From Leonard Darwin to Horace Darwin [20 October 1872]

Dear Jim.

The srew machine is as follows. There is a clock or small steam engine (which ever answers best) that turns a wheel at the rate the srew would go if the ship went say 20 miles an hour., just a little quicker than it would ever go through the water. There is a similar wheel worked by the screw shaft; now these two wheels work a wheel just the same as in George’s machine, making it go one way when one is faster, the other way when the other is faster. When the wheel goes one way a connection is made and by electro magnet the valve is shut

Thus far the whole idea is mine own. In order to make this connection there are two inventions one mine; and one Georges   Georges letter shews his   It is very good by I feel mine is better.

All these wheels are loose on the axles

[DIAG HERE]

A is the wheel that turns differently when the speed is greater to less than the given one.

B is a light wheel that is turned only in one direction by the lower one— this being done by steel spring and catch, or [DIAG HERE] by the silent feed.

Now the axle a is glass and the projection b is glass. This lower wheel B is fastened to a heavier wheel C by the spring at b. Now the working is as follows. When B begins to move (in the only direction it can) the spring b is compressed and the two nuts at d brought in contact and the circuit completed. When the wheel leaves off going round, and goes the least thing back, the spring at b presses the two apart and the circuit is broken and steam let on again. Those things on the top are breaks to make certain that the spring b shall be compressed to make contact.—and to prevent the wheel getting up momentum.

In Georges plan there would be more wear and tear and less certain for he must have the heads of the pins quite covered with the noconductor or else the spring would catch it coming over and make the [DIAG HERE] connection when going the wrong way.; and if it were going to quick the spring would jump from nonconductor to nonconductor and make no connection.

This might be obscured

Which do you think best?

L Darwin