Practical Use Of The Scales In Drawing Plans, Etc. To take measurements from scales is a simple matter. Suppose the drawing, of which the dimensions of various parte are required to be taken, is drawn to a scale of " 1 inch to the foot;" and suppose that a certain distance from point to point of any given line in the drawing is taken in the compasses, then, by applying it to the scale, as, say, that in fig. 6, which is a scale of 1 inch to the foot, while one leg of the compasses is in the point 4, while the other reaches to the point 6 in the last division of inches, then the measurement of the distance in the compasses, and by consequence that of the part represented in the drawing, is shown to be 4 feet 6 inches. Again, suppose that to a general plan a scale of " 10 feet to three quarters of an inch " is attached, and the actual length of a line taken in the compasses from the drawing be required to be known; if by applying the compasses to the scale, as in fig. 7, Plate XXXVIlla., the one leg of which being at the division marked 50, and the other reaches to the point 5 on the division to the left; then the distance is known to be 55 feet.
To lay down measurements from a scale is the exact converse of the above, and is simply done. Thus, suppose that on the line a e, fig. 1, Plate XXXVIIIb., it is desired to lay down a line, as a b, representing the side of a box, as a b c d, and that the drawing is to be made to a " scale of ¼ of an inch to the foot." First, draw the line a e along the edge of the square, in a light pencil line; if the length of the side of the box, as a b, is to be 8 feet 9 inches, then on the scale, as in fig. 10, Plate XXXVIIIa., put the point of one leg of the compasses in the division to the right, marked 8, and draw out the compasses till the point of the other leg reaches exactly to the point indicating the ninth division on the division of inches to the extreme left of the scale; then take this distance, and with one point of the compasses, on the line a e, at a, measure from a to b, this will give a line in length equal to 8 feet 9 inches, as desired. The depth of the box, as a c, which we shall suppose to be 1 foot 2 inches, is measured from the scale in fig. 10, Plate XXXVIlla., in the same way, and the mode of drawing it is as follows: - Suppose that the edge of the square is coincident with the line a b, previously drawn, move the square so that the edge be a little below the line, as f g in fig. 1, Plate XXXVIII6.; then take the "set square," as represented by the dotted lines at h, and, putting the base on the edge of the square, as g f, slide the set square till the perpendicular of the base be coincident with the point b, on the line a e, and draw a line along the edge b d; then slide the " set square " along the edge of the " T-square," till its perpendicular be coincident with the point a, in the line ae; next, from the scale in fig. 10, Plate XXXVIlla., take the distance in the compasses of 1 foot 2 inches, by measuring from the first large division marked 1 to the second small division in the part o, 12; and, with this distance in the compasses, set one leg in the point c, and with the other mark a point in the line ac, at c ; next, move the " T-square" up the board till its upper edge be coincident with the point c, and draw a line along the edge cutting the line b d in the point d; the outline of abcd will then be drawn, and the lines ab,cd will be parallel to each other, as will also ac, b d. Dimensions, when marked on drawings, are usually put in, as shown in fig. 1,
Plate XXXVIIIb., between the marks, as --------- with a dotted line; the acute angles of the marks being the limits of the line of which the dimensions are figured.* In some drawings, owing to the complications of the parts, or to preserve the drawing itself from being marked with figures, the dimensions are indicated in the manner shown in fig. 1, Plate XXXVIIIb .; the lines, as c a, d b, being extended in dotted lines to a short distance beyond the drawing, and the dotted line put between the marks ---------» as shown. The other measurement in this diagram is indicated in like manner at k e. In finished drawings these dimension marks <......>• should be put in neatly and carefully. This will best be done by the aid of the " set square," as shown in fig. 2, Plate XXXVIIIb. Thus, let a b be the dotted line terminated by the dimension marks at a and b; let c d represent the upper edge line of the " T-square," and the dotted triangle, d e f, the " set square," the base, e d, of which is placed on the edge, c d} of the '•' T-square f adjust the " set square " so that its hypothenuse, e f. is coincident with the point b; then along the edge draw a short line, marked in the diagram by a strong black line; the corresponding angular line is drawn in at a, by sliding the u set square " along the edge of the " T-square," till the point in the hypothenuse is coincident with the point a. The reverse angular line is put in by reversing the position of the " set square," as shown by the dotted lines, g c h; the angular lines should all both be of the same length. In place of putting to drawings the scale in the manner as indicated in fig. 10, Plate XXXVIlla., it is the practice of some architects and builders to write merely on the drawing the scale to which it is made, as " scale, 1 inch to the foot," " scale, ½ inch to the foot," and so on. Some make the matter more simple still, by merely writing " ⅛th scale,"
