On the Means of generating Motion for Mechanical Uses
From the original document in the letter to Thomas Jefferson at the Library of Congress.
As the limit of the mechanical powers, properly so called, is fixed in nature no addition or improvement otherwise than in the application of them, can be made. To obtain a still greater quantity of power we must have recourse to the natural powers, and for usefulness combine them with the mechanical powers. Of this kind are Wind and Water, to which has since been added Steam. The two first cannot be generated at pleasure. We must take them where and when we find them. It is not so with the Steam Engine. It can be erected in any place, and act in all times, where a well can be dug and fuel can be obtained. Attempts have been made to apply this power to the purpose of transportation, as that of moving carriages on land and vessels on the water. The first I believe to be impracticable, because, I suppose, that the weight of the apparatus necessary to produce Steam is greater than the power of the Steam to remove that weight, and consequently that the Steam engine cannot move itself.
The thing wanted for purposes of this kind and if applicable to this may be applicable to many other, is something that contains the greatest quantity of power in the least quantity of weight and bulk, and we find this property in gun-powder. When I consider the wisdom of nature I must think, that she endowed matter with this extraordinary property for other purposes than that of destruction. Poisons are capable of other uses than that of killing.
If the power which an ounce of gunpowder contains could be detailed out as steam or water can be, it would be the most commodious natural power because of its small weight and little bulk, but gun-powder acts, as to its force, by explosion. - In most machinery operations the generating power is applied to produce a rotatory motion on a Wheel, and I think that gunpowder can be applied to this purpose. But as an ounce of gunpowder, or any other quantity, when on fire, cannot be detailed out so as to act with equal force through any given space of time, the substitute in this case is, to divide the gunpowder into a number of equal parts, and discharge them in equal spaces of time on the wheel, so as to keep it in nearly an equal and continual motion, as a boy’s whipping top is kept up by repeated floggings. Every separate stroke given to the top acts with the sureness of explosion, but produces, as to continual motion, the effect of uninterrupted power.
When a stream of water strikes on a water wheel it puts it in motion and continues it. Suppose the water removed and that discharges of gunpowder were made on the periphery of the wheel where the water strikes would they not produce the same effect?
I mention this merely for the simplicity of the case. But a wheel on which gunpowder is to act must be filled for that purpose. The buckets or boards placed on the periphery of a water wheel are the whole breadth of the stream of water; but the parts corresponding to them on a gunpowder wheel should be of Iron and concave like a cup, and of no larger size than to receive the whole of the explosion. The back of them should be convex or oval, because in that shape they meet with less resistance from the air. The barrels from which the discharges should be made, should, I think, be in the direction of a tangent with the cups. But if it should be found better to make the discharge on the solid periphery of the wheel, the barrels should be a tangent of a circle something less than the periphery of the wheel. A wheel put and continued in motion in this manner is represented by holding the axis of a wheel in one hand, and striking the periphery with the other.
If acting on the solid periphery of the wheel should be found preferable to acting on the Cups, the wheel should be shod with Iron, the edges should be turned up, and the middle part be fluted cross. By this means the explosion cannot well escape sideways and the fluting will be preferable to a plain surface.
That the power of any given quantity of gunpowder can be detailed out by this means to act through any given quantity of time, and that a wheel can be put and continued in motion thereby, there is, I think, no doubt. Whether it will answer profitably in practice is another question. But the experiment, I think, is worth making, and the more so because it appears one of the things in which a small experiment decides almost positively for a large one, which is not the case in many other small experiments. I think the wheel for a great work should be large, 30 or 40 feet diameter, because the explosions would give too much velocity to a small one, and because the larger the wheel is the longer the explosion would rest upon it and the motion will be less irregular.
The machine which it seems to come into competition with is the steam engine. In the first place, a steam engine is very expensive to erect. In this only a few Iron barrels are required. In a Steam engine the expence and consumption of fuel is great, and this is to be compared to the expence of gunpowder, with the advantage, that the interest of the money expended on erecting a steam engine goes towards the expence of the gunpowder.
A steam engine is subject to be out of order, and for this reason they frequently have two, that when one is repairing the other can supply its place, or all the works dependent upon it must stand still. But nothing of this kind can happen to the gunpowder engine, because if a barrel burst, which is all that can happen, its place can be immediately supplied by another; but if a boiler bursts there must be a new one. But I will not take up your time with calculations of this kind. The first thing to know is if the experiment will succeed.
If in your retirement from business you should be disposed to vary your mechanical amusements I wish you would try the effect of gunpowder on a wheel. I suppose on a wheel of two or three feet diameter the smallest bored pistol there is, about the size of a quail, would give it considerable velocity. The first experiment will be to observe how long it will revolve with one impulse, and then with two. If the wheel revolves perpendicularly, fast to its axis, and a cord be fastened to the axis with a weight to the end of the cord which, when the wheel is in motion, will wind on the axis and draw up the weight the force with which it revolves will be known.
Perhaps there may be some difficulty in starting a great wheel in motion at first, because gunpowder acts with a shock. In this case, might not gunpowder be mixed with some other material, such as is used to make sky rockets ascend, because this lessens the shock and prolongs the force. But I conceive that after the wheel is in motion, there will be scarcely any sensible shock from the gunpowder.
As it is always best to say nothing about new concepts till we know something of their effects I shall say nothing of this till I have the happiness to see you which I hope will not be long and which I anxiously wish for.
Thomas Jefferson President of the United States
Paris 25 June 1801