Practical Geometry.
Fig. ½. Fig. 21. Fig. 22.
7.—BY MEANS OF TWO PICKETS,
to ascertain the height of an object.
[Vide 2nd Plate, Heights, and Distances, [Fig. 1.]]
Let two pickets C D (4 feet), E F (6 feet), be placed with their bases in the line C A passing through A the height required, and move them nearer to, or farther from each other, until the summit B of the object is seen in the same line as D, and F, the tops of the rods. Then, by the principles of similar triangles,
As D H (= C E) : F H :: D G (= C A) : B G.
To which add A G = C D for the whole height A B.
Thus, supposing C E to be 6 feet, F H 2 feet, and C A 150 feet, the proportion will be,
As 6 : 2 :: 150 : 50 feet.
Then 50 + C D will be the altitude required.
DISTANCES.
1.—BY MEANS OF THE SEXTANT,[52]