INSTRUMENTS FOR MEASURING LAND AND CIVIL WORKS DIRECTLY—CHAINS—VARIOUS TELLERS—STANDARD CHAINS—ARROWS—DROP ARROW—VICE FOR ADJUSTING CHAIN—CAINK'S RULE FOR INCLINES—STEEL BANDS—WIRE LAND MEASURES—COMPENSATION SYSTEMS—LINEN TAPES—OFFSET RODS—PINE STANDARD RODS—RODS WITH IRON CORE—BEAM COMPASS RODS—COINCIDENT MEASUREMENTS—COMPENSATED RODS—BASE LINE APPARATUS—COAST SURVEY LINES—PERAMBULATOR—PEDOMETER—PASSOMETER—SOUNDING CHAINS—SOUNDING LINES—TELEMETERS—HAND RODS—RULES.
715.—The Instruments Generally Employed for Measuring Land are chains, steel bands, and tapes. Where roads are roughly measured, pedometers are commonly used. Where very exact measurements are required, rods have been used. Rough approximate measurements are obtained by stepping, with the use of the passometer to count the steps.
716.—Land Chains.—Although these are made in many qualities the forms vary very little. They are too well known to need much description. In the British Isles and some of our colonies the chain of 100 links, equal to 66 feet, the invention of Edmund Gunter about 1620, is generally used, 10 square chains (100,000 square links) giving the statute acre, presenting a decimal system of measurement much in advance of any other at the present time. The best land chains are made of steel, which is afterwards hardened and tempered to spring temper, in the process of which the surface is burnt off with asphalt varnish in order to produce a covering to resist the rusting effects of moisture. Steel chains are made light and strong. The light chain, of No. 12 Birmingham wire gauge, weighs under 5 lbs. The strong chain, of No. 8 B.W.G., weighs about 12 lbs. A light chain of 50 links, of weight under 3 lbs., is sometimes used with the complete chain of 100 links for taking offsets.
Fig. 330.—Land chain and arrows.
All the best chains, whether of steel or iron, are made with long links formed by turning up the ends of a length of wire. Three small oval links are placed between each pair of long links. These three interval links are found to cause the chain to kink less than when only two are used. Each oval link is sawn through at the meeting line, which is brought up on one flat side of the oval in bending it from the wire. The saw-cut forms the point of adjustment. The small link is afterwards re-sawn and closed to shorten it, or forced open to lengthen it. There are generally four swivels in the length of the chain, two of which are at the handles: these prevent the chain from becoming twisted in turning the handles over in use. A swivel is shown Fig. 331 at S. Iron chains are sometimes galvanized to prevent rust. This process, however, makes the chain much more brittle, and cannot be recommended. It may be noted that all link chains lengthen with use.
717.—Tellers are small pieces of brass suspended to the chain by a spare link placed at every ten links. They divide the chain decimally from either end equally. Proceeding from one end of the chain the tellers read 10, 20, 30, 40, 50, and the other end they read by subtraction from the complete chain: 100 - 10 = 90, 100 - 20 = 80, 100 - 30 = 70, and 100 - 40 = 60. Fig. 331 shows detached pieces of chain with value of the tellers figured under. S inserted swivel. The 50 teller shows the link attachment. A shows the position at which the arrow or other mark is placed to commence or finish the chain measurement, the handle being included in the first link. These tellers are liable to catch and get dragged off in chaining. When this chain is used abroad, or far from home, it is well to have an extra set of tellers to repair losses.
Fig. 331.—Gunter's land chains.
718.—Inserted Tellers.—This form of teller is preferred by many, Fig. 332. It is much less liable to get dragged off, but it is not considered quite so distinct, and it is a little liable to get clogged with grass and weeds.
Fig. 332.—Inserted tellers.
719.—The author's design for inserted tellers is shown Fig. 333. These are perhaps quite as distinct as the last. The holes in wet weather fill up with mud and the surfaces keep bright, so that they remain very readable. There is much less drag, and the chain therefore wears longer.
