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ENLARGING AND REDUCING.

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there is a strong prevailing idea that any kind of printing will do on a plan, and a great fancy is expressed for stencil-plates. This is decidedly wrong, as the neater the writing the more effective the plan. Stencil-plates are convenient for marking sacks or the address of voyageurs upon those clean deal boxes one sees outside the trunk manufacturer's, but in the drawing office (except of course where work is done at so much an hour) they are out of place.

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The title of a plan should be carefully set out from a centre line, and the letters, especially the large ones, Fig. 322. pencilled faintly, for which the template, Fig. 322, will be found very useful, giving as it does the angle of the slanting portions of the various letters.

Scales. The best kind of scales for plotting are divided into chains and tens of links on one side, and equivalent feet on the other, so that the mark of two chains would be 132 feet on the feet scale, and the same applies to the offset scale.

I do not suppose the scale-maker could offer any other explanation why 2-chain, 3-chain, and other such scales should be marked 20, 30, 40, &c. True it is they are sometimes used by engineers to plot work to 20, 30, 40 feet to an inch, but it is well to bear in mind that the scales marked 10, 20, 30, 40, 50, and 60 are really 1, 2, 3, 4, 5, and 6 chains to an inch, and the subdivisions are each ten links, and equally on the "feet" side, the 1, 2, 3, 4, &c., represent 100, 200, 300, 400, &c., feet, the greater subdivisions 10 and the lesser 5 feet each.

Enlarging and Reducing Plans. It is often necessary to enlarge or reduce either whole or portions of surveys. For reliable purposes, the most satisfactory method is to replot the work to a larger or smaller scale from your field notes. But this may not always be possible, consequently in these days of "labour saving," we have appliances for expeditiously accomplishing these results. As this work would be incomplete without a description of the pantagraph and eidograph, I have elected to quote from an excellent authority upon the subject *- an author to whom I have already referred (ante, pp. 73, 74). But although I do so, it must not be inferred that I entirely approve of either instrument, against the use of which I have somewhat of a prejudice, added to which I do not consider their great cost always justifies their adoption.

Pantagraph." The Pantagraph (Fig. 323) consists of four rulers, AB, AC, DF, and EF, made of stout brass. The two longer rulers, A B and a c, are connected together by, and have a motion

* "Drawing and Measuring Instruments," p. 65, by J. F. Heather, M.A. Crosby Lockwood & Son, London.

round, a centre at A. The two shorter rulers are connected in like manner with each other at F, and with the longer rulers at Dand E; and, being equal in length to the portions A Dand A E of the longer rulers, form with them an accurate parallelogram, ADFE, in every position of the instrument. Several ivory castors support the machine parallel to the paper, and allow it to move freely over it in all directions. The arms, A B and DF, are graduated and marked,, &c., and have each a sliding index, which can be fixed at any of the divisions by a milled-headed clamping screw, seen in the engraving. The sliding indices have each of them a tube, adapted either to slide on a pin rising from a heavy circular weight called

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E

Fig. 323.

the fulcrum, or to receive a sliding holder with a pencil or pen, or a blunt tracing-point, as may be required.

"When the instrument is correctly set, the tracing-point, pencil, and fulcrum will be in one straight line, as shown by the dotted line in the figure, and which may be proved by stretching a fine string over them. The motions of the tracing-point and pencil are then each compounded of two circular motions, one about the fulcrum, and the other about the joints at the ends of the rulers upon which they are respectively placed. The radii of these motions form sides about equal angles of two similar triangles, of which the straight line в с, passing through the tracing-point, pencil, and fulcrum, forms the third side.

"The distances passed over by the tracing-point and pencil, in consequence of either of these motions, have then the same ratio, and, therefore, the distances passed over in consequence of the combination of the two motions have also the same ratio, which is that indicated by the setting of the instrument.

THE PANTAGRAPH AND EIDOGRAPH.

