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3. Another agent, used for the creation of momentum, is the gravitating power of Water. This is used only as a stationary agent. Its advantages are, that it is cheap, tolerably constant, and frequently, is capable of exerting great mechanical force. Its disadvantages are, that it is stationary ; that is, that it can be used only in situations where it has been created by nature. Hence, it is frequently at a considerable distance from the seaports whence the manufacturer derives his supplies, and whence he exports his products. In such cases, the cost of transportation must be deducted from the profits of the establishment, and is of course, to this amount, a diminution of their value.

Water cannot always be commanded in sufficient quantity. Very few mill-seats are secure from the liability to suffer from the want of water. This is a great inconvenience, inasmuch as, in seasons of drought, a large number of the laborers must be unemployed, and a large portion of the expenses of the establishment must be incurred, without yielding any remuneration to the proprietor.

Another disadvantage of water power is, that it is liable to danger from inundation. Though this may be guarded against, in many cases; yet, it frequently can be done only at an expense which greatly reduces the cheapness of the agent. Notwithstanding these disadvantages, water power will probably be always used, where great mechanical force is required ; where the machinery to be employed is simple, and where the operation does not require the greatest possible nicety of execution.

4. The power, however, most commonly in use at present, is Steam. Its advantages are, that it can be used to create any required degree of mechanical

that it is perfectly under human control; that it may be created in any place where fuel can be obtained; that it can be used at will, either as a stationary, or a locomotive power; and that it can be made to act with perfect regularity. Its only disadvantage,

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is its expensiveness. The machinery by which it is generated is costly, and requires frequent repairs; and the fuel, by which it is maintained, is a very serious item of consumption. The price of engines, however, will be gradually reduced, as the demand for them increases. And it is probable, that, by im

, provement in their construction, the consumption of fuel will be greatly diminished; while increased facilities for transportation will materially reduce its price. The introduction of steam power has greatly reduced the price of fuel in Great Britain.

The question whether steam or water power should be used in any particular case, is, I suppose, to be decided by their relative expensiveness. This will be decided, principally, by the place in which the power may be required. Water power will generally be the cheaper where it can be procured in abundance, and sufficiently near to a market or to tide water. But where it is variable in quantity, or is at a considerable distance from the place of delivery, the cost of transportation will frequently overbalance its other advantages, and render steam power the more economical. Machinery, propelled by steam, can be erected and carried on upon a wharf, or in the midst of a city; and hence it avoids all the cost of unnecessary transportation. Machinery, propelled by water power, can be erected only at the place where the water power exists, and, of course, is subject to all the expense of transportation between that place and the market.

The ADVANTAGES of inanimate over animate natural agents, are several.

1. Inanimate agents can, within a small compass, and with comparatively little weight, produce a vastly greater amount of momentum, than animate agents. Thus, a steam engine, of one hundred and fifty or two hundred horse power, occupies but a small space, and forms but a small part of the cargo of a vessel.

But so great a number of horses could scarcely be carried in any vessel designed to

transport either freight or passengers; and, besides, no mechanical arrangement has yet been devised, by which such a number of animals could conveniently be employed upon one operation.

2. They are continuous ; that is, they are never liable to fatigue, and never need rest. Animals must spend the greater part of their time in feeding or in repose. Specially is this the case, if they are work

. ed rapidly. During this time, the labor which they perform must either be suspended, or else other animals must take their place. A horse cannot labor severely for more than eight hours in twenty-four. Hence, if the uninterrupted labor of horses were required for twenty-four hours, three relays must be provided. Thus, if a boat were required to perform a voyage in twenty-four hours, she must employ three relays of horses; that is, a steam boat, worked by a power equal to that of one hundred and fifty horses, would require four hundred and fifty horses, in order to create the necessary momentum.

3. Hence, there is a great gain in Economy. The first cost of inanimate is generally less than that of animate agents; they are liable to no diseases; they require no food, and create expense only while they are performing their work. Were the labor now performed by steam, to be performed by horses, the price of the ordinary necessaries of life would be quadrupled, and many articles of ordinary use would be placed out of the reach of any but the most opulent. Nor is this all. The substitution of inanimate for animate power, has a great tendency to reduce the cost or to increase the supply of all agricultural products. Suppose that, by the use of steam, one thousand horses can be dispensed with. A horse requires for sustenance, throughout the year, as much agricultural produce as would support eight men. If, then, these one thousand horses can be dispensed with, there may

be produced, on the land which was formerly employed for the production of hay, as much grain as will support eight thousand men. This must, at first, re

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duce the price of grain; and the result would be, that the district would support eight thousand more men than before.

4. There is also, commonly, a gain in personal safety. Inanimate agents act under laws which may be known and obeyed, and of which the results may be commonly foreseen and guarded against. Animals are endowed with passions and will, which we can frequently neither control nor influence. Besides, the greater expensiveness of the individual machines employed in the use of inanimate agents, renders it for the interest of the proprietor, to employ men of experience and responsibility to manage them. This very sensibly diminishes the risk. When we reflect upon the vast amount of traveling by steamboats and railroads, it must be evident, that, notwithstanding the accidents to which they are liable, a vastly greater amount of human life would be sacrificed, if the same number of persons were transported by horses. It is also to be remembered, that the use of steam is yet in its infancy, and that greater experience and skill will materially reduce the number of accidents to which this mode of conveyance is at

present liable.

5. Inanimate agents can be used without the infliction of pain. Inanimate agents are insensible. Where the labor to be accomplished is either severe, or where it requires great speed, animals must be rapidly consumed. This exposes them to great suffering. A horse in a stage coach can rarely travel, rapidly, more than ten miles a day; and most horses will endure even this labor but for a short time. From this suffering inanimate power is exempt. It never endures pain from being over driven.

6. Animate power decreases with velocity. Hence, we must soon arrive at a point beyond which it can no further be used to create momentum. resent the tractive force of a horse, when moving at two miles an hour, at 100, his force, at the rate of three miles, will be 81; at the rate of four miles,

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64; at the rate of five miles, 49; at the rate of six miles, 36; while at the top of his speed, he can carry nothing more than his own weight. An engine, on the contrary, may be made to work as powerfully at one degree of velocity as at another. In all cases, therefore, in which both great power and great velocity are required, inanimate power must, of necessity, be employed.

From these causes, we see that inanimate is rapidly taking the place of animate power, both where stationary and where locomotive force is required. By the additional speed which it is capable of producing, it gives rise to great economy of time. This, to all persons engaged in active employments, is a consideration of vast moment. Being a continuous agent, it is also enabled to act with the greatest certainty. Hence, men may adjust their transactions, in different places, with entire precision. This is also another source of economy, both of time and of capital. And, besides, notwithstanding the expensiveness of the arrangements for the use of locomotive forces, yet the amount of additional traveling, to which they give rise, is so great, that the expensiveness of transportation between different places is, in general, materially diminished.

II. Of the natural agents by which momentum is

It is obvious, that a great addition is made to human power, where the agents for creating momentum have been discovered. But this is not all. Several combinations of matter may be formed, by which mere human force may be greatly assisted, and which, by being united with the agents for creating momentum, may greatly increase, and vary, and give adaptation to, its utility. These are called the mechanical powers, which are treated of at large in works on Mechanics and Natural Philosophy. In their simple form, they are the lever, the wheel and axle, the inclined plane, the screw, the pulley, and the wedge. They are variously combin



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