J094

THEORY AND PRACTICE OF ENGINEERING.

Book II.

Dynamic units.

A man carrying wood up a ladder, his own weight being 56, and that of hisload 53- - - - - - - - -109

A coal porter on a ladder, his own weight being taken into account - 112 to 120

A man applying his strength to a rope over a pulley to raise a ram - 75

The same—another experiment - - - - - - 40

The same — another experiment - - - 48

A man drawing water from a well by means of a cord - - - 71

A man working the handle of a winch - - - - -116

A man drawing a boat with traces - - - - 110

A unit of transport being the weight of a cube metre of water transported 1 metre on alevel road, we have —

Dynamic units.

A man travelling over a level plain, his own weight being 70 kilogrammesx 50 kilometres -------

A soldier loaded with 20 to 25 kilogrammes, and marching 20 kilometres f

a day -------

A Roman soldier making a forced march of 40 kilometres a day - - -I

Hawkers with crochets, the weight of the man not being taken into account \

Men drawing sledges - -

A man drawing a boat on a canal 50,000 kilogrammes transported 11kilometres - - - -

3500

1800

1900

4400

4800

792

880

627

s : —

Daily Labour mea-

- 550,000

sured by the draught

Useful

and the road goneover.

5600

3420

Effect.

_

585

1684

842

1185

595

2948

675

800

1560

Waggon horse -

Poste horse -

Mill horse -

Ditto -

Ditto - - -

Ditto -

Ditto -

Barge horse -

We see by the first summary that a man who mounts a ladder freely without any loadcan furnish a quantity of action double that which he affords with crochets, cords, pulleys,or winches.

In considering the useful effect of a wood porter in houses, it is expressed by the number56, which is only the quarter of 225, the measure of the first daily action : it results thatif a man mounts a staircase freely, and lets himself fall by means attached to the extremityof the cord of a pulley, he would elevate a weight equal to his own at the other extremityof the cord; he would produce nearly the same useful effect as four men lifting the sameweight on their backs or with crochets.

It will be conceived that a man could elevate a load by a ladder greater in proportion asthe weight is equally distributed over all the active parts of his body, but the weight ofthe man himself fulfils this condition to its greatest possible extent. This theory confirmsthe result of experiment on the best employment of the greatest daily labour a man iscapable of.

Applying the same reasoning to a horse, it is easy to calculate the useful effect producedby his mounting a slope. Let us suppose that he elevates himself in a day to 20 kilo-metres, his weight being about 250 kilogrammes, he would elevate the same weight to thesame height by letting himself fall, and bis daily labour would be expressed by 5000, anumber six times greater than that which expresses the daily labour of a horse drawing aboat, and which we generally regard as the mean action answering to a numberof horses. There would therefore be a great advantage in employing a horse as a weightcapable of lifting, even when we reduce the height to 10 kilometres, which he might mountin a day.

The Measure of the greatest Effort of an animated Mover _We have seen that the daily

labour of animated mover depends on three elements: the pressure which he exercises onthe resistance to be overcome; the velocity of the centre of application of the pressure ;and lastly, the duration of the pressure. There is such a relation between the two ele-ments pressure and velocity, that if one attain its maximum the other becomes nothing.The greatest velocities observed among men and horses are respectively 8 and 15 metresper second ; a man running with this velocity consumes in half a minute the labour he iscapable of in a day. The horse consumes the same in 7 or 8 minutes, and if he be