* The figures, as "¼" put to the foot of the diagrams to follow in this volume, are meant to denote the scale to which the drawings are made. Thus, in fig. 1, "¼" means that the scale of the drawing is "¼, or one-fourth of an inch te the foot." or "one-eighth scale;" or "1/12th scale," or "one-twelfth scale." This does not mean that the ⅛th scale, for example, is " ⅛th of an inch to the foot," but that it is ⅛th of a foot, or " equal to a scale of 1½ to the foot." A1/12 th scale is thus equal to 1 inch, as there are 12 inches to the foot, and is equal, therefore, to a scale of " 1 inch to the foot;" a 1/24th scale is equal to " half an inch to the foot;" a 1/6th scale equal to " 2 inches to the foot." But in all cases it is by far the most satisfactory method to draw a properly divided scale to each drawing. The easier methods above named go on the assumption that in the office, scales (on ivory or box-wood) of various sizes are at hand, from which the specific dimensions of certain parts can be taken; but drawings are often referred to in the actual carrying out of the work, in circumstances where these scales are not available, so that it is better to put a properly divided scale to each drawing as recommended. At all events, this should be done in the drawings of pupils beginning practice. Scales of tenths, as in figs. 7 and 13, Plate XXXVIlla., are, as already stated, used for laying down drawings of general plans, as block plans, where the measurements are long. As a useful lesson in drawing, and as further exemplifying the use of scales, we shall suppose fig. 3, Plate XXXVIII6., to represent the plan of the ground upon which a house is to be erected. The scale to which this is drawn being that in fig. 7, Plate XXXVIII&., which gives 10 feet to ¾ of an inch, the first thing to be done is to draw a line representing a b in fig. 3, Plate XXXYIIIb., along the upper edge of the " T-square," the blade of which is parallel to the lower edge of the drawing board - the butt or head of the "T-square" being thus placed on the edge of the right-hand end of the drawing board. The length of the line a b is marked in the drawing as shown to be equal to 35 feet. This is taken from the scale in fig. 7, Plate XXXVIIIa., by putting one point of the compasses in the division marked " 30," and extending the other to the point "5," in the division to the extreme left of the scale. Then, from any point on the line a b, fig. 3, Plate XXXVIIIb., as a - this point being selected so as to put the drawing when finished as nearly in the centre of the paper as possible - mark off the distance taken from the scale to the point, as b, fig. 3, Plate XXXVIIIb.; the length of the line a b will then be equal to 35 feet, measured from the scale, fig. 7, Plate XXXVIIIa. The next point is to obtain the position of the point c in the drawing, fig. 3, Plate XXXVIIIb. On the drawing which is being thus copied extend by a very fine and light pencil line - so that it can be easily erased - the line d c to some distance beyond the point c, as, say, to the point e. Next, at right angles to the base line a b, draw another line, lightly put in by a pencil line, so as to cut the line d c extended in e. On the paper on the drawing board draw now a line from a (or, rather, from the points on the drawing board corresponding to the point a in the copy, which is supposed to be fig. 3, Plate XXXVIIIb.), perpendicular to a b; this can be done by shifting the " T-square " so that the blade will be run parallel to the end of the board, the head or butt running along the lower edge of the drawing board; or, if the line is not too long, the " set square" can be used, as described in connection with fig. 1. Take from the copy the distance a e, and measure it on the scale, fig. 7, Plate XXXVIlla., and set off, from a on the drawing board, this distance, cutting the line a e in the part e. Through e draw along the edge of the square - which is again shifted, so that its blade shall be in its original position, that is, parallel to the lower edge of the drawing board - a line e f; this line will correspond to the same line in the copy, fig. 3, Plate XXXVIIIb., and will be the same distance from the line a b. Take in the compasses the distance e c from the copy, and measure it from the scale, fig. 7, Plate XXXVIlla., and from the corresponding point e on the drawing board, set off this distance from ae to c; the position of the point c will thus be obtained, and, if the operations have been correctly performed, the length of the line a c, when measured from the scale, fig. 7, Plate XXXVIlla., will be found to be as marked - 33 feet 6 inches. In practice, where the copy is to be the same size as the original, the length of the lines a e and e c need not be measured from the scale, but simply transferred from the copy to the drawing board, as above described. The next operation is to measure from the scale the distance c d 22 feet, and transfer it to the drawing board, or, rather, the paper on its surface. On examination of the copy the line d g will be found to be exactly at right angles to the line c d. The " set square " should then be brought into use, and by it the line d g should be drawn of same length, and on it the distance taken from the scale - namely, 13 feet, set off from d to g. The line g h will be found, on examining the copy, to be parallel to a b; draw, then, on the paper the line g h at right angles to d g, or parallel to a b, and make it equal to 7 feet; join h b, and the plan is complete. The line b h is not at right angles to the line a b; and the accuracy of the drawing will be tested by measuring this; and if the drawing be correct, it will be found to be 20 feet. But in place of the copy being accurately drawn - as it is supposed to be, in fig. 3, Plate XXXVIIIb. - the case may be supposed that the copy might be a rough outline sketch, something like the form of fig. 3, with the dimensions or measurement marked on it; in this case, if the pupil was desired to make an accurate drawing to scale of this rough sketch, no such facilities for ascertaining the position of the point c in relation to the point b a would be afforded such as we have described. The pupil would therefore have a very different process to go through before he could make his drawing. We have also stated that by examination of the copy he could ascertain whether the line d g was or was not at right angles to c d. This could only be done if the copy was accurately drawn, and very simply by placing the copy on the drawing board, and marking the base line parallel to the edge of it, by means of the " T-square," and then shifting the square to test the line d g. Examination like this can, after a little practice, be very quickly made. But, if a rough sketch was provided, the line d g might be put in obliquely, as also the line g h. The pupil will find in the volume noted on page 14 full instructions how to draw from rough sketches, or from the ideas of his own mind, which, in the case of original work, take the place of rough sketches. For the method of constructing and of using "diagonal scales," see the volume noted on page 14.