others so constructed that instead of the frame working upon a tongue, the groove is made to receive strips of very thin box-wood, upon which are divided scales of from 1 to 6 chains to an inch, and the various Ordnance scales. Mr. Stanley, of Great Turnstile, has brought out this scale, which, together with six or eight strips, is made to fit into a case, the whole cost being £2 18s. 6d., and supplied with each set are specially-prepared sheets of divided paper. Areas by different Scales to Plan.-The scale illustrated in the plate is of so simple and reliable a character that it commends itself; and whilst it is desirable, in an office where computation on a large scale is carried on, to have computing scales of the various scales in vogue, yet it is quite possible to arrive at an accurate estimate of the area of property drawn to a different scale from that of the computer. For instance, suppose we have a plan 5 chains to an inch, the area of which it is desired to ascertain, but our computing scale is 3 chains to an inch. As an example, we will assume that the operation of computation gives a result of 6 A. 2 R. 0 P. with the scale. Now, as 5 chains to an inch is much smaller than 3 chains, then the area will necessarily be greater, so that if we treat it as a rule-of-three sum we shall get the correct result. In examinations, I regret to say, this question has been a source of trouble and embarrassment to many students, who, even if they are happy in thinking of the proportion, quite forget that it will not be as three to five; but, as they are dealing with areas, it is as the square of three is to the square of five, so is the known area to that required. So that, having the area with the 3 chain scale of 6 A. 2 R. Ō P., we proceed as follows: 32: 52: 6 a. 2 r. 0 P. = 7 a. 3 R. 26 P. = area of the plan drawn to a scale of 5 chains to an inch. The cost of a computing scale similar to the one illustrated is £1 5s. Planimeter. There is another method of ascertaining the areas of a plan by what is known as the planimeter, invented by J. Amsler, Professor of Mathematics at Schaffhausen, and manufactured by Messrs. Elliott Brothers, of St. Martin's Lane, the cost of which is £3 3s. But it is a very delicate instrument, and the slightest dirt or rust will throw it out of gear. "It consists essentially of two arms jointed together, so as to move with perfect freedom in one plane, and a wheel which is attached to one of the arms, and turning on this arm as an axis, records by its revolutions the area of the figure traced out by a point on the arms to which it is attached, while a point on the other arm is made a fixed centre, about which the instrument revolves." For a full description of * its various parts, and of the method of using it, I cannot do better than refer the reader to Heather's "Drawing and Measuring Instruments," p. 80. Like all instruments the object of which is to save labour, the planimeter, from the very delicacy of its construction, has to be used with the greatest care; and for ordinary practice it is hardly advisable to adopt it, on account of its great liability to injury. For myself, I cannot help saying that I much prefer to take off the quantities of land either by triangles or with a computing-scale. INDEX. ABNEY level, 33; how to use it, 34 Acute angle, 85 Adjusting the allowance for slope, 22; for collimation, 74 Angle, acute, 85; of buildings, 16; chain, 24; complemental, 98; of Apex, inaccessible, 217 Arc of circle, 85; vertical, 50 Arrows, 3; how to use them, 12; As to the chain, 166 Ascertaining areas on the ground, Avoid crossing fences as much as BACK sight, 175 Ball and socket arrangement, 46 Base, 93; base lines, 23 Basis of formulæ for sines, &c., Beam compass, 234; how to use it; Be careful not to cut fences unne- Begin at bottom of page to work, 140 Boning lines with laths, 139 Box sextant, 59; how to use it, 61 Broken ground, 135 Buildings, how to measure, 16; as to, Centering-plates of theodolite, 43 Chain-men, 10; should be instructed Chain survey of part of Wimbledon Check-lines, 24: obviated, 146; check Chord, 92 Circumferentor, 40 Circle, 85; centre of, 85; diameter Clamps, 48; Troughton's arrange- ment, 49 Clear up ground after you, 144 Close paling, 133 Closing a traverse, 159 Collimation, adjustment of, 74; line Colours, 242 Colour saucers, 242 Combined telescope clinometer and Compass, prismatic, 38; of theodo- Computation scale, 262; various Cotangent, 91 Co secant, 92 Coversed sine, 92 Cross-sections, 197 Cross-staff, 29; how to use it, 15 Crossing hedges, 12 Curve, length of, 214; impeded Curves of contraflexure, 222 DATUM, 177; ordnance, 177 Décamètre chain, 3 Diaphragm of level, 73; theodolite, 51 Distances, inaccessible, 25; in level- Direction of line, 135 Ditch and hedge, 13, 132 Divisions of Gunter's chain, 3 Do not erase figures, 169; cut down Double box sextant, 62 Draw scale on plan, 229 Drawing pen, 237; how to hold, 237 Duties of follower, 11; of leader, 10 EARTH'S curvature, 171 Eidograph, 249; to adjust, 252 Enter every ten chains, 139 Equilateral triangle, 86 Equipment of office, 231; of survey- Eye-piece, 72 FALL, 175 INDEX. Fence, how to measure, 13; Field-book, 6, 132, 135; ordnance, First a chain survey, 148 Instructions to staff-holder, 194 KEEPING the level book, 185 269 LAND quantities, 256; by tri- Lamp-posts, as to, 169 Laths, bundle of, 5 Laying out a base, 66; down survey Level, Abney, 33; adjusting, 175; Level ground, what is, 20 Levelling, 171; different kinds of, Limb or lower plate, 46 Limits of offset, 18; of radii, 211 ranger. 31; of ranging out, 19 |