This Instrument (Fig. 1.) usually made of Brass or Wood, consists of four Rulers; the longest one AB being about 20 Inches in Length, so fastened together at the Places A, F, D, E, that the Parallelogram AFDE may vary it’s Species, while the Sides remain the same, that is, that any two adjacent Sides may make all variety of Angles from 180 to no Degrees. Upon the opposite Sides, AE, DF are fixed two sliding Sockets *e*, *f*; in these Sockets likewise goes the Ruler GC, a little longer than AB; these Sockets may be fastened to any parts of the Sides AE, FD, by means of Screws underneath, and the Ruler GC may be fastened in them by two Screws a top, so as to be always parallel to the Sides AF, ED; which opposite Sides AF, ED are each in Length equal to EB. But the Sides AE, FD, are each a little longer, *viz.* by twice the width of the Sockets, so that when the Sockets are shoved home to the Side AF, the Distance from the Middle of the Socket *e*, to the Middle of the End E of the Ruler DE may be equal to the Middle of the End E from the Point B: and the Length of the Ruler GC is such, that when the Point C is drawn out as far as possible, so that the Point L near the Middle of it falls upon the Middle of the Side DF; the Part CL is equal to the Part EB, and the three Points C, D, B, are in the same streight Line, which they must always be, when the Instrument is fit for use. Upon each of the opposite Sides AE, FD, are three Scales of Divisions into 100 Parts, beginning at the Width of the Socket’s Distance from the farther Edge of the Side DE, and ending at the same Distance on the hither Side of the opposite Side FA; the outermost Scale of Divisions is of 100 equal Parts, the middlemost is made by extracting the Square Root of each of the Numbers between 1 and 100, and multiplying that Root by 10, as to find against what division of the equal Parts 25 must stand on the middle Scale. It’s Square Root 5 multiplied by 10, is 50; wherefore 25 must stand against 50 on the equal Parts, and so of others. Just after the same manner is the inner Scale made, *viz.* by extracting the Cube Root of each Number between 1 and 100, and multiplying the Root by 100. The same Scales of Divisions are likewise upon the part, or half CL, of the long Rule GC, beginning near the Point C. The first of these Divisions of equal Parts are for fixing the long Rule CG, and the Sockets, so as to readily describe a Figure, whose Circuit shall have a given Proportion to the Circuit of a given similar Figure. The second, for the Description of a Figure whose Area shall have a given Proportion, to the Area of a given similar Figure. And the third, for the Description of the representation of a Solid that shall have a given Proportion to a given Solid.

D*d* is a turned Brass Pillar about \(\frac{3}{4}\) of an Inch long, with a Worm at the End to go into a very even Table; the Sides FD, DE of the Parallelogram are fastened to the Top D of this, so as to be any ways moveable horizontally. And at the Ends B, C, are equal round Holes wherein are put a Steel Point C*c* for moving over every part of a given Figure or Draught, and a Pencil Point B*b* for describing a Figure or Draught similar to a given one. Lastly, The turned Brass Pillar A*a* has a free horizontal Motion about the Point A, and a little Wheel *a* upon the Bottom of it to run upon the Table, like one of the Legs of a Child’s *Go-Cart*. The Lengths of these Pillars and the two Points C*c*, B*b* must be such, that in all Positions of the Parallelogram the Points *d*, *c*, *b*, *a*, may be in one Plane, *viz.* upon the Surface of the Table, or of a Paper laid upon it.

The Use of this Instrument is contained in the Description, sufficiently to be apprehended by any intelligent Person.