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The back sights being greater in amount than the forward sights, it is evident that the bench mark at B was higher than the bench mark at A by the difference of the two sums.

LEVELS FOR THE FORMATION OF A SECTION.

Next to the running levels, the most simple case that can occur is, to take the levels of a line of country where the ground plan is already made, and the exact line of section determined upon, and in some instances picketed out. It is then only necessary, in addition to what is required for running levels, that the distance

between the levelling staves, or the whole distance at every station from the starting point, be measured. The instrument should be placed, as usual, as near as can be at an equal distance from each staff; but it is not essential that it be placed in the exact line between them, unless it should happen to prove the most advantageous position. Plate II. represents an example of this kind of work, the survey of the land having been completed, and the plans of the fields, etc. drawn; the strong black line A B was the direction determined upon as the most suitable for a portion of an intended line of railroad, and the section was accordingly taken; a bench mark had been previously agreed upon at each extremity (A and B), from whence other surveyors could take up the levels, and carry them onwards in both directions.

First, a staff was placed on the bench mark at A for a back station, and another staff was held up for a forward station, in the adjoining field, but exactly on the line as marked down on the plan, a copy of which the surveyor had in his possession; the instrument was then set up, as near as could be estimated, or the level of the ground would admit, at an equal distance from each staff, so as to be able to read them both; the adjustment of the instrument for observation, as described at page 35, was carefully attended to; and the reading of the staves noted. As soon as the observations were made, the distance from staff to staff was measured with a Gunter's chain, which completed the first level.

The measurement of the distances can be more con

veniently performed, and with a great saving of time, by two additional assistants, who can be measuring, whilst the surveyor proceeds to direct the man who held the back staff in the last case, to take up a forward station precisely on the line as laid down on the plan. The staff which was the forward station in the last case now becomes the back station, and the instrument must be set up so as to read both stations as before, and as nearly equidistant from them as can be; by the time the instrument is adjusted, and both the staves read off, the assistants would have completed the measurement from the bench mark A to the first forward staff, and be ready to continue on to the second one; whilst this is doing, the instrument and back staff can be carried forward and set up, &c., as before; by a continued repetition of a similar process, the whole line A B was levelled.

The measuring assistant should report to the surveyor the total distance of each forward staff from the bench mark at A as soon as it is determined, or, if thought more convenient, he may keep a book to enter the distances in, which should be ruled in two columns, one for his distances, and the other for references to them; as a, b, c, etc., or the numbers 1, 2, 3, etc., placed opposite; and if the observer makes similar notes in his book to each pair of sights, there can arise no mistake in placing the corrrect distances opposite the corresponding levels, when the measurer makes his return.

The following is a copy of the field-book of the

example given in Plate II.; showing the manner of keeping it, and also the method adopted of reducing the levels to obtain the actual heights of each station, with regard to the starting point, for the purpose of drawing the section; which we shall then explain.

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