~Iain
A log is a piece of wood. That is the origin of "log" meaning "journal".

It is nautical in origin.
A ship's log was a device for measuring the speed of a ship through the water. The log was a "a thin quadrant of wood, loaded so as to float upright in the water, and fastened to a line wound on a reel". It was dropped over the side of the ship. It would float in a fixed position in relation to the sea while the ship moved through the sea. The line was allowed to unreel. The length of line that was unreeled in a particular time was a measure of the speed of the ship through the water.
Measurements of the ship's speed were recorded in a "log-book". "Log-book" was, and is, abbreviated to "log".
The meaning of log-book has been extended to a record of things other than a ship's speed.

Peter Duncanson, UK
(in alt.english.usage)
The line was allowed to unreel. The length of line that was unreeled in a particular time was a measure of the speed of the ship through the water.

And the line was knotted at fixed intervals. The length of line paid out was measured by counting the number of knots that went over the rail. Hence "knot" as a measure of speed.

John Varela
Trade NEW lamps for OLD for email.