Dear Pat Re: >As far as I know power and energy were clearly distinguished as two quite >separate and distinct physical realities late in the 1700s or early in the >1800s (I would like to have an exact date but this is the best I can do at >present).
I am inclined to think that it must go back a bit earlier than that, at least to a point where the principal of conservation of energy or momentum was established. Isaac Newton effectively did that in his third law of motion (popularly known as "action and reaction are equal and opposite") in 1687. In that context power is a measure of the rate at which energy is converted from one form to another. I may be wrong but I find it hard to imagine Newton and his contemporaries not having some idea that energy is a quantifiable property of matter according to its state (Einstein later showed that matter and energy are interchangeable) and the concept of power as we now know it.
