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.
Fig. 366.—One of the two end supports of the band, showing tension weight, with cord running over the ball-bearing pulley.
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