Fig. 333.—Stanley's inserted tellers.
720.—Feet Chains are usually made 100 feet, more rarely 50 feet. They are generally made in foot lengths, but sometimes for flexibility are preferred in 6-inch lengths. They are commonly made of No. 8 B.W.G. steel or iron. The weight of 50 feet is 6 lbs.; 100 feet, 11 lbs. If made of light steel, No. 12 B.W.G., the 100 feet weighs 6 lbs.
721.—Mining Chains used in mineral districts are made generally 10 fathoms, or 60 feet, 6-inch links counted off by tellers in fathoms. They are made entirely of brass. The weight is about the same in brass as steel—No. 8 B.W.G., 9 lbs. Occasionally they are made extra strong, No. 7 B.W.G.; weight 12 lbs. In coal mines Gunter's chains are generally used.
722.—Metre Chains are made 20 or 25 metres long. They are marked with tellers at every two metres with a plain ring at the metre. The tellers are generally of the inserted kind, Fig. 332. In taking measurements the sign of the teller is doubled: thus the ordinary 1 or 10 is counted 2 metres; the 2, 4, and so on. 20-metre chains in light steel, No. 12 B.W.G., weigh 4½ lbs.; strong, in No. 8 B.W.G., 9 lbs. 25-metre, light, 6 lbs.; strong, 11 lbs.
A land chain is generally secured for carrying by a leather strap with a buckle. Occasionally it is carried in a sailcloth bag with a strap over the shoulder.
723.—Standard Chains.—These are of the same form as the ordinary steel chain, but all the links are hard soldered after being adjusted link by link. They are not intended to be used for regular chaining, except it be for laying down rough base lines. Their special employment is to test chains, or to set out with two pegs on a straight piece of ground a standard length or station where the common chains in use may be tested daily. A standard chain is commonly enclosed in a box with a lock to prevent its accidental use for an ordinary chain.
724.—Arrows.—These are sometimes called pins. Ten form a set. They are shown with the chain in Fig. 330, and are commonly made of the same wire as the chain—No. 8 B.W.G. They are much better made one gauge stouter (equal to about 1/7 inch), and preferably of hardened steel than of iron. The common length is 15 inches. Where heath, stubble, or woodlands prevail 18-inch are better for use, and in some exceptional cases even 2-feet are very convenient. Surveyors going to new countries are recommended to take the longer arrows as well as those supplied with the chain. It is common either to tie a short length of scarlet webbing upon each ring of the arrow or to sew a piece of red flannel or bunting upon it to find it easily in long grass. Arrows are sometimes carried in a quiver with a strap over the shoulder, Fig. 334, which leaves the hands of the fore chainman free to remove obstructions where they occur.
725.—Drop Arrow, Fig. 335. Where ground is very hilly it is common to roughly level the chain by holding the lower position shoulder high, either by guess work or by using any kind of rough hand level or clinometer to ascertain this. The arrow is then dropped, and the point, held at first lightly in the ground, is pressed hard down or another arrow supplanted for it. The chain in this case is used in odd multiples of links as they occur, of which record is taken separately at each station. In going downhill a drop arrow answers very well. In going uphill a plummet to the last arrow is better. Some use the drop arrow as a plummet, carrying for this purpose in the pocket a piece of fine whipcord, with a bent hook tied to one end, to be used when required.
726.—Examination and Adjustment of Chains.—Respectable makers send out chains tested to within half of one of the small links of standard, that is, within a quarter of an inch; but in use this error may increase either by the bending of the long links of the chain, when it becomes shorter, or in the more general case of friction from wear and from strain, by which it becomes longer. In London, standards are fixed upon the pavement in Trafalgar Square and at the Guildhall. These standards are also fixed at many municipal town halls. Surveyors very commonly lay down a standard on the pavement, or by pegs on a level gravel path. Where a peg is used it should be driven home nearly to the surface. It should if possible be made of a piece of heart of oak 12 inches long and about 2½ inches square. The standard length, which may be set off by a standard chain or new steel tape, should be from a saw-cut across the centre of one peg to a similar cut on the other. It is well also to have the centre space (50 links) indicated by a smaller peg.
