PARALLELISM IN FURNITURE.

25

Let us now examine the application of the propertiesjust pointed out to the construction and movement of bo-dies.

Having a body of an invariable form, ABCD, fig. 10,pi. 2, let us suppose that it is moved forward in such amanner, that all its points situated in the right line A/impmove in the direction of A mnpa : every other point, B, or

C, or D, of the figure ABCD will also move along aright line, B6, C c, D d, parallel to A a. In fact the bodynot changing its form during the motion, each point, B, C,

D, always remains at the same distance from the right lineA a ; consequently it describes a right line parallel toA mnpa.

This geometrical property is very often made use of byworkmen of all descriptions.

The movement of drawers in their respective frames is onthis principle. The drawers, fig. 11, pi- z, of tables, bu-reaus, cupboards, or other furniture, are guided in theirmovement by a frame, of which the right-angled joiningsrepresent so many parallel right lines, A a, ]ib, D d, C c.When the drawer is pushed in or drawn out, if the pieceof furniture is well made, that is to say if the parallelismin every part is exactly preserved, the drawer is adjustedat every point, as it moves backwards and forwards, and isno where impeded in its motion. The parallels beingalways comprised between the same parallels, and beingin consequence equal, represent the distance of the variouspoints of such a drawer in its various positions.

Artisans have frequently occasion to draw a line in themiddle, between two other lines, and parallel to them : forthis purpose, as well as for many others, they will find thefollowing instrument, the invention of Mr. Palmer, ofgreat use.

In fiff. 12, pi. 2, A is a square bar of hard wood, having the twosliding cheeks B D, fixed tight to it. The cheek B is fixed on one endof the bar, while the cheek D slides upon it, but may be madefast at any required point by the thumb-screw C. At the cheekB, a common scribing point is fixed in the bar, and with this, and