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the Folkstone road; but on account of the curvature of the earth, it was apparently depressed to the same level.

But the effect of the earth's curvature is modified by another cause, arising from optical deception; namely Refraction. An object is never seen by us in its true position, but in the direction of the ray of light which conveys the impression or image of the object to our senses. Now the particles of light, in traversing the atmosphere, are, by the force of superior attraction, refracted or bent continually towards the perpendicular, as they penetrate the lower or denser strata; and consequently they describe a curved track, of which the last portion, or its tangent, indicates the apparent elevated situation of a remote point. This trajectory, suffering almost a regular inflexure, may be considered as very nearly an arc of a circle, which has for its radius seven times the radius of our globe; in consequence of which, the distance at which an object can be seen by the aid of refraction, is to the distance at which it could be seen without that aid, nearly as 14 to 13, the refraction augmenting the distance at which an object can be seen by about a thirteenth of itself. Hence, to correct the error occasioned by refraction, it will only be requisite to diminish the effects of the earth's curvature, or height of the apparent above the true level, by oneseventh of itself. Thus for our example of Dover Castle, of 13.5, or 13.5 1.93 feet nearly, to be substracted from 13.5, which leaves 11.57 feet for the

height of Dover Castle above the level of a ce point on the Folkstone road.

The following Tables show the reduction of the parent to the true level, both for the curvature o earth only, and also for the combined effects of cu ture and refraction. The first gives the correc corresponding to distances expressed in miles, and second for distances in chains.

Table of the Difference of the Apparent and True Lev Distances in Miles.

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Table of the Difference of the Apparent and True Level for

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The correction for distances greater than those given in the latter Table may be computed by the following rule, the same by which the Table itself was computed:

Rule. To the arithmetical complement of the logarithm of the diameter of the earth, or 2.3788603, add double the logarithm of the distance in feet, the sum will be the logarithm of the correction for curvature in feet and decimals; from which if one-seventh of itself be subtracted, the result will be the combined correction for curvature and refraction.

The practice of levelling is one of the most delicate operations that fall within the province of a surveyor, requiring the utmost possible circumspection to avoid the numerous sources of error to which he is liable. More especially, as it is seldom possible for him, after levelling over a long tract of country, to conjecture in what portion of the work his error lies, if he should then find that he had been so unfortunate as to commit any, and, not unfrequently in such cases, sufficient time cannot be spared to go over the ground again; as, for instance, when a section is required within a very limited time to produce before a Parliamentary committee, either to support or oppose any measure submitted to their consideration. We have witnessed an instance where such a committee, during their inquiry into the merits of a certain proposed line of railroad, had brought before them a rival contemplated line with pretensions to great superiority; but it had been so hastily

surveyed, that the learned counsel who had the supporting of the measure, acknowledged, in his opening address, that a trifling error at some unknown part of the line had been detected, which did not exceed fifty feet. We hardly need add, that the rival line was rejected.

The importance of extreme accuracy may also be felt, when it is known that from the section, the engineer has to make his calculations of the quantity of earthwork, in cuttings and embankments, necessary to carry into execution the intended measure, whether of a canal, a railway, or turnpike road, and of course the accuracy of the estimated expense is involved in it; and further, the fitness of the ground itself for such works is determined from the section; that is, whether the inclinations, which the undulations of the ground admit of being introduced, are suitable for the purpose either of a railway or turnpike road. And if the object be the formation of a canal, the section must show what extent of lockage will be required; not only affording a key to the expense, but also the possibility of its execution. We do not throw out these suggestions to alarm the mind of the young beginner, by bringing before him a fearful responsibility, but that he may understand the ultimate object of his labors, and to induce him, by carefulness and attention, to merit that confidence which is sure to be reposed in those who are known to possess such habits.

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