Faraday to Joseph Denman   27 May 1854¹

Royal Institution | 27 May 1854

My dear Sir

Your letter is full of interest and I feel great delight that any conversation in which I had part should be connected with so just an application of the principles of natural philosophy as has been made by His Royal Highness Prince Albert in the cases of the paddle wheel & the propeller²[.]

You will be aware from the communication of his Royal Highness that all practical result may be referred to the following facts. A disc when rotating, resists any force tending to alter its place so as to change the plane of its rotation far more than if the disc were not rotating: and the resistance is the greater as the body is heavier;- as the parts have greater velocity or momentum & therefore as they are further from the axis of rotation, and as the change of plane is greater. Now the force of the paddle wheels & their positions in relation to a steam ship are such that they cannot but affect its rolling; and their tendency will be to diminish it. You will understand that the endeavour is not to preserve any particular plane as regards the horizon, but that in which the disturbing force finds the rotating disc; so if a wave causes the vessel to roll, the revolving bodies will tend to resist this roll; as the vessel endeavours to recover itself the tendency will be to resist the recovery also; but on the whole, the rolling will be obstructed and diminished. I have always considered that paddle wheels resist and diminish rolling by the hold the descending side takes (like a hand in swimming) upon the water; but I have not the slightest doubt, now, that they will act also by the effect His Royal Highness has pointed out. What the proportion may be I cannot say; or to what extent the weight of 70 tons disposed in forms about 32 feet in diameter & revolving once in 2 seconds would affect a ship of 2300 tons. But I should expect it would be very appreciable, & should not be surprised if it may form a considerable part of any superiority which paddle wheels have over screws.

The screw you refer to, though it would revolve with twice the velocity of the paddle wheels has only half their diameter & a third of their weight; so that it would present much less resistance to change of plane than the latter. Besides this it is at the extremity of the vessel; and therefore perhaps 6 or 8 times as far from the horizontal transverse line about which the ship tends to revolve when pitching, as the paddle wheels are from the horizontal longitudinal line about which the ship tends to revolve when rolling;- for the short motions of the roll will be much more resisted than the long motion of the pitch, because the place of rotation in the first case, is more quickly changed. I do not think that the screw would tend to increase rolling otherwise than as it would replace the paddle wheels which tend to diminish it.

The suggestion of his Royal Highness in regard to a central fly wheel is highly philosophic & perfectly justified by natural principles[.] At the same time I cannot undertake to say what amount of effect it would produce in any given case. Still the experiment could be made so simply and progressively that I think any marine engineer could ascertain the point practically in a very few days.

Suppose a boat with a heavy disc or fly wheel fitted up in the middle this being attached by running bands to an axle & handles in the fore or aft part so that a man (or two men if needful) could get the fly into rapid rotation; the boat being of such size that a third person standing across or from side to side could by the action of his limbs sway her right & left. He might do this when the fly is still, and also when in quick motion. He would soon find the resistance to his efforts in the latter case, and then a judgment might be formed as to the result of a larger experiment & as to the application to a ship. If more convenient two fly wheels might be used one on each side of the boat and the gear & men be in the middle; but the first experiment ought to be made with a boat that can be easily & quickly rocked or the results will not be so instructive as they might be.

Though I have spoken thus far of a disc revolving in a vertical plane yet it is of course evident that a horizontal or any other plane may be selected provided that the axis of rotation is perpendicular to the length of the boat.

Supposing that a great disc or fly wheel were revolving in the inside of a vessel parallel to & in the same direction as the paddle wheels, and a wave were to affect the vessel, rolling her so as to depress the Starboard side, the resistance set up by the disc would not be direct, but would have an oblique result tending to turn the ships head to Starboard. Has any thing of this kind been distinguished by the man at the wheel? Probably he could not tell it from the effect due to immersion of the Starboard Paddle wheel:- in the boat experiment it ought to be sensible.

I am | My dear Sir | Very Truly Yours | M. Faraday

Captn. Jos Denman | &c &c &c