EDWARD We are all aware, that engines for dividing straight lines, are of much less importance than those for graduating cir cles and arcs; yet we should not think that justice was done to our article, were we to omit an account of one for the former purpose.
Encouraged by his former success, and having in pros pect farther rewards, of which he was not disappointed, Dlr Ramsden constructed the first right-line engine, an account of which was published in 1779, by order of the Board of Longitude. Of this, the following is a copy, ac companied by reduced drawings of the original figures.
Experience having evinced the great utility of the en gine for graduating circles, it encouraged me to attempt a similar method, whereby lines of equal parts, sines, tan gents, secants, Exc. might be divided with equal ease and correctness.
By the engine hereafter described, any line of equal parts, &c. may be divided without an error of the part of an inch ; and, as this can be done by any indifferent person, and so very expeditiously, its uses for dividing all sorts of navigation scales, sectors, &c. must be obvious ; especially when it is considered, that from the incorrect ness of the present method of dividing, these valuable in struments are of less use than they might be.
This engine consists of a strong plate of brass, movea ble on two edges of an iron frame. To facilitate its motion, the friction is considerably diminished by the application of three rollers to the under side of the plate : the iron frame is supported on a strong mahogany stand.
One edge of the brass plate is ratched or cut into teeth, of which there are exactly twenty in an inch, and it is mov ed along the iron frame by an endless screw, having exactly the same number of threads in an inch ; these threads fit into the teeth on the brass plate. Each revolution of the endless screw round its axis will move the plate of an inch along the iron frame.
A small wheel is fixed on one end of the screw, having its circumference divided into 50 parts, which are again subdivided into five parts by a vernier ; therefore, when the screw is turned on its axis, one of the primary divisions, the plate will be moved of an inch along the iron frame ; if the screw be turned to the coincidence of one division on the vernier, the plate will be moved of an inch, and so of the rest ; and the line on the plate to be divided, which terminates the spaces moved by the brass plate, may be drawn on it, or on any instrument fastened on the plate, with the greatest accuracy, by a point or tracer fixed in a proper frame, whereby it has a rectilinear motion, any lateral shape.
Sometimes it may be necessary to lay down lines on in struments which are not commensurable with English inches ; such as are the feet, inches, Exc. of most other countries : this is done by inclining the line to be divided to make an angle with the direction of motion of the plate, by an apparatus to be described hereafter ; if the tracer be set to draw lines at right angles to the direction of mo tion, or to the side of the plate, then the line to be divided will be as much longer than the space the plate has moved, as the secant of the angle of inclination is longer than the radius ; but if the tracer be set to draw lines at right angles to the line to be divided, then divisions on that line will be short er than the space the plate has moved along the iron frame, as much as the cosine of the angle of inclination is shorter than the radius.
(Plate CCLXXX.) Fig. 1. Represents a plan of the di viding engine.
Fig. 2. An elevation.
Fig. 3. A section on the line AB.
Fig. 4. A section on the line DE.
Fig. 5. The underside of the plate A, represented in Fig. 1.
Able. Like parts are marked with the same letter in each of the Figures.
(Fig. 1.) A represents a strong brass plate, 27 inches long, four inches broad, and of an inch thick ; work ed exceeding flat, and of the same thickness throughout, with its two edges exactly parallel.