Synopsis
This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1873 Excerpt: ...at a specified distance from the telescope. When these wires are thus adjusted to include a certain interval, as a foot for instance, upon a rod placed at a distance of 100 feet, it is found that they will cover half a foot at one half that distance, or two feet at a distance of 200 feet, and Bo on in very nearly the same proportion. By this means the distance of the rod from the instrument can be measured or set off, without the use of a chain, and with astonishing accuracy and ease. Indeed, we have been frequently assured that with a powerful telescope, such as we have often placed on our transit instruments, or such as we are now describing, distances can be measured with even greater accuracy than by a chain, especially when the surface of the ground is broken or intereected by deep ravines. The two small screws by which the movable wires are adjusted, have their heads upon the outside of the washers of the cross wire screws, and can thus be moved by the surveyor with a simple screw driver, one being slackened and the other tightened, until the interval between the wires is made precisely as desired, when the little movable ferule is slipped over all, and the wires protected from any derangement. When measurements are to be recorded in chains and links, the wires should be made to cover a foot at a distance of 66 feet, if recorded in feet, they should cover the same interval at a distance of 100 feet. The rod used with the micrometer should be gradaated to feet and decimals of a foot, and provided with two targets, the upper one being fixed at some definite point, while the lower one can be moved as the surveyor requires, the distance between the two targets being accurately read off by the vernier of the movable one. In using the micrometer, the upper ...
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