SURVEYING.
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of the bottom of the roadstead, river, lake, or harbourBy letting fall a perpendicular from every point, and con-necting all the extremities by curve lines in both direc-tions, we may obtain a number of profiles sufficient to giveus an idea of the form of the bottom.
In practice, when it is required to survey any river orroadstead, certain points are selected, both on different sidesof the river and in the direction of its course: a boat is thenrowed from point to point in both directions, describing, infact, the horizontal lines just mentioned, on the surface ofthe water. As the boat moves onward, or frequently stop-ping in its progress for the purpose, a skilful leadsman keepscontinually sounding, or letting fall, by a line and a lead, aperpendicular from the surface of the water to the bottomof the river. The depth is marked at every spot, and anaocurate register kept of every sounding. By laying downthese various depths on paper, making proper allowancefor the rise and fall of the tide, the motion of the boat,and numberless other things which practice teaches, it isplain that we can draw a tolerably correct representation ofthe bottom of a river, or a part of the sea shore, or evenof the great ocean itself, as far as the bottom can be readi-ed by a plumb line. The more accurately we can deter-mine the situation of the floating boat, and the more nu-merous we make the soundings, the more accurate is theknowledge we acquire of the surface of the earth, con-cealed from our view by the water. That knowledge doesnot in fact extend beyond the number of points preciselyascertained; but its importance is very great to the artof navigation. By pursuing the geometrical method herepointed out, we have acquired an accurate knowledge ofthe figure of a large portion of the earth which is continu-ally covered with water. Not only does that knowledgeenable the hardy navigator fearlessly to venture close tothe land, almost at times rubbing the side of his vesselagainst the rocks, or stirring up the mud with her keel,but it enables him, when no land is in view,—when neitherthe sun nor the stars are to be seen, to ascertain, in many
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