727.—The Chain to be Adjusted should be first examined and its long links set straight by means of a hammer on a flat, hard stone or anvil, after which the error will be, if it has been much used, that it is too long. It should be then laid in direct line on the standard just described, and stretched lightly with a pull of about 7 lbs., and then left to rest. Assuming it too long, the centre of the chain should be observed to ascertain which half is of the greater length, then short links should be taken out at distributed distances, if more than one be required, by twisting the link open in a vice, and opening and closing another link to restore the chain.
Fig. 336.—Stanley's vice for adjusting and repairing land chains.
728.—Chain Vice.—The links of steel chains can seldom be twisted open without breaking, and broken links cannot be restored by steel links. Iron links answer, but they are very stiff to twist open. Generally it will be found best for professional men to repair the chain with spare brass links. These wear very well. Where a smith is near with his vice and a light hammer the links are readily opened. It often occurs in open districts and abroad that no smith's shop is to be found. To meet these cases the author has constructed a special vice, as shown Fig. 336. This vice is let into a piece of hard wood—an old oak post answers admirably. In stone districts it is perhaps better to let it into a stone and fix it by pouring hot lead round it. The part B is used for an anvil for straightening the links. The vice V holds the link edgewise very firmly by bringing up the slide J by means of the screw S. The link may then be knocked open by the pane end of a light hammer. The link is closed again in the same manner. If the vice be left out of doors the screw should be well greased and the whole covered with a leaden cover. The weight of the vice is about 6 lbs. It is made of cast iron with chilled face, or the jaws are faced with steel.
729.—Opening and Closing the Chain for Use.—The chain is most readily unfolded by taking the two handles in the hand and walking away from it as it lies on the ground. It is most convenient to place it about 45°, and half a chain length from the first station, each chainman taking a handle and moving to his position. The only danger in undoing a chain is from two chainmen taking one handle each and walking in opposite directions, in which case, if there happens to form a kink, the opposite movement of the two men will probably stretch or break the chain. In closing the chain it is taken by the middle links and folded up two links at a time till the handles are reached. If the links be placed consecutively in position round the axis formed by the first links, it may be folded up very compactly in a twisted form ready for the strap, by which it is carried, to be passed round it.
730.—Chaining is performed by two chainmen, termed the leader and follower. The follower, having pressed a stake into the ground for a starting point, then places the centre of the outside of the handle of the chain against it. The leader takes ten arrows in his right hand and one handle of the chain in his left, and walks directly towards a point which is to be the termination of the measurement, stopping at nearly the length of the chain, examining the chain to see that it is straight. He then places an arrow lightly outside the centre of his handle. The follower looks over this arrow to the distant station to see whether it is in direct line. If it be not so, he waves his right or left hand once, twice, or thrice for 1, 2, or 3 inches for movement to right or left. The follower picks up the arrows consecutively as left by the leader, and when he has the ten, 10 chains have been measured, which is then recorded in the field-book, or earlier than the ten if a shorter distance or object completes the measurement. It is most important to observe that if an arrow be taken for the first station, the follower having ten counts nine only for the first ten. To prevent accident it is therefore safer to start from a stake or other landmark, not one of the arrows. Some surveyors advise eleven arrows. If eleven be used, one should be distinctly marked from the rest so as never to be counted. This may be done by omitting the red webbing tie, or using a green tie for the odd arrow. The French always make the drop arrow the eleventh arrow, which is never counted in direct chaining.
Fig. 337.—Caink's rule for correcting inclines.
