Front Page Titles (by Subject) Limits to the Intensity of Labour. - The Theory of Political Economy
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Limits to the Intensity of Labour. - William Stanley Jevons, The Theory of Political Economy 
The Theory of Political Economy (London: Macmillan, 1888) 3rd ed.
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Limits to the Intensity of Labour.
I have mentioned (p. 170) that labour may vary either in duration or intensity, but have yet paid little attention to the latter circumstance. We may approximately measure the intensity of labour by the amount of physical force undergone in a certain time, although it is the pain attending that exertion of force which is the all-important element in Economics. Interesting laws have been or may be detected connecting the amount of work done with the intensity of labour. Even where these laws have not been ascertained, long experience has led men, by a sort of unconscious process of experimentation and inductive reasoning, to select that rate of work which is most advantageous.
Let us take such a simple kind of work as digging. A spade may be made of any size, and if the same number of strokes be made in the hour, the requisite exertion will vary nearly as the cube of the length of the blade. If the spade be small the fatigue will be slight, but the work done will also be slight. A very large spade, on the other hand, will do a great quantity of work at each stroke, but the fatigue will be so great that the labourer cannot long continue at his work. Accordingly, a certain medium-sized spade is adopted, which does not overtax a labourer and prevent him doing a full day's work, but enables him to accomplish as much as possible. The size of a spade should depend partly upon the tenacity and weight of the material, and partly upon the strength of the labourer. It may be observed that, in excavating stiff clay, navvies use a small strong spade; for ordinary garden purposes a larger spade is employed; for shovelling loose sand or coals a broad capacious shovel is used; and a still larger instrument is employed for removing corn, malt, or any loose light powder.
In most cases of muscular exertion the weight of the body or of some limb is of great importance. If a man be employed to carry a single letter, he really moves a weight of say a hundred and sixty pounds for the purpose of conveying a letter weighing perhaps half an ounce. There will be no appreciable increase of labour if he carries twenty letters, so that his efficiency will be multiplied twenty times. A hundred letters would probably prove a slight burden, but there would still be a vast gain in the work done. It is obvious, however, that we might go on loading a postman with letters until the fatigue became excessive; the maximum useful result would be obtained with the largest load which does not severely fatigue the man, and trial soon decides the weight with considerable accuracy.
The most favourable load for a porter was investigated by Coulomb, and he found that most work could be done by a man walking upstairs without any load, and raising his burden by means of his own weight in descending. A man could thus raise four times as much in a day as by carrying bags on his back with the most favourable load. This great difference doubtless arises from the muscles being perfectly adapted to raising the human body, whereas any additional weight throws irregular or undue stress upon them. Charles Babbage, also, in his admirable Economy of Manufactures, has remarked on this subject, and has pointed out that the weight of some limb of the body is an element in all calculations of human labour.
"The fatigue produced on the muscles of the human frame," says Babbage, "does not altogether depend on the actual force employed in each effort, but partly on the frequency with which it is exerted. The exertion necessary to accomplish every operation consists of two parts: one of these is the expenditure of force which is necessary to drive the tool or instrument; and the other is the effort required for the motion of some 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."1
It occurred to me, some time since, that this was a subject admitting of interesting inquiry, and I tried to determine, by several series of experiments, the relation between the amount of work done by certain muscles and the rate of fatigue. One series consisted in holding weights varying from one pound to eighteen pounds in the hand while the arm was stretched out at its full length. The trials were two hundred and thirty-eight in number, and were made at intervals of at least one hour, so that the fatigue of one trial should not derange the next. The average number of seconds during which each weight could be sustained was found to be as follows:—
If the arm had been thus employed in any kind of useful work, we should have estimated the useful effect by the product of the weight sustained and the time. The results would be as follows, in pounds- seconds:—
The maximum of useful effect would here appear to be about seven pounds, which is about the weight usually chosen for dumb-bells and other gymnastic instruments. Details of the other series of experiments are described in an article in Nature (30th June 1870, vol. ii. p. 158).
I undertook these experiments as a mere illustration of the mode in which some of the laws forming the physical basis of Economics might be ascertained. I was unaware that Professor S. Haughton had already, by experiment, arrived at a theory of muscular action, communicated to the Royal Society in 1862. I was gratified to find that my entirely independent results proved to be in striking agreement with his principles, as was pointed out by Professor Haughton in two articles in Nature.1
I am not aware that any exact experiments upon walking or marching have been made, but, as Professor Haughton has remarked to me, they might easily be carried out in the movements of an army. It would only be necessary, on each march which is carried up to the limits of endurance, to register the time and distance passed over. Had we a determination of the exact relations of time, space, and fatigue, it would be possible to solve many interesting problems. For instance, if one person has to overtake another, what should be their comparative rates of walking? Assuming the fatigue to increase as the square of the velocity multiplied by the time, we easily obtain an exact solution, showing that the total fatigue will be least when one person walks twice as quickly as he whom he wishes to overtake.
In different cases of muscular exertion we shall find different problems to solve. The most advantageous rate of marching will greatly depend upon whether the loss of time or the fatigue is the most important. To march at the rate of four miles an hour would soon occasion enormous fatigue, and could only be resorted to under circumstances of great urgency. The distance passed over would bear a much higher ratio to the fatigue at the rate of three, or even two and a half miles an hour. But, if the speed were still further reduced, a loss of strength would again arise, owing to that expended in merely sustaining the body, as distinguished from that of moving it forward.
The Economics of Labour will constantly involve questions of this kind. When a work has to be completed in a brief space of time, workmen may be incited by unusual reward to do far more than their usual amount of work; but so high a rate would not be profitable in other circumstances. The fatigue always rapidly increases when the speed of work passes a certain point, so that the extra result is far more costly in reality. In a regular and constant employment the greatest result will always be gained by such a rate as allows a workman each day, or each week at the most, to recover all fatigue and recommence with an undiminished store of energy.
THEORY OF RENT
[]Babbage, On the Economy of Machinery and Manufactures, sec. 32, p. 30.
[]Vol. ii. p. 324; vol. iii. p. 289. See also Haughton's Principles of Animal Mechanics, 1873, pp. 444-450. The subject has since been followed up with much care and ability by Professor Francis E. Nipher, of the Washington University, St. Louis, Missouri, U.S. Details of his experiments will be found in the American Journal of Science, vol. ix. pp. 130-137; vol. x., etc.; Nature, vol. xi. pp. 256, 276, etc.