The bars are placed in strong wooden boxes, to the bottoms of which are fixed the plates that hold the brass rollers upon which the bars are supported, Fig. 362 F, and the central stay E mentioned before prevents any displacement of the bars when the rods are held by the rollers RR′. To protect the tongue A, which projects beyond the boxes, there is a special covering nozzle having a hole and cover over the dot. A level is placed on one of the bars, which is seen through a window in the lid of the box. At the ends of the box plates are fixed for supporting the tripod of the double compensated microscope, Fig. 361 D, employed to observe the standard points of one pair of rods brought by adjustment to true position MM′. A pair of sight vanes which shut down are placed on the ends of the box for setting the rods approximately in line.

Two Rigid Tripod Stands Fig. 361 S are used to each of the rods placed under the rollers Fig. 362 F upon which the bars are supported in the box. The tripods carry a universal slide-rest by which the rod may be adjusted to position both in horizontal and vertical planes Fig. 361 A. Six rods were used for the Ordnance Survey at one time, and were designated by the letters A B C D E F. The weight of each rod complete with microscopes in its case is 136 lbs.

Compensated Microscopes.—The compound microscopes, Fig. 361 MM′, used with the Colby apparatus form a complete separate instrument, consisting of two microscopes placed parallel to each other and united together for reading the rods when they are brought with their standard points the distance apart that separates the axes of the two microscopes. In the intermediate space between the two microscopes, and parallel with them, a telescope T is fixed on the same piece of apparatus, with adjustment for reading a point on the ground G perpendicular to the measuring rod. The microscopes are held apart by two bars of brass and iron 7 inches long, 0·5 inch broad, and 0·375 inch thick, which are placed at 2·5 inches apart and secured with the telescope, which forms the fixed centre, by collars to the bodies of the microscopes. The difference of expansion of the iron and brass maintains the separation of the microscopes at their foci at one distance with every change of temperature of the air. The object-glasses are of 2 inches focus. The microscopes are brought to adjustment and bearing by levelling on a tribrach whose base is fixed firmly to one of the rod cases, and by lateral adjusting screws. Special microscopes are used with each of the six rods of the Colby apparatus, and are distinguished by the letters M N O P Q R S. The weight of each compound microscope is 5 lbs. Very full particulars of the Colby apparatus with engravings of all parts, are given in "The Ordnance Survey Account of the Measurement of the Lough Foyle Base."

In measuring a base line a piece of nearly level land is selected, and the rods are supported upon the trestles or tripod stands at about 3 feet from the ground. The heights of the upper surfaces of the tripods are ranged by a theodolite or level for all intermediate points between the two ends of the line. Generally twelve trestles are employed with these rods, which are fixed firmly to the ground at every station by legs well rammed in, Fig. 361 HH′. The cases containing the rods, or the rods themselves, are made sufficiently strong to be supported upon two points only without serious deflection.

The Colby system of measurement of base lines varied in detail has been employed by nearly all the nations of Europe and in America.

760.—Modern Base-line Apparatus.—The introduction of "Konstat" steel (highest grade Invar) tapes and wires has revolutionised the method of measuring base lines. These tapes offer a means which is far superior to anything obtained by measuring bars, because they combine the advantages of great length and simplicity of working, with more precision than the shorter laboratory standards, providing that suitable apparatus is used in applying them to their work. Base lines may now be rapidly measured with long "Konstat" steel tapes so that much longer lines are laid down than was formerly the practice when measured with bars, with the result that any errors that may be introduced do not affect the ultimate expansion so much owing to the greater length of the base.

The coefficient of expansion of "Konstat" steel is under ·0000005 per degree Fahrenheit, so that provided accurate means of suspending the tape and reading it and transferring the readings to a plate properly let in the ground are used, we have a most exact and rapid method for this important work.

The tapes are usually 100 feet or 30 metres long, but 300 feet or 100 metres are often used. The tapes are a few feet longer than these measurements, so that the rings are well clear of the reading lines. A silk cord is attached to these rings and passes over the end suspension supports, one of which is shown at Fig. 366. These are made with two steel bars rigidly mounted on two tripods; upon the bars a sliding carriage is mounted carrying a pulley running on ball bearings with a vertical motion for final adjustment of the tape for height. A weight is attached to the other end of the silk cord to give the same tension as that under which the tape was divided.

