Under almost all the arches there are great numbers of drip-shot piles, or piles driven into the bed of the water-way, to prevent it from being washed away by the fall. These drip-shot piles considerably contract the water-ways, at least ⅙ of their measured breadth, or about 39⅓ feet in the whole.
So that the water-way will be reduced to 196⅔ feet.
Now b = 926; c = 196⅔; v = 3⅙; 4a = 64,3596.
Then 25b ⁄ 21c = 23150 ⁄ 4130 = 5,60532.
And 5,60532² = 31,4196; and 31,4196 - 1 = 30,4196 = (25b ⁄ 21c)² - 1.
Also vv = (19 ⁄ 6)² = 361 ⁄36; And vv⁄4a = 361⁄36 × 64,3596 = 0,15581.
Then 30,4196 × 0,15581 = 4,739 feet, the fall sought after.
By the most exact observations made about the year 1736, the measure of the fall was 4 feet 9 inches.
Example II. For Westminster-Bridge.
Altho’ the breadth of the river at Westminster-bridge is 1220 feet; yet, at the time of the greatest fall, there is water thro’ only the thirteen large arches, which amount to 820 feet: to which adding the breadth of the twelve intermediate piers, equal to 174 feet, gives 994 for the breadth of the river at that time: and the velocity of the water just above bridge (from many experiments) is not greater than 2¼ feet per second.