[48] To strengthen a beam, &c. which is required to support a great weight over a cavity, or ditch.—Place a prop, or short skid, under the centre of the beam, and pass a strong rope, or chain, over the beam lengthways, and under the skid, hauling it very tight, and making fast.
[49] In Lieut.-Colonel B. Jackson’s scientific “Treatise on Military Surveying, &c., &c., &c.,” Portable trigonometry without logarithms, is thus introduced—
“The following useful application of Trigonometry, by means of the natural sines, tangents, &c., is taken from an early number of that valuable periodical, ‘The Mechanics’ Magazine,’ and will be found particularly suited to the purposes of the military surveyor.”
[50] For further information on Surveying, and Reconnoitring, reference should be made to the highly-valued publication, entitled “A Treatise on Military Surveying, including Sketching in the Field, Plan Drawing, Levelling, Military Reconnoissance, &c.,” by Lieut.-Colonel Basil Jackson, containing a full account of every surveying instrument, and the right adaptation of them.
[51] 1. The Reconnoitring protractor is not intended to supply the place of the Theodolite, or other expensive instruments, when very great accuracy is required in surveying, or in trigonometrical observations; but, in the hands of officers accustomed to the use of it, bearings may be rapidly taken, heights and distances ascertained, roads traversed, &c., &c., with sufficient accuracy for a military survey, or reconnoissance.
The protractor has a tripod, on which it is to be steadily fixed for taking angles, &c.; but the instrument can nevertheless be used without the tripod; and mounted officers may, after a little practice, make a reconnoissance with the protractor alone, especially if they are able to measure, or calculate the distance of base lines, by the length of the paces of their horses.
2. A survey, &c., may be very rapidly taken in the field, by laying drawing-paper on the face of the protractor, under the marginal scale, fixing it firmly by means of drawing-pins in the sides, and using, at the first station, the edge of the index as a ruler to set off on the paper, at once, by observation through the sights, the angles of the objects whose distance is required; drawing a base line parallel to the tube side of the instrument, and also lines at the angles found. At the second station, the paper must be moved a few inches, for a base line to be drawn; at the termination of which (the second station) the index is to be directed to the objects, as before, and lines are to be produced until they intersect those drawn at the first station: thus the position of the objects will be obtained; and, by using the scale on the index for the length drawn for the measured base line, as well as for the lines directed to the objects, their respective distances will be ascertained.
3. The reconnoitring protractor, and all other instruments for surveying, &c., &c. can be readily obtained from Messrs. Elliott, 56, Strand, London.
[52] Or Reconnoitring protractor.
[53] To erect a perpendicular, vide [“Practical Geometry.”]