To prevent catenery light intermediate stands, as shown at B Fig. 367, are used at about every ten feet; these have a rising cross piece with guides which are adjustable for height and sideways to support the tape in perfect alignment. Having the tape properly suspended the reading instruments, C Fig. 367, are placed in position at either end. These are mounted on rigid-framed stands and provided with levelling screws, cross levels, transverse screw motions and movement in azimuth, with clamp and tangent motions and aligning telescopes. A powerful microscope is rigidly fixed over a little table over which the tape passes and reads its division with great exactness, coincidence with the division being made by the traversing screw. By the side of the reading microscope, and in exact collimation with it, a plumbing telescope is rigidly fixed, and this sights down to a transferring apparatus, D Fig. 367, which is over the plate let into the ground.

The transferring apparatus is a spring centre punch rigidly mounted truly vertical on a supporting plate having transverse motions, cross levels and levelling screws. The top of the centre punch has a small platinum disc let in a recess, and upon this disc very fine cross lines are marked. This apparatus is placed on the ground over whatever has been let in to receive the mark, it is then levelled and the cross lines upon the punch top brought by means of the transverse motion screws exactly to coincide with the spider web of the plumbing telescope, and in this position the centre punch is lightly struck with a mallet which marks the plate let in the ground in the exact position of the centre of the cross lines at its top, so that if now the transferring apparatus be removed the cross webs of the plumbing telescope would cut the dot marked in the plate by the centre punch. This method is far more exact than any hanging plumb-bob, as even if they are screened to prevent swinging very few hang with the point perfectly true. In laying down a base line No. 1 reading and plumbing instrument is set up and levelled over the starting end block, which is usually of hard stone or granite set on a firm foundation, with a copper plate let in its top about the centre, the line having been previously set out with a theodolite, and the intermediate stations being roughly measured with an ordinary steel tape. At each intermediate station or length of Konstat tape used a teak post is driven into the ground and a zinc plate screwed upon its top; the other end block is similar to the starting one. The Konstat tape is now mounted between the end suspension supports, one being outside the starting end block and the other outside the first teak post which has been put in for the first length. No. 2 reading and plumbing instrument is set up over this post, and No. 1 and No. 2 are aligned upon each other by their aligning telescopes, and the Konstat tape adjusted over the little tables under the microscope of each; the intermediate stands are then put in and adjusted for height to prevent catenery, and the guide pieces are brought up to the tape on either side and clamped to prevent side deflection by wind. The tape being properly suspended it can be easily moved with the fingers lengthways, as it is suspended at either end by silk cords over ball-bearing pulleys. It is brought in position with its starting end division somewhere under the microscope of No. 1 reading apparatus, and the microscope is then brought into exact coincidence by the traversing screws. The transferring apparatus is put on the granite block with the centre punch in the field of view of the plumbing telescope and then levelled; the cross lines in the top of the centre punch are then brought to exactly coincide with the plumbing telescope webs by the transverse motion of the transferring apparatus, the centre punch is struck and the mark thus made in the copper plate has a line engraved through it. The transferring apparatus is then removed to the position under No. 2 reading and plumbing apparatus. No. 2 microscope is made to coincide with the end division on the Konstat tape by its traversing screws, the centre punch of transferring apparatus, brought by its traversing screws to coincide with the webs of the plumbing telescope and struck, marks the first section. No. 1 reading and plumbing apparatus is then transferred to the next post, No. 2 remaining over the first section post, the first end suspension stand is transferred outside No. 2 post and the tape mounted between as before, the traversing motion of No. 2 reading apparatus must not be touched, but the end division of the tape brought to coincide with its microscope web by shifting lengthways. No. 1 microscope at the further end is adjusted to coincide by its traversing screws and the transferring apparatus as before, and so on until the entire length is measured, the last centre punch mark on the copper plate let in the further end block or stone having a line engraved through it.

A few 1-100th of an inch divisions, or 1-10ths of a millimetre, are divided on either side of one end division of the Konstat tape so that any allowance for expansion or contraction may be made under the microscope at the time, but with Konstat tapes this is very small indeed. With fairly level ground any slight differences of level can be allowed for in setting up the stands, so that the tape remains level; if the difference is too great for this the difference of hypo and base must be calculated. Thermometers are used, generally one suspended on the tape at each end.

