This section is from the book "The Boy Mechanic Vol. 2 1000 Things for Boys to Do", by Popular Mechanics Co.. Also available from Amazon: The Boy Mechanic, Vol2: 1000 Things for Boys to Do.
A reliable letter scale that can be easily made is shown in the sketch. It consists of a wide-neck bottle filled with water into which the weighing device is inserted. This latter part is made of a light piece of wood weighted on the lower end, to keep it in a stable, upright position, and a piece of cardboard is tacked to the other. The wood is placed in the water, and known weights are used on the cardboard while calibrating.
The first line is marked at the water level when there is no weight on the cardboard, and then a known weight placed on the top and another mark made at the water level, and so on, until a sufficient number of 1/2 -oz. and ounce-divisions have been marked. The wood should be well coated with shellac varnish before it is placed in the water. - Contributed by Francis Chetlain, Chicago.