When the line of travel changes, it does not pay to keep an old hotel up to the same state of repair as when it had a great patronage. Old factories sometimes may better be allowed to depreciate while the price of repairs is invested in more prosperous industries. In a declining neighborhood the houses fall into decay, the owners seeing that "it would not pay" to keep them up. § II. DEPRECIATION IN RENT-EARNING POWER OF AGENTS KEPT IN REPAIR [Sidenote: Repairs can not always prevent ultimate decay of agents]