761.—Perambulator.—A very ancient instrument, described by Vitruvius as being among the effects of the Emperor Commodus; it was used by hand, or attached to a carriage to measure distances. The instrument is at present used as formerly for measuring roads. Upon pavements and asphalt roads it measures accurately, where by reason of traffic it is sometimes a difficult or very slow process to use the chain. The plan of manufacture is varied considerably. The author makes the felloe of the wheel in segments of well-seasoned mahogany in two rings, Fig. 368. These are rivetted together from side to side in such a manner that the grain of the wood is crossed as much as possible to prevent lateral warping. The tyre, which is 6 feet in circumference, is made of hard rolled brass 1 inch by ¼ inch thick. The spokes are light steel tubes covered with brass tube, and screwed into a brass hub. The axle of the wheel is placed in a steel fork which is formed by screwing, by means of a winged nut, two bars of about 18 by 1½ by 3/8 inches upon a boss formed at the end of the steel stem of the turned wood handle. Made in this manner the handle may be easily detached and placed flatwise upon the wheel, so that the whole may be packed in a square deal case of moderate dimensions for transport.

The Registering Part of the Instrument, Figs. 369, 370. The axle is protruded through the fork on the left-hand side, and thence through the registering box supporting one of its ends. The other end of the box is supported by a stud which fits into the side of the fork. The axle in the part contained within the box is cut into a screw, Fig. 370 S, of about sixteen threads to the inch. The screw works in the edges of a pair of discs R, placed one upon the other upon the same axis; these are cut on their edges with teeth to form worm wheels in which the screw upon the axis of the wheel works. The upper disc has 110 teeth. This therefore moves one revolution by 110 turns of the wheel. It is divided into 110 divisions at its circumference, but is figured 20 yards to 220 yards or 1 furlong, so that each division represents 2 yards, corresponding with the circumference of the wheel, Fig. 369 O. The divisions are read by a point attached to the side of the box shown at the top of the figure. Single yards are shown by the intermediate position of the pointer between the divisions, but single feet may very well be estimated approximately. The lower disc is cut with 111 teeth. The ratio 110 to 111 gives a differential displacement of one tooth only after 110 revolutions of the wheel, or of 220 yards traverse. The two discs take, therefore, by revolution over the surface 220 × 110 = 24,200 yards or 13·5 miles before they return to the same relative position as at starting. This is, therefore, the space this perambulator will traverse without resetting. To enable the lower disc to be read the upper disc is cut away for half the interior circumference of its circle. A part of the upper disc is formed into a point, to read direct from the centre into divisions on the lower disc, in furlongs up to 13½ miles.

The Measuring Box is covered with glass for protection. The box can be taken off by removal of the milled-headed screw at any time to set it back to zero, but in practice it is often found more convenient to spin the wheel round to zero or an even mile of the outer circle, and record differences of reading, if this can be done in the distance within the record of 13 miles of the lower disc. The screw and axis, which are of hard steel, should be occasionally oiled with watch oil to keep the perambulator in good working order.

762.—The reviser has designed a light form of perambulator on the bicycle wheel principle. It is shown at Fig. 371, and is very light and portable. The rim of the wheel is of gun-metal and is usually made two yards in circumference. It is fitted with a counter which denotes two yards to every revolution, and the distance is given in number of yards only. The handle is detachable from the fork for packing, and the whole is contained in a light pine case. The wheel is also made two metres and ten links in circumference.

763.—Pedometer.—Used for roughly ascertaining distances passed over in walking. This ingenious instrument was the invention of William Payne in 1831 (patent No. 6078). It is the size of an ordinary watch, and has a similar face; but between the figures, which indicate miles 1 to 12, there are four divisions only, to indicate quarter miles. The pedometer is slung up by a loop, Fig. 371, H fixed upon the handle, which in use is passed over the edge of the waistcoat pocket so as to keep the instrument in an approximately vertical position.

764.—The Registering Apparatus consists of a pendulum, Fig. 372, P placed horizontally by being supported by a delicate spring L to its highest position, where it rests against a stud. The action of the pendulum is caused by its following the motion of the body in stepping, until stopped by the foot reaching the ground, when the momentum attained by the pendulum carries it from its upper position of rest where it is sprung against the stop to its lower free position, where it is stopped by a screwed adjustable stud S, shown under it. The axis of the pendulum is free upon the axis of the ratchet wheel R. When the pendulum falls, a fine spring, fixed to its upper surface, drops its end into the teeth of the ratchet, moving over two or three teeth, which are held against retrograde motion by the spring pawl D. When the pendulum rises, the ratchet is moved forward the number of teeth that the spring at first slipped over. The ratchet is connected with a pair of geared wheels, not shown, the axis of the second of which forms the axis of the hand. In this manner each oscillation of the pendulum is communicated to the index hand. The ratchet is made with extremely fine teeth, so that by adjustment of the screw stud S a greater or less number of these teeth may be taken by one beat of the pendulum, and thus the mileage rate may be adjusted approximately to the step. This is done, however, very imperfectly, as the variation of the average steps of men amounts to one or two inches, and the difference from the number of teeth taken will scarcely indicate less than three inches in the step.

