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When the boiling point at the upper station alone is observed, and for the lower the level of the sea, or the register of a distinct barometer is taken; then the barometric reading had better be converted into feet, by the usual method of subtracting its logarithm from 1-47712 (log. of 30 inches) and multiplying by 6, as the differences in the column of" barometer" vary more rapidly than those in the "feet" column.
Assuming 3000 inches as the average height of the barometer at the level of the sea (which is however too much), the altitude of the upper station is at once obtained by inspection in Table I., correcting for temperature of the stratum of air traversed, by Table II.
In moderate elevations, the difference of one degree in the temperature at which water boils, indicates a change of level of about 500 feet, nearly equivalent to what would be shown by a difference of 0.6 of an inch in a mercurial barometer.
SHADING AND ENGRAVING TOPOGRAPHICAL PLANS.
AFTER all the mechanical portions of the survey, (including the horizontal contours if they have been traced instrumentally,) have been plotted to the required scale; the features of the ground, and any other detail that may have been sketched in the field, are transferred to the original plot for the commencement of the finished plan, supposing one to be required either to be preserved as a drawing, or for the purpose of engraving. This is generally finished with a brush, either in Indian ink or sepia; but a grea want of one general system of topographical plan-drawing is here felt, particularly as regards the method of expressing the features of the ground in a manner at once easy of execution and generally intelligible.
The different disposition of the light affords the means of varying the system of shading hills. Where it is supposed to descend in parallel vertical rays upon the ground, each slope evidently receives less light, or, relatively speaking, more shade, in proportion to its deviation from a horizontal plane, on which the maximum of light falls. Mr. Burr, in his "Practical Surveying," devotes a chapter to the scale of shade to be applied to plans finished on this supposition, which however he candidly acknowledges to be an impracticable theory; but it leads him to the very just conclusion, that hills are generally shaded much too dark to give anything like a natural representation of their various slopes, which defect has also the additional fault of confusing the appearance of the drawing, and impairing the accuracy of the outline. The slopes drawn upon this system have evidently no light or dark sides, which causes a monotonous effect; and yet, on the same plan, both trees and houses are constantly represented with shadows.
SHADING AND ENGRAVING TOPOGRAPHICAL PLANS.
The other system of supposing the light to fall obliquely upon the ground (as in nature), either at one fixed angle or at an angle proportioned to the general character of the slopes*, is decidedly favourable to the talent of an artist; but there are two objections its general adoption in plans of an extended survey: first, the difficulty of execution; and secondly, its ambiguity, even when correctly drawn, except to those accustomed to the style. The slopes directly opposed to the light would evidently receive a greater portion of illumination than the summits of the highest hills; and, in fact, the whole arrangement of the disposition of the shades is quite different from what it would be under a vertical light, as is seen by exposing a model of any portion of ground to a strong light from a partially-closed window. The practice of copying the effects of light and shade from models is the best introduction to this system of shading ground, and is in fact indispensable before attempting to finish a plan +.
The method now most generally practised in topographical plandrawing partakes of both these systems; the light is considered as falling nearly vertical, but sufficiently oblique to allow of a decided light and shade to the slopes of the hills, trees, &c. The hills are shaded, not as they would really appear in nature, but on the conventional system of making the slopes darker in proportion to their steepness; the summits of the highest ranges being left white. This arrangement, though obviously incorrect
* Mr. Burr proposes an angle of about 15° for a flat country, and 40° for mountainous districts; the angle of oblique light ranging between these two extremes according to the nature of the ground.
† Mr. Dawson, whose talents and energy have done so much towards bringing the sketching and shading plans of the Ordnance Survey to the present state of perfection, was the principal advocate of this system of oblique light; and some of the copies, from models of large tracts of country drawn by Mr. Carrington, at the Ordnance Map-office, in the Tower, are hardly to be distinguished from the models themselves, when they are both placed in the proper light.
These and the preceding remarks apply solely to shading with the brush; the methods of delineating slopes by the pen and pencil having been explained in the last chapter. The Ordnance Surveys of the North of England are finished on this system for the engraver, even though the ground may have been instrumentally contoured. These maps, however, are at present engraved upon the same scale as those of the old surveys of the southern counties, 1 inch to 1 mile, though plotted upon that of 6 inches, in order to have the whole maps of England uniform in scale and in execution.
SHADING AND ENGRAVING TOPOGRAPHICAL PLANS. 117
in theory, has the advantage of being generally understood even by those not accustomed to plan-drawing, and is also easy of execution: it is that now adopted in finishing the plans of the Ordnance Survey, and from which the features of the ground are engraved on the vertical system of etching, as being much the easiest, although not so for sketching in the field.
Trials have also been made to render the patent process of engraving by a machine, known by the name of "Anaglyptograph," which answers so beautifully for giving a correct representation of a cast, or basso-relievo, available for topographical designs. A surprising relief is produced by this method of engraving, but it renders the general surface of the plan so dark as to obscure the accuracy of the outline; and as it is necessary that a model should be previously made of the feature to be represented, it is only suited to small portions of irregular ground.
Attempts have likewise been lately made to introduce some system of engraving that may combine as far as it is possible the accuracy of horizontal contours with the effect of etching, which it is hoped will before long be brought into practice.
In finishing detailed plans on a large scale, stone or other permanent buildings are generally coloured red (lake or carmine). Wooden or temporary structures are tinted with a shade of Indian ink. Water is always coloured blue. Where distinctions between public and private buildings or property are required to be shown, different colours must be used and explained by references on the drawing; the same remark applies to the distinction between buildings erected and those only contemplated. The most usual conventional signs have already been alluded to in pages 68 and 69.
THE preceding chapters will, it is believed, be found to contain all necessary information connected with the survey of any tract of country, whatever degree of accuracy or detail may be required; but in a newly-established colony, or one only partially settled, the primary object in view, in commencing an undertaking of this nature, is not the same as in that of a thickly peopled and cultivated country. In the latter case, the surveyor aims at obtaining, by the most approved methods consistent with the time and means at his disposal, data for the formation of a territorial map showing the position and extent of all roads, towns, provinces, counties, and, where the scale is large, parishes, and even the boundaries of property and cultivated or waste land; as well as the features of the surface of the ground, and all natural and artificial divisions, together with the collection of a variety of other useful geological and statistical information. In a new country only the natural lines and features exist;—the rest has all to be created.
The first operations then, required in a perfectly new settlement, are, the division into sections of such size as may be considered best adapted to the wants of settlers, of the land upon which they are to be located; and the marking out the plan of the first town or towns, the sizes and positions of which will of course be regulated by local circumstances and advantages; whilst the first rural sections will naturally be required either in their immediate vicinity, or contiguous to the main lines of communication leading to the different portions of the province, whose local importance is the earliest developed.
In the case of a small settlement established upon the coast of any country, for the immediate reception of settlers who require to