Mathematical Instruments
Book IV. Additions. Ch. III.

Of the Construction and Use of the Surveying-Wheel.

Fig. 8

This Instrument consists of a wooden Wheel, shod with Iron, to prevent it’s wearing, exactly two Feet seven Inches and a half in Diameter, that so it’s Circumference may be eight Feet three Inches, or half a Pole.

Fig. 9

At the End of the Axle-Tree of this Wheel, on the left Side thereof, is, at Right Angles to the Axle-Tree, a little Star, about three fourths of an Inch Diameter, having eight Teeth. Now the Use of this Star is such, that when the Wheel moves round, the said Star’s Teeth, by falling at Right Angles into the Teeth of another Star of eight Teeth, fixed at one End of an Iron Rod (Q) causes the Iron Rod to move once round in the same Time the Wheel hath moved once round. Therefore every time you have drove the Wheel half a Pole, the Iron Rod goes once round.

This Iron Rod, lying along a Groove in the Side of the Body of the Instrument, hath on the other End a square Hole, in which goes the square End b of the little Cylinder P. This Cylinder is fastened underneath the upper Plate H, of a Movement, covered with a Glass, placed in the Body of the Instrument at B, yet so, that it may be moveable about it’s Axis, having the End a cut into a single threaded perpetual Screw, which falling into the Teeth of the Wheel A, being thirty-two in Number, when you drive the Instrument forwards, causes the Wheel A to go once round at the End of each 16th Pole. The Pinion B hath six Teeth, which falling into the Teeth of the Wheel C, whose Number is sixty, causes that to move once round at the End of each 160th Pole, or half Mile. This Wheel carries round a Hand, once in 160 Poles, over the Divisions of an annular Plate, fixed upon the Plate H, whose outmost Limb is divided into 160 equal Parts, each tenth of which is numbered, and shews how many Poles the Instrument is drove.

Again; The Pinion D, which is fixed to the same Arbre as the Wheel C is, hath twenty Teeth, which by their falling into the Teeth of the Wheel E, which hath forty Teeth, causes the said Wheel E to go round once in 320 Poles, or one Mile; and the Pinion F, of twelve Teeth, falling into the Teeth of the Wheel G, whose Number is 72, causes the Wheel G to go once round in 12 Miles. This Wheel G carries another lesser Hand once round in 12 Miles, over the Divisions of the innermost Limb of the aforesaid annular Plate, which is divided into twelve equal Parts for Miles, and each Mile subdivided into Halves and Quarters (that is, into eight equal Parts, for Furlongs), with Roman Characters numbering the Miles.

The Use of this Instrument is such, that by driving the Wheel before you, the Number of Miles, Poles, or both, you have gone, is easily shewn by the two Hands. And so this Instrument, together with a Theodolite or Circumferentor, for taking of Bearings, is of excellent Use in Plotting of Roads, Rivers, &c. For having placed your Wheel and Circumferentor at the Beginning of the Road you design to plot, which call your first Station, cause some Person to go as far along the Road as you find it straight; and then take a Bearing to him, which set down. This being done, drive the Wheel before you to the Place where the Man stands, which call the second Station, and note, by the Hands of the Dial-Plate, the Distance from the first Station to the second, which set down. Again, Having placed your Circumferentor at the second Station, cause the Man to go along the Road ’till he comes to another Bend therein. And from the second Station take a Bearing to the Man at the third, which set down. Then drive the Wheel from the second Station to the third, and note the Distance, which set down. And in this Manner proceed ’till you come to your Journey’s End. Then in Plotting the Road, you must observe the same Directions, as are given in Plotting the Example of Use IV. of the last Chapter.

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