# A treatise on the principal mathematical drawing instruments employed by the engineer, architect and surveyor. With a description of the theodolite, by H.J. Castle

John Weale, 1845 - 153 σελίδες

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Σελίδα 38 - This done, press the protractor gently down, which will fix it in position by means of very fine points on the under side. It is now ready to lay off the given angle, or any number of angles that may be required, which is done by turning the pinion d till the opposite vernier reads the required angle. Then press downwards the branches ee, which will cause the points to make punctures in the paper at opposite sides of the circle ; which being afterwards connected, the line will pass through the given...
Σελίδα 87 - From 8 times the chord of half the arc, subtract the chord of the whole arc ; one-third of the remainder will be the length of the arc nearly.
Σελίδα 24 - The above engraving represents this instrument, which consists of a beam, AA, of any length required, generally made of well-seasoned mahogany. Upon its face is inlaid throughout its whole length a slip of holly or boxwood, aa, upon which are engraved the divisions or scale, either feet and decimals, or inches and decimals, or whatever particular scale may be required. Those made for the use of the persons engaged on the ordnance survey...
Σελίδα 54 - When a measure is taken on any of the sectoral lines beginning at the center, it is called a lateral distance ; but, when a measure is taken from any point on one line to its corresponding point on the line of the same denomination on the other leg, it is called a transverse or parallel distance. The divisions of each sectoral line are contained within three parallel lines, the innermost being the line on which the points of the compasses are to be placed, because this is the only line of the three...
Σελίδα 25 - ... plotted *. Two brass boxes, B and c, are adapted to the beam ; of which the latter may be moved, by sliding, to any part of its length, and fixed in position by tightening the clamp screw E. Connected with the brass boxes are the two points of the instrument, G and H, which may...
Σελίδα 81 - ... it dries. Sometimes strong paste is used instead of glue ; but as this takes a longer time to set, it is usual to wet the paper also on the upper surface to within an inch of the paste mark, care being taken not to rub or injure the surface in the process. The wetting of the paper in either case is done for the purpose of expanding it; and the edges being fixed to the board in its enlarged state, act as stretchers upon the paper, while it contracts in drying, which it should be allowed to do...
Σελίδα 45 - AD: DE; so that if AD be the half, third, or fourth part of AB, then DE will be a half, third, or fourth part of BC ; and the same holds of all the rest. Hence, if DE be the chord, sine, or tangent of any arc, or of any number of degrees to the radius AD, then BC will be the same to the radius A B.
Σελίδα 37 - The centre of the instrument is represented by the intersection of two lines drawn at right angles to each other on a piece of plate glass, which enables the person using it to place it, so that the centre, or intersection of the cross lines, may coincide with any given point on the plan. If the instrument is in correct order, a line connecting the fine pricking points with each other would pass through the centre of the instrument, as denoted by the before-mentioned intersection of the cross lines...
Σελίδα 69 - The fine thread represented as passing from the pencil quite round the instrument to the tracing point at c, enables the draughtsman at the tracing point to raise the pencil from the paper, whilst he passes the tracer from one part of the original to another, and thus to prevent false lines from being made on the copy.
Σελίδα 107 - ... before it. The existence of parallax is very inconvenient, and, where disregarded, has frequently been productive of serious error. It will not always be found sufficient to set the eye-glass first, and the object-glass afterwards. The setting of the object-glass, by introducing more distant rays of light, will affect the focus of the eye-glass, and produce parallax or indistinctness of the wires, when there was none before. The eye-piece must, in this case, be adjusted again. Generally, when...