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.
Fig. 367.—Two reading and plumbing instruments, C, C; transferring instrument, D; and one of the adjustable intermediate supports, B.
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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.
Fig. 368.—Perambulator.