limb of the animal 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 strips of that material into pieces of such a breadth that when bent round they just enclosed the lace.  Two pieces of steel have recently been fixed to the side of the shears, by which each piece of tinned-iron as soon as it is cut is bent into a semi-cylindrical form.  The additional power required for this operation is almost imperceptible, and it is executed by the same motion of the arm which produces the cut.  The work is usually performed by women and children; and with the improved tool more than three times the quantity of tags is produced in a given time.(1*)

35.  Whenever the work is itself light, it becomes necessary, in order to economize time, to increase the velocity.  Twisting the fibres of wool by the fingers would be a most tedious operation:  in the common spinning-wheel the velocity of the foot is moderate, but by a very simple contrivance that of the thread is most rapid.  A piece of catgut passing round a large wheel, and then round a small spindle, effects this change.  This contrivance is common to a multitude of machines, some of them very simple.  In large shops for the retail of ribands, it is necessary at short intervals to ‘take stock’, that is, to measure and rewind every piece of riband, an operation which, even with this mode of shortening it, is sufficiently tiresome, but without it would be almost impossible from its expense.  The small balls of sewing cotton, so cheap and so beautifully wound, are formed by a machine on the same principle, and but a few steps more complicated.