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It is always preferable to make the subtense as large as possible, as the larger it is the more accurate the result will be. All distances set out by this method are base distances, no matter what the difference of level may be, and such figures for divisors should be used as give the gradients below 100. Gradients between 12 and 65 are the best and quickest to work with, and with care more accurate results are obtained than with chaining.
Thus, at one time, a distance may be set out or measured, the difference of level taken, and also the gradient ascertained, and the drum can instantly be set to zero and all ordinary levelling operations continued.
If preferred the gradient drum can be divided to percentage gradients ·001 to 8 instead of ordinary gradients 1 in 12 to 1 in 1,200.
603.—Gradioplane.—This is a new instrument, specially designed by the reviser for very accurate underground surveying, such as is required for large sewage work or water works, long tunnels, or any work requiring a very rigid and accurate instrument, with a very powerful telescope for measuring all horizontal and small vertical angles.
The horizontal circle is 6 inches diameter, and reads by two verniers to 20 seconds of arc, or it is fitted with micrometer microscopes reading to five seconds of arc if desired.
In the former case it carries a floating bevelled aluminium ring compass divided to ¼ degrees, reading by microscope, and in the latter a long trough compass.
Vertical angles are measured by a very accurate form of gradiometer screw carrying a drum with open extended scale in exactly the same manner as the foregoing instrument, and the remarks regarding that and its working apply equally to this instrument. The telescope, which is 14 inches long and carries a 1¾ inch object glass is so mounted that it will revolve in the plane of any inclination set by the gradiometer drum, and is provided with a locking arrangement for fixing it absolutely true for fore or back sight, and it carries a long sensitive spirit bubble to enable it to be used as a most accurate level and for rapid levelling; this may instantly be set level by the drum at any sight without troubling to level the instrument. The diaphragm is fitted for subtense measurements.
A further refinement is fitted to the telescope when desired, by which any grade may be instantly divided into any desired number of parts; this is effected by means of a horizontal circle fitted to the stage under the telescope, which is read by a vernier attached to the telescope. This circle is divided from 0 when the telescope is fixed at zero round each way to 90 degrees into 100 parts, the vernier divided to read 100ths or 10ths of each division. It will be seen that when the telescope is in line with the gradient drum, that is at zero, it will be raised or depressed to whatever grade is indicated upon the drum, and is then capable of being revolved 180 degrees for taking a back sight, when it sights the opposite grade to that which it does when at zero. When it has revolved 90 degrees only the telescope becomes level at any grade, and therefore at any position it is set between zero and 90 degrees it sights a part of that grade; supposing the telescope at zero is set by the gradienter drum at 1 = 1,000 then by revolving the telescope from that to level, it passes over 100 parts of that grade, each of which may be subdivided by the vernier to 100 parts again, consequently 100 × 100 × 1,000 which equals 1 in 10,000,000 or any desired number of 10,000,000ths may be readily set by means of the clamp and tangent fitted to the telescope, or if the grade be set by the gradient drum to 1 = 100 then 100 × 100 × 100 equals 1 in 1,000,000, or any other grade which is set by the gradient drum may, with equal ease, be divided by 10,000 or any other proportion that the horizontal circle vernier may be divided to give.