This simple piece of apparatus is useful for testing the verticalness of walls, posts and other parts of structures.
It can be quickly made out of a 6-foot length of floorboard, planed smooth all over and given quite straight and parallel edges. A line is marked with the gauge down the centre from end to end, and blackened with graphite or Indian ink so that it can be seen easily.
Eight inches from what is to be the bottom edge a pear-shaped hole for a plumb-bob is cut in the centre. At the top end three fairly wide saw-cuts are sunk into the end, to a depth of half an inch or so – one in the centre line, and one on each side of this. These are for the plumb cord, which is passed through the centre cut and wound round through the others.
Just below the cuts, on the face, a transverse slip of hard, smooth wood, J-inch thick, is nailed, to hold the cord clear of the faco when this is perfectly vertical; and just above the bob-hole is a loop of string or leather, or a wire stirrup, for the cord to pass through and restrain the movements of the bob.
The last should preferably be of brass, and have a fine point, the tip of which will be on the centre line when the plumb-rule is vertical laterally.
If a spirit-level is let into one edge, the rule will serve also for levellinj.
Plywood. This consists of three or more veneers of wood (usually birch) glued together with their grains running in different directions. . It may be bought in thicknesses ranging from jfc inch up to 1 inch, according to the number of plies. Though not entirely exempt from warping, especially if thin, plywood warps less than ordinary wood, and, owing to its construction, does not tend to warp in any particular direction.
The crossing of the grains prevents splitting, and makes it much stronger than ordinary wood of the same thickness. The glue used – its composition is generally a trade secret – is very resistant of damp, and the wood may be soaked in water for many hours without separation of the plies. Thin plywood is used widely for fretwork, since its toughness enables very delicate designs to be cut without fear of breakage. It comes in very handy also for the backing of pictures, and the making of toys, dolls-houses, etc. Plywood ½ inch or ½ inch thick is excellent for the tops of tables, and for drawing boards. The material is specially useful in large sizes, as sheets measuring several feet both ways can be bought in any thickness, and the troublesome business of making gluod joints is thus avoided.
Care has to be taken in planing up the corners, in order to avoid pulling out edges of plywood. The plane blade must splinters of some of the plies, planing be very sharp, as it is always cutting should always be done from the corner some plies across the grain; and at the inwards.