765.—Passometer.—This instrument was originally invented by the author as an improvement upon the pedometer (1868). The instrument, Fig. 373, is not intended to indicate miles or any distance, it simply counts the number of steps taken. The action is just the same as the pedometer, but the ratchet teeth are larger and less liable to miss a tooth, as it is made to take one tooth only at a single step. The dial arrangement is entirely changed. The steps are numerically indicated by a separate hand which reads into the graduations up to 50 steps upon a small dial. Each revolution of the small hand reads through gearing one division of the central hand, which moves over the complete circumference of the dial, reading up to 25,000 steps. This is the extent of indication. It is necessary in continuation beyond 25,000 steps to take a record of progression per 25,000 where a greater distance is required to be measured.

766.—The average step may be estimated perhaps within 1 or 2 per cent. by training in walking several miles steadily, counting the steps, always remembering that we take shorter steps uphill and when we are tired. But the mean step of the individual under all the different circumstances is the only rule that can be followed.

767.—Sounding Chains used for coast surveys are generally made of iron, but sometimes of brass. They are usually made of 10 fathoms entire length. The links are 1 inch, and the feet are indicated by tellers. The form of teller designed by the author is shown in Fig. 374 for the 3. A leaden weight, similar to that shown Fig. 375, is used upon the end of the chain—of 28 lbs., for ordinary coast work, or heavier if there are strong currents. The chain is contained in a strong wooden box.

A very elaborate apparatus with steel wire line has been made for deep-sea sounding by Lord Kelvin and others; but this subject is beyond the province of the present work.

768.—Sounding Lines, used for survey of shallow coasts and harbours, are made of water-laid line of fine green hemp, about ¾ inch circumference, Fig. 375. White tapes are inserted as tellers at every foot, and red tapes at every fathom. 3 to 6 fathoms are the ordinary lengths employed. If the water is shallow the fathoms are easily counted, but if thought necessary knots may be tied to indicate the number of fathoms on the red tellers. The weight is about 7 lbs. for 50 feet line, about 15 lbs. for 100 feet. The under side of the weight is commonly recessed to take tallow when it is desired to bring up a specimen of the bottom, if this is loose sand or mud.

769.—Coast Survey Lines.—For surveying distances, from point to point of soundings along a coast, lines of fine copper wire rope marked with tellers at 50 and 100 feet are commonly used. The line is generally allowed to rest on the bottom of shallow water, and is floated up by means of attached corks in deep water. It is usually laid and picked up by means of a reel fixed at the stern of the surveying boat. The lengths of line used vary from 1000 to 5000 feet.

770.—Telemeters.—These scarcely enter within the practical limits of surveying instruments, but as several attempts have been made to introduce their use it is necessary to mention them. The general attempt has been to measure a great distance, 1000 feet or more, by means of the angles subtended from the ends of a short base to a distant point. This base in the telemeter of Piazzi Smyth is 60 inches; Colonel Clarke, 72 inches; Otto Struve, 73·5 inches; and Adie, 36 inches. The angles are usually taken upon the principle of the sextant by coincidence of image. Very much greater success has been attained recently by Messrs. Barr and Stroud by means of their range-finder of 54 inches base. The author, as far as his information reaches, is assured that no instrument of the class is satisfactory for surveying purposes. Further, the subject is one to which he has devoted some study, and designed two telemeters.[55] One of these appeared to him for a time satisfactory within certain limits. The base in this instrument was 50 feet, formed of a fine pianoforte wire stretched between two observing telescopes, the tension of the wire directing the one telescope to a right angle, and the other telescope to an arc which read either degrees and minutes or absolute distances in the eye-piece to the direction in which the telescope was pointed. In first trials this instrument was found fairly satisfactory; but subsequently in windy weather the deflection of the wire rendered the action of a pair of instruments quite unreliable.

There are some instruments, as Colonel Gautier's telemeter used in the French army, which depend upon combined reflectors placed normally at 15° to 45°, as in the apomecometer, art. 693, but with a tangent screw to give a small motion of displacement to one mirror which reads on a scale of calculated distances to angle from a certain base measured between two stations of observation. A very similar instrument, invented by Labez, has one reflector only at 45°. These instruments may be useful for measuring approximate distances for range in the army, but can scarcely rank as surveying instruments, the box sextant, art. 664, being in every way a superior telemeter for the purpose when a measured base can be fixed and well-known trigonometrical calculation used.