731.—Caink's Rule for correcting inclines in chaining is the invention of Mr. Thos. Caink, C.E., of Malvern, Fig. 337. It is made four-fold, each fold being one link. The link is divided decimally along the inside of the rule. On the outer edge of the rule there is a scale marked degrees, a part of which is subdivided where the scale is open to read closer, that is, to 20 or 30 minutes. These degree divisions, which read up to 16° on one side of the rule, indicate the space from the end of the rule to be allowed in addition for the same degrees of inclination of the land up to 4 links of measurement. On the opposite side of the rule the inclination scale is carried from 16° to 22° 10′. For these higher numbers the length of the rule is first set off, and then plus such part of the rule as is indicated by the position marked upon it of the required number of degrees.
732.—To Use Caink's Rule.—The follower has a clinometer of one of the kinds shown, Figs. 260 or 264. He notes at starting the position upon the face or body of the leader that corresponds with the height of his own eye. He takes the inclination of the land to this point of the leader's body while he is standing upright at one end of the chain and the leader standing at the other, noting the number of degrees shown by the clinometer. He then places the rule in the direction of the chain, with the number of degrees indicated, in front of the arrow, and moves the handle of the chain to this position. For the sake of verification, if he has a second arrow he may place it in the new position, which gives the true allowance. In either case the leader moves the chain forward by the amount required and places his arrow ready to continue the work. By this method it is seen that there is no after calculation or separate record necessary for undulating land, but the true horizontal position is given correctly at each chain measured. The same form of rule is made for feet and metres.
733.—In mountainous countries the eight links of the rule is insufficient allowance for common inclinations. Such countries are measured much more accurately by some system of subtense measurement, for which see Chapter XII.; but where a small piece of sudden steep inclination occurs half a chain may be taken, and the number of degrees indicated upon the rule be doubled, so that the full rule, instead of taking 22° only, will take 44°.
734.—Steel Bands for measuring, termed steel band chains, are made in various forms in this country, and sold by nearly all opticians. They are much lighter than chains of equal strength, and are made of standard length. They are also lighter to use, being smooth and without any projection. On the other hand the reading is less distinct than with the chain, and they need more careful usage in chaining. They also require oiling before being put by. From the thinness of the metal they are altogether more delicate and less durable than the chains for hard wear; but it is thought by many to be a compensation that they are always of true length.
Figs. 338, 339, 340.—Steel bands and tapes.
Fig. 341.—One link of steel band.
735.—The bands commonly used for land measuring are made 3/8, ½, 5/8, and ¾ inch wide, of Nos. 26 and 24 B.W.G. in thickness, respectively. The chain is divided into links by a small stud riveted through the centre of two small washers, a large stud being placed at the fives and an oval plate held by two rivets at the tens, which are numerically indicated in plain engraved figures, as shown in detail, Fig. 341 b, or perforated with holes indicating the number of tens. These band chains are made in links, feet, metres, or to any foreign measure to order, and of any length corresponding with land chains. Weights, approximately—100 feet: ¾ inch, 7 lbs.; 5/8 inch, 4¾ lbs.; ½ inch, 4 lbs. 100 links: ¾ inch, 4¾ lbs.; 5/8 inch, 2¾ lbs.; ½ inch, 2¼ lbs. 20 metres: ¾ inch, 5 lbs.; 5/8 inch, 4 lbs.
736.—Steel band measures are also made with divisions throughout, etched upon them with acid in such a manner that the divisions and figures stand in relief up to the original surface, whereas the new surface, which is etched back to form the ground, appears dull. The brightness of the figures and divisions on the dull ground makes them easily read. These bands are divided into links, feet and inches, metres and decimeters, or closer quantities either on one or both sides of the band as required. With the etched band there is perhaps a little risk of weak places from over-etching, although these bands are most carefully made, but perhaps this is not greater than in the inserted stud band, where weak places are necessarily caused by the loss of width at the points where the holes are made for the studs, wherein moisture hides after use in damp weather.
737.—The steel bands have handles the same as a land chain. They are wound upon a steel cross, Fig. 340. They are commonly placed in a wind-up case similar to that of an ordinary measuring tape, but in steel, provision being made that one of the pair of handles may be secured about the position of the axis of the tape for winding it up. In Fig. 338 the axis is made very large, so that the handle may be pressed in from an opening in one side of it. The newest idea is to cut a slit in one side of the plate up to the centre, as shown, Fig. 339. In this case the handle and band are put in from the side, so that the axis is no larger than is necessary to take the handle. A strap is placed on the side of the case for holding it. This is shown cut off to admit sight of the handle.