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"Our engraving (Fig. 323) represents the pantagraph in the act of reducing a plan to a scale of half the original. For this purpose the sliding indices are first clamped at the divisions upon the arm marked; the tracing-point is then fixed in a socket at c, over the original drawing; the pencil is next placed in the tube of the sliding index upon the ruler DF, over the paper to receive the copy; and the fulcrum is fixed to that at B, upon the ruler a B. The machine being now ready for use, if the tracing-point atc be passed delicately and steadily over every line of the plan, a true copy, but of one-half the scale of the original, will be marked by the pencil on the paper beneath it. 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. The pencil-holder is surmounted by a cup, into which sand or shot may be put, to press the pencil more heavily on the paper, when found necessary.

"If the object were to enlarge the drawing to double its first scale, then the tracer must be placed upon the arm DF, and the pencil at c; and if a copy were required of the same scale as the original, then, the sliding indices still remaining at the same divisions upon D F and a B, the fulcrum must take the middle station, and the pencil and tracing-point those on the exterior arms, A Band A C, of the instrument."

The Eidograph. *-"The pantagraph just described requires four supports upon the paper, and from this cause, and from its numerous joints, its action is apt to be unsteady. An instrument to avoid these defects was invented by Professor Wallace in 1821. This instrument (Fig. 324), called the eidograph, is more regular in its action than the pantagraph, as will be readily understood from the following description of its construction, by which it will be seen that there is only one point of support upon which the entire instrument moves steadily and regularly; and the joints, if we may so term them, consist of fulcrums fitting in accurately ground bearings, the motion round these fulcrums being capable of adjustment for regularity as well as accuracy. It also possesses the further advantage over the pantagraph, that it may be set with equal facility to form a reduced copy bearing any proportion whatever to the original, while the pantagraph can only be set to vary the relations between the original and the copy in the few proportions which are specifically marked upon it.

"The point of support of the eidograph is a heavy weight, н, formed exteriorly of brass and loaded internally with lead, and having three or four small needle points to keep it steady on the * Heather's "Drawing and Measuring Instruments," p. 70.

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paper. The pin, forming the fulcrum upon which the whole

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Fig. 324.

instrument moves, projects from the centre of this weight on its

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upper side, and fits into a socket attached to a sliding-box, к. The fulcrums are ground to fit very accurately. The centre beam, c, of the instrument fits into and slides through the box k, and may thus be adjusted to any desired position with respect to the fulcrum, and then fixed by a clamping screw attached to the box. Deep sockets are attached to each end of the centre beam, into which are accurately fitted the centre pins of the two pulley wheels J J. These pulley wheels are made most exactly of the same diameter, and have two steel bands, I 1, attached to their circumferences, so that they can move only simultaneously, and to exactly the same amount. By means of screw adjustments these bands can have their lengths regulated so as to bring the arms of the instrument into exact parallelism, and, at the same time, to bring them to such a degree of tension as shall give to the motions of the arms the required steadiness, which forms one of the advantages of the instrument over the pantagraph. The arms, A and B, of the instrument pass through sliding boxes upon the under side of the pulley wheels, these boxes, like that for the centre beam, being fitted with clamping screws, by which the arms can be fixed in any desired position. At the end of one of the arms is fixed a socket with clamping screw, to carry a tracing-point, G, and at the end of the other is a socket for a loaded pencil, D, which may be raised when required by a lever, FF, attached to a cord which passes over the centre of the instrument to the tracing-point. The centre beam c, and the arms, a B, are made of square brass tubes, divided exactly alike into two hundred equal parts, and figured so as to read one hundred each way from their centres, and the boxes through which they slide have verniers, by means of which these divisions may be subdivided into ten, so that with their help the arms and beam may be set to any reading containing not more than three places of figures. A loose leaden weight is supplied with the instrument to fit on any part of the centre beam, and keep it in even balance when set with unequal lengths of the centre beam on each side of the fulcrum.

"The pulleys, JJ, being of exactly equal size, when the steel bands 1 I are adjusted so as to bring the arms of the instrument into exact parallelism, they will remain parallel throughout all the movements of the pulleys in their sockets, and thus will always make equal angles with the centre beam. If, then, the two arms and the centre beam be all set so that the readings of their divisions are the same, a line drawn from the end of one arm across the fulcrum to the end of the other arm will form with the beam and arms two triangles, having their sides about equal angles proportionals, and being, therefore, similar; hence any motion communicated to the end of one arm will produce a similar motion at the end of the other, so that the tracing-point being moved over

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