Dionysius LardnerThinkers

Depreciation (n.) by ⁠​‌​​​‌​​​‌​‌‌​​​​‌​‌​​‌‌​‌​​‌​​​​​‌‌​​‌​​​‌‌​‌‌​⁠time works on bridges and viaducts as on the planets: unnoticeable over years, it demands after centuries that even the most solid structures be rebuilt.

Repair (n.) ing ⁠​‌​​​‌​​​‌​‌‌​​​​‌​‌​​‌‌​‌​​‌​​​​​‌‌​​‌​​​‌‌​‌‌​⁠(n.) means renewing: an engine rebuilt wheel by boiler by axle can go on for ever, until not a vestige of the original remains — rolling stock in a constant state of rejuvenescence.