738.—The French make the handle generally T-shaped and hollow in the cross part, which renders it very light and perhaps less cramping to hold. The arrows are very commonly held by loops to the cross on which the band is wound. This general arrangement is very portable and convenient to carry; it is shown Fig. 342.
739.—Wire Land Measures.—Where long open stretches of new country are to be measured, it is common to employ a steel wire chain, of 5 chains or of 500 feet in length, fitted with a pair of strong cross handles only.
740.—The author has made many chains of 500 links; in Fig. 344 a part of one is shown full size. This band, as we may term it, is wound upon a reel in an iron case, Fig. 343. A spring brake is placed at the position A, which holds the reel and prevents the band from springing out into loose hoops when it is run out. The 50 and 100 links are indicated by short lengths of brass tube placed over the band—single at the 50 links, but numerically indicated by number of bands as 2, 3, and 4 chains. In Fig. 344 a 50 and a 300 links are shown; weight, 3½ lbs. This flat, narrow, steel band chain was unknown until introduced to the notice of the profession in the first edition, 1890. It is now in very general use, and lengths may be had from stock of 2, 3, 4, or 5 chains, or 200, 300, or 400 feet wound upon a steel cross.
Fig. 345.—Richmond's tension handle.
741.—Richmond's Tension Handle.—Various devices have been employed for giving equal tension to chains and bands to ensure equality of measurements. Salter's spring balance has been very commonly used attached to one handle of the chain to give a uniform pull, say of 15 lbs. This appears to answer very well. Mr. Richmond, surveyor, of Sydney, has devised a very simple plan for tension of light bands, which, being lighter and attached, is much more convenient than Salter's balance. This is shown Fig. 345. The band passes through a fitting in the centre of the handle, and a spiral spring is fixed to this and the band at a short distance along it. By pulling the handle a given tension can be applied, which is shown by the mark it reaches towards the end of the band. This is adjusted to standard length, and a small notch is placed in the centre of the end, from which a plummet may be suspended if necessary.[54]
The engraving is of a slightly modified form by the author, in which a thin tubular cap covers the free end of the band to save this exposed part from accidents.
Fig. 346.—Copper case thermometer for suspending to a band chain.
742.—Chain and Band Thermometer.—Where very great accuracy of chain or band measurement is aimed at, temperature is taken to allow for expansion of the metal. A thin plain glass thermometer of the clinical form is the most sensitive of any. This is carried in a wooden pull-off case lined with indiarubber. When it is used it is placed upon the ground by the side of the chain. The delicacy of the clinical form of thermometer is often objected to by the practical surveyor, hence there are several other forms with boxwood and ivory scales. These are not very satisfactory, as the boxwood and ivory retain the heat of the body, from being carried in the pocket, for a long time after exposure. The author has enclosed the clinical form of thermometer in a copper case with open face, Fig. 346. The copper being a good conductor of heat, this is very sensitive to the temperature of the air. Two turn-down hooks are placed at the ends of the tube to suspend it on the band. The thermometer stem has two indiarubber caps, so that it will bear dropping on grass. It is contained in the same form of pull-off case as the clinical.
Fig. 347.—Littlejohn's temperature handle.
743.—The coefficient of expansion for steel between 32° and 212° Fahr. is about ·000012, which is less than ·01 inch per degree per chain. Temperature corrections can therefore be recognised only upon very exact work, appreciable only when long bands of the Marchant type, lately described, of from 5 chains to 10 chains in length are used.
744.—Mr. Littlejohn has patented an adjustable handle for temperature. This is divided for allowance for the 100-feet or other band for every degree Fahr. or centigrade. Fig. 347. The handle is set to the temperature as it changes during the day. It offers, perhaps, the highest refinement in ordinary land measurements.
