Where straight-line depreciation is spoken of it is assumed that the worth of a machine depreciates an equal part of its first cost each year. For example, if the life of a plant is assumed to be 20 years, straight-line depreciation will assume that the plant loses 1/20 of its original value annually. The present worth of a plant under this assumption would be the product of its first cost and the ratio between its remaining life and its total life. This method of estimating depreciation and worth is frequently used, particularly for short-lived plants and for simplicity in bookkeeping, but it is less logical than the method given above. =89. Number and Capacity of Pumping Units.=—In order to select the number and capacity of pumping units for the best economy, a comparison of the costs of different combinations of units should be made and the most economical combination determined by trial. The principles outlined in the preceding articles should be observed in making these comparisons. In a steam pumping station, when the number of units operating is less than the average daily maximum for the period, steam must nevertheless be kept on a sufficient number of boilers to operate the maximum number of pumps. This, and corresponding standby losses must not be overlooked, as they may show that a smaller number of larger units is ultimately more economical. TABLE 33 SUMMARY OF FLUCTUATIONS OF SEWAGE FLOW AT A PROPOSED PUMPING STATION ─────────────────┬─────────────────┬─────────────────┬───────────────── Number of Days │Flow in Thousand │ │ Loads Occurred in│ Gallons per │ │ One Year │ Minute │ Lift in Feet │ Horse-power ─────────────────┼─────────────────┼─────────────────┼───────────────── 1│ 293│ 6.0│ 450 8│ 163│ 8.6│ 354 15│ 119│ 10.0│ 300 18│ 106│ 10.6│ 284 23│ 88│ 11.2│ 249 31│ 69│ 12.2│ 211 32│ 65│ 12.4│ 204 45│ 51│ 13.4│ 173 41│ 50│ 13.5│ 169 30│ 45│ 13.8│ 158 28│ 44│ 13.9│ 154 23│ 40│ 14.2│ 143 21│ 38│ 14.4│ 137 18│ 35│ 14.6│ 129 12│ 29│ 15.0│ 111 8│ 24│ 15.6│ 95