I have already observed that the intensity of muscular exertion, cannot be measured, like its duration, by any general scale or standard. When we wish to give any clear ideas of it, either positive or comparative, we are obliged to resort to the effect produced. The same, indeed, is the case in the mensuration of mechanical energies; as when to shew the operative force of a steam engine, we speak of a four-horse or a ten-horse power: the known effect of the one, serves to measure and define the force of the other. So when we say, that a man has carried so many stone weight, has walked or run so many miles in a given time, or has threshed out in a day so many bushels of corn, we may form just ideas, comparative ones at least, of the easiness or intensity of his labours, because we know how much other men usually carry, or walk, or thresh out, when they exert their strength in the same modes of action. But when the descriptions of human labour in question are not familiar to us, nor the effects produced by them commensurable with any known standard, even this resort is in great measure precluded.

It is evident, therefore, that in the present division of my work, the same simple modes of demonstration that I have resorted to in the preceding sections cannot have place, I cannot establish or refute general propositions as to intensity or ease, by computing and comparing the effect of particular admissions; because neither the one nor the other have any determinate or clearly definable meaning.