Fig. 348.—Stanley's repairing sleeve.
745.—Repairing Sleeve for Steel Bands.—The reviser has patented a sleeve which will be found useful, as by its use a broken band can be immediately and permanently repaired in the field without the use of tools. They are made to fit all sized bands, but it is necessary that the correct sized sleeve should be used. One of these sleeves is shown attached to a band at Fig. 348.
In order to effect a repair it is merely necessary to clean the broken ends of the band, and insert them into the sleeve, then hold a lighted match under it until the soldering material is melted, when the repair is completed.
The central hole in the sleeve is to enable the user to see when the broken ends are in contact, and the other two are to indicate when the soldering material is melted, which takes place when it either bubbles up in or runs away from these holes.
746.—Linen Tapes.—This most useful implement, Fig. 349, is one of the most unsatisfactory measures the trader and user has to do with. It consists, as is well known, of a tape oiled, painted, and varnished, which is rolled up in a leather case when out of use. When the weather is moist it shrinks, and when dry it expands. If it be too heavily painted it becomes brittle and rotten; if it be lightly painted it remains more flexible, but is more affected by moisture. A good tape bears very well a stretching force of 7 lbs. to 14 lbs., but if strained over this it is permanently stretched. There is no plan known to the author by which these defects can be remedied. Numerous attempts have been made—often valueless or worse—some, although popular, mere claptraps, such as the insertion of wire. The best tapes for strength and permanency are made entirely of green, hand-made, unbleached flax. The tape is said to come from Holland to this country. These are at first oiled with a drying oil (boiled linseed oil), and when seasoned for a month or so, painted once or twice with white lead colour—not too thickly. The printing is more permanent if done in oil; but the tape is somewhat more flexible if the figures are stencilled in Indian ink and the whole afterwards thinly varnished over with copal varnish. The great secret for preserving the tape is to use it very carefully and only in fine weather. In wet weather for taking offsets a light steel 50-link chain is quite as convenient as the tape, and safer.
Tapes are divided into links, feet and inches, metres, and all measures as required. A decimal yard is commonly placed on tapes for measuring earth work. For use with the chain a 66-feet tape is usually employed, but many think a 33-feet better, using the chain for dimensions above this. For measuring buildings, 50-feet or 100-feet tapes subdivided to inches are employed.
747.—Steel Tapes.—Thin steel tapes, 3/8, ½, and 5/8 inch wide are in very extensive use. They are more accurate and more costly than linen tapes, but less flexible and less durable. Where dimensions are important they should always be used for short measurements. In all cases it is advisable for a surveyor to keep a steel tape for examination of the lengths of linen tapes in use. They are made to all the measurements of linen tapes.
748.—Pocket Steel Tapes 6 feet to 12 feet, Fig. 350, are used more generally by mechanical engineers. These tapes, which are very light, are held open by a catch, and closed by a spring.
Fig. 351.—Jointed offset rod, top and centre.
749.—Offset Rods are generally made 10 links long, either in one piece or jointed in the centre with a bayonet joint. They are about 1-1/8 inches in diameter, diminishing towards the top to 7/8 inch, and made either of yellow pine or ash. A hook is commonly put at the top, Fig. 351, which takes the handle of a chain to draw it through a hedge or other obstruction. The author's plan of making this is shown at H. The lower end of the offset is shod with a steel or wrought iron socket point, so that it may be set up in the ground and used if required as a picket. Bands are painted alternately black and white at every link. Square or flat rods are occasionally used for the same purpose, but they are not generally so convenient.
The offset is Used in the manner of an ordinary rule to take rectangular short measurements from the chain as it lies upon the ground, commonly in order to obtain the contour of irregular outlines.
750.—Measurement by Rods has become less general than formerly, from the greater accuracy of Konstat or Invar steel tapes, by which practically correct base lines may be laid down. For geodetic works requiring the greatest accuracy the bases have been laid with rods of various forms. These rods will be briefly described. It is only in the construction of iron bridges, roofs, etc., that rods are at present generally employed in the work of the civil engineer.
751.—Pine Standard Rods, made of straight-grained pinewood seasoned five or six years and then well soaked in linseed oil, make good standard rods. The ordinary length in use is 10 feet by 1¾ inches square. If the rod be used for butt measurement the ends are tipped with gun-metal in which a turned steel stud is hard-soldered. The stud is afterwards ground to true face in a lathe, and left of standard length at 60° Fahrenheit (15·5 centigrade), Fig. 352. A disc of brass 1 inch diameter is inlaid at every foot for 5 feet from one end of the rod, with a line at the true foot. These rods, after the work upon them is finished, are lightly French polished to keep them clean and to prevent the effects of moisture. The effect of temperature upon deal was found by Roy to be about the same as upon glass—·0000085, average of total length per degree centigrade, which is about three-fourths that of iron.
752.—Where butt rods are used for continuous measurement, it is necessary that they be brought very carefully together. In base line measurement three or four are used, but for metal work or masonry two 10-feet only are generally employed. It is necessary that the rods should lie upon a straight surface or be supported in a straight line. In bringing them together, a piece of indiarubber 1/8 inch or so in thickness temporarily placed at one end will prevent any palpable disturbance of the percussion if the fixed rod be well weighted. One 5-feet more fully divided butt rod is very commonly supplied with a pair of 10-feet rods for supplemental measurement.
753.—Angle-piece.—A solid angle-piece with two planes at right angles is very convenient for use with butt rods to give means of scribing the true length down to a surface, Fig. 353 S.
754.—Butt Rods with Iron Core.—Where rods are to be used for preparing iron work it is better to have an iron core through the rod, that may expand and contract with the metal on which they are used. The rods that the author has designed for this purpose are made out of a length of seasoned pine 2¼ inches square, sawn down and turned cut sides outwards to prevent warping. A 10-feet length of iron steam tube about ½ inch diameter is painted several times and then bound round with paper soaked in paraffin. This is placed in a pair of meeting grooves, as shown in section Fig. 355. The two pine flitches are cross-tongued together and glued up with the inserted tube between them. The tube has a turned steel cap placed over each end, Fig. 354, and this is ground in a lathe to true standard at the temperature of 60° Fahr. A steel pin is placed through the centre of the rod to indicate 5 feet. The finished size of the rods is 2 inches square.
The author has made these rods in sets, consisting of two 10-feet and one 5-feet packed together with an angle-piece, Fig. 353 S, in a deal case.
755.—The 5-feet Rule is of steel, ¾ inch by ¼ inch, inlaid in a piece of dry pine, altogether of only half the thickness of the rods, so that it stands the correct height for central butt measurement, Fig. 356. The rule is divided into feet and inches, with one foot to eighths. A centigrade thermometer is placed in one of the rods to indicate the prevailing temperature, and a small piece of scale showing amount to be allowed in 10 feet per degree centigrade for temperature above or below 15·5° centigrade is engraved upon the thermometer scale. The coefficient for the expansion of wrought iron is given by Lord Kelvin as ·000019 mean per degree centigrade.
Figs. 354, 355, 356.—Butt rods with iron core.
Where a long length is laid down for a base line or other purpose, it is better to take the thermometer reading at each measurement and defer correction to the completion of the work; the temperature errors may then be added together as a total, and the space allowance may become a measurable quantity. For example, say ten 10-feet lengths give by these united centigrade degrees, plus and minus, shown at separate readings + 167°, and that the standard of the rods is true at 15·5°. Then 167 - (10 × 15·5) = + 12° per foot total allowance, that is, 12° × 10 feet × ·000019 = ·0228 feet or ·2736 inches to be added. In measuring iron of course no correction has to be made.
756.—Beam Compass Measurements are occasionally preferred for iron work. In this case the beam is moved from centre punch mark to mark along a surface by the beam producing a scratch for the forward position in which to place the punch mark. Rods of pine are commonly employed. Figs. 357, 358 will sufficiently illustrate the instruments.
Fig. 359.—Coincidence rods.
757.—The Method of Coincidence in measurements by rods has often been applied to measurement of base lines. The plan consists in allowing one rod, or a lighter continuous part of it, to pass the other rod, so that a line cut to standard on one rod may be read into one on the other. The best plan to do this is to have a scale fixed along the face of one rod near its end, as shown Fig. 359, and to have an extension from the other end of the second rod to pass alongside this scale, so that two lines may be brought into coincidence. The rod B has a fixed scale b placed on top of it at one end. The rod A has a scale protruding from it. This scale may be jointed with a good ground joint at J for portability. The rods are laid lightly together, and any final adjustment is given by light taps with a small hammer or mallet upon one or the other side of the stud P until exact coincidence of the lines shown at b is brought about. This tapping operation appears a rather rough process, but practically it is very exact.
Fig. 360.—Bessel compensated rod.
758.—Compensated Rods.—The plan used by Bessel for the measurement of a base upon the shores of the Baltic in 1836 is looked upon as a model of the most perfect work of its kind. The rods were composed of two bars of iron having surfaces accurately planed, with a similar bar of zinc placed between them. The bars were laid one on the other, but not in contact, the surfaces being kept apart by glass plates, upon which they could slide with little friction. The linear expansion of zinc per degree centigrade is about ·0000292 (Fizeau); that of iron much less than half this—about ·0000119 (Thomson). The bars are attached to each other in such a way that the expansion of the zinc may act in the opposite direction to the expansion of the iron. The form followed for the construction is shown in Fig. 360 where II′ are the iron bars, Z zinc. The length of the zinc required for compensation between the junctions is found in the equation—
(S + Z)(0000119) - Z(0000292) = 0,
S being the total length of the standard rod in feet, and Z the length of zinc in feet required for compensation. This plan is that adopted for the compensation of pendulums. For the verification of a rod it may also be made to form the rod of a pendulum, by which temperature expansion and contraction upon the system will be clearly indicated by difference of time rate in the change of temperature during night and day. This test becomes important where great precision is aimed at, as the expansion in metals varies according to their purity and state. The standard lines in rods made upon this model are placed upon small inserted discs of platinum placed near the ends, which are read by microscopes in coincidence upon a pair of rods.
759.—Colby Compensated Rods, the invention of Major-General T. F. Colby, who was for twenty-seven years superintendent of the Ordnance Survey, upon which these rods were used. Each rod is composed of one rod of iron and one of brass, which are so arranged in pairs that the difference of expansion of these metals shall act to diminish the amount of entire expansion at the points measured, a quantity equal to its increase by temperature, in a manner to be described.
The Rods are each made 10 feet 1·5 inches long, 5 inches broad, and 1·5 inches deep. Fig. 362 i is a side elevation of one rod, Fig. 363 ib plan of iron and brass rods, Fig. 365 ib perspective view. By this it will be seen that the rods are placed edgewise. The distance apart is 1·125 inches. They are supported in the middle upon rollers, Fig. 362 F. They are firmly fixed together at their centres by transverse steel cylinders, Fig. 363 RR′ 1·5 inches diameter, each rod being left free to expand or contract from the neutral central point independently of the other. The neutral point is formed of a T-piece E, Fig. 363, fixed firmly on the bottom of the box bx. At the extremity, and at right angles to each of these bars, is a flat steel tongue, Figs. 364, 365 A, 6·2 inches long, 1·1 inches broad, and 0·25 inch thick, which projects 3·25 inches from the side of the iron bar i. The tongue A is jointed by double conical pivots at f and f′, which form axes perpendicular to the surface of the tongue, allowing it to be inclined to slightly different angles to the direction of the bars according to the expansion or contraction the system experiences by heat. The pivots are 0·5 inch diameter, and are placed at 2·3 inches from the end of the tongue next the brass bar. On the tongue at P, flush with its upper surface, a small stud of platinum is inserted, upon which a small dot is struck to form the point of standard measurement.