United States Naval Academy

A Text Book on Surveying, Projections, and Portable Instruments United States Naval Academy

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. 1876 edition. Excerpt: ...the top, or a convenient opening in the side of the box, technically called the reading point Besides the reference of the reading point to the bench mark by a line of levels, it is necessary to record the distance from the zero of the staff, or some other stated division, to the water line of the float, in order that the elevation of the bench above the surface of the sea may be computed for the readings of the gauge; and as the water line of the float may change with the saturation of the staff, or other cause of alteration in weight, this distance should be occasionally measured and recorded. If the holes at the foot of the gauge are too small, or become partially closed by weeds, the ingress and egress of the water will not be free, and the observations become erroneous. On the other hand, if the holes are large, the undulations of the sea will affect the float and staff. The effect of inadequate water holes is to increase the intervals, reduce the ranges, and alter the magnitudes of all the inequ lilies of the recorded tide. 82. To the box gauge is sometimes attached a self-registering apparatus, so arranged that the rising or falling float carries a pencil with which it describes a curve upon paper wrapped about a cylinder revolving by clock work. The Saxton self-registering tide gauge is described in the Coast Survey Report of 1853. In this gauge the motion of the pencil is horizontal, and the paper unrolls from one cylinder and rolls up on another, the receiving cylinder being revolved twice in a day, and a mark being pricked upon the paper at each half hour. The curves on the paper are reducedto figures by the use of scales; and in order to determine the absolute heights represented by the curves it is necessary to refer to a staff gaue...