producing the action. In driving a nail into a piece of wood, one
of these is lifting the hammer, and propelling its head against
the nail; the other is, raising the arm itself, and moving it in
order to use the hammer. If the weight of the hammer is
considerable, the former part will cause the greatest portion of
the exertion. If the hammer is light, the exertion of raising the
arm will produce the greatest part of the fatigue. It does
therefore happen, that operations requiring very trifling force,
if frequently repeated, will tire more effectually than more
laborious work. There is also a degree of rapidity beyond which
the action of the muscles cannot be pressed.

33. The most advantageous load for a porter who carries wood
up stairs on his shoulders, has been investigated by M. Coulomb;
but he found from experiment that a man walking up stairs without
any load, and raising his burden by means of his own weight in
descending, could do as much work in one day, as four men
employed in the ordinary way with the most favourable load.

34. The proportion between the velocity with which men or
animals move, and the weights they carry, is a matter of
considerable importance, particularly in military affairs. It is
also of great importance for the economy of labour, to adjust the
weight of that part of the animal's body which is moved, the
weight of the tool it urges, and the frequency of repetition of
these efforts, so as to produce the greatest effect. An instance
of the saving of time by making the same motion of the arm
execute two operations instead of one, occurs in the simple art
of making the tags of bootlaces: these tags are formed out of
very thin, tinned, sheet-iron, and were formerly cut out of long