[ 497 ] 
And — c : b : : v : — v the veloc. per fee. in the 
25 21 C 1 
water-way between the obftacles. . . Pnncip . V. 
Alfo 2 a 1 : vv : 
acquire the vel. v. 
a : — the height fallen to 
4 a ° 
I. & II. 
And 2 
2 S b 
21 c 
z 29 b 
y. vv \ : a : — 
21 d 
X 
— the height 
4 <2 
fallen to acquire the vel. v I. & II. 
J L ’ — — is the meafure of the fall 