They are
equally true for men, animals, or machines; and are wholly independent of
the way in which the power is applied: whether, for instance, a man
carries his burden, or draws it, or rows or punts it in a boat, or winds
it up with a crank or tread-mill.
Travellers might well turn the theory to account on their own behalf;
they are well situated for testing its truthfulness, by observing the
practices of the countries in which they are travelling. Reliable facts
upon the extreme distances that can be travelled over, day after day, by
people carrying different loads, but equally circumstanced in every other
respect, would be very acceptable to me.
The formulae are as follow:--Let b be the burden which would just
suffice to prevent an animal from moving a step; d the distance he could
travel daily if unloaded. Also, let b1 be some burden less than b; and
let d1 be the distance to which he could travel daily when carrying b1.
Then b1 d2 = b(d-d1)2. (1)
Again, the "useful effect" is a maximum, if b1d1 is a maximum. When
this is the case, then
b1 = 4/9 b. (2)
And
3 d1 = d. (3)
In other words, an animal gets through most work in the day if he carries
4/9 of the greatest load he could just stagger under; in which case he
will be able to travel 1/3 of the distance he could walk if he carried no
load at all.
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