771.—The simplest and best telemeter for surveying purposes is the subtense telescope, and all good, up-to-date surveying instruments have their telescopes so fitted, but for those who do not carry an instrument with a telescope the reviser has designed a small subtense telemeter, Fig. 376, which consists of a small telescope fitted with subtense points, and mounted in a collar which has vertical and horizontal motions and a centre socket to fit a Jacob's staff. The stadia is set to read 1 in 100. The telescope has rack and pinion focussing and may be revolved in its socket so that the stadia rod may be read held either horizontally or vertically. It is packed in a leather holster case, and a four-fold 10-feet spring-pointed stadia rod is supplied with it divided into feet, tenths, and hundredths.

772.—Hand Rods, although used more generally by building surveyors, are extremely useful also to the civil engineer and land surveyor for town work among buildings and in mines. They are made 5 feet in length, less generally 10 feet. The 5-feet are made of single blades of lancewood or of two jointed to fold. The 10-feet are always jointed and made much stouter than the 5-feet. The 5-feet are generally sold in pairs.

773.—Ordinary 5-feet Rods are divided to every 3 inches, with feet only stamped with numbers, as shown Fig. 377. Where the rod is jointed the best form of folding joint is shown in the figure in section and plan. The spring S is sunk into the face of the rod at the joint on one side, and springs into a groove (housing) in the other side so as to lock the joint when it is either open or closed. The most useful dimension for the rod is 1 inch by 1/6 inch. Rods are nearly always made of lancewood, but they are preferred dyed black for neatness by many surveyors. A pair of rods is usually carried in a cowhide case. They are also often carried in the stem of a walking-stick hollowed out for the purpose. The rod or rods in this case are made much lighter, generally ½ inch by 1/8 inch for a pair of rods, or 7/16 inch by 7/32 inch for a single rod. The single rod is to be preferred in this case for its extra strength.

774.—Fully Divided Rods.—The author has made rods for many years divided to single inches. These measure from both ends—one end direct as Fig. 378 and the other end reversed by turning the rod over as Fig. 379. By this plan the rod gives direct measurement in feet, inches, and parts from either end, and the division is always placed outwards against the work, so that measures may be taken from either end by turning the rod over sideways, without turning it end for end.

775.—Connecting Link for Rods, which weighs only 1 oz. and may be carried loose in the pocket, is often found convenient for measuring heights, as it permits the ends of a pair of rods to be brought together, Fig. 380. By this means the arm will raise the rods about 7 feet, and with 10 feet, the height of the pair of 5-feet rods, this will make 17 feet of measurement. When the 10-feet is set against a wall, its height, if 20 or 30 feet, may be guessed very approximately by standing at a distance from it.

776.—Slip Jointed Rod.—This form is less general, but it is a very convenient form of rod. The jointing is effected by two loops which are fixed to the centre end of one part of the rod in such a manner that the other part may slide through the loops. When the rod is extended to 5 feet there is a stop which prevents further extension, and a spring to keep it at this exact position, Fig. 381. The outside of the rod is divided into feet and inches. The inside is divided so that any addition to the half rod, produced by extending it; may give the measurement from end to end of the rod at this position, thus:—The half rod being 2 feet 7 inches closed, if the loose side be drawn out 19 inches the rod from end to end will be 4 feet 2 inches, which will be indicated by the division and figuring inside the rod. This is very convenient for measuring openings such as doorways or passages.

777.—Brace-piece.—A 10-feet rod is sometimes made with a brace-piece, which folds up inside the rod. This brace-piece is jointed to fix both half rods to 90° when it is desirable to use the rod as a square.

778.—Civil Engineer's Rule is made fourfold in both boxwood and ivory, Fig. 382. The most convenient size is 1 inch wide. Some of the profession prefer them narrow for lightness—¾ inch; and some wide for strength—1¼ inches. This rule is generally well made, with German-silver joints and outside joint-plates. The divisions placed on the rule outside are inches in eighths and tenths; the inside, the ordinary architects' scales, 1/8, ¼, ½, 1, and four chain scales, 20, 30, 40, and 50. A 10 is got by halving the 20; 60, by doubling the 30. A protractor reading to 5° is divided on the head. With silver joints and in fine ivory this rule is often made a presentation instrument.