This section is from the book "Cassell's Cyclopaedia Of Mechanics", by Paul N. Hasluck. Also available from Amazon: Cassell's Cyclopaedia Of Mechanics.
It is supposed that a method is required of taking cross sections of a tidal river. Soundings must be taken from a boat, with a lead plummet having a round plate above to rest on the mud. The position of each section will he marked in turn by two station poles on the same bank, so that the true line can be sighted from the boat, but the position of each sounding must be obtained by observing the bearings to, or angles between, certain fixed and permanent points, as a chimney stack, steeple, tree, house, etc. Two observations at each point will generally be sufficient. By noting the time of sounding and comparing with the time of high and low water, the proportion between the total rise and fall and that which had then occurred will be found. The actual height of each tide must be recorded at a tide gauge on shore as long as the survey lasts. The following is a sample of the entries: -
No. of Section, 5. High water, 2 p.m.
Taking Cross Sections of Large River.
No. of Sounding.
Depth in feet.
Chimney 345. Tree 87
360. „ 74
10. ,. 60
Chimney to tree 520 ft., bearing 104, that is east of magnetic north taken with prismatic compass (mag. var. N. 16 W.). The accompanying figure shows the plotting of these three soundings with the construction lines left in. After the soundings are all corrected they can be entered on the plan, and then sections made in any required direction. When the angle has been taken between the fixed stations instead of their bearings, the method of plotting is different and also more complex. In the present method, if the bearing from the sounding to chimney is 345°, that is north 15° west of the magnetic meridian, or north 15 + 16 = 31° west of the true meridian, the bearing or direction from the chimney to the sounding will be the reverse of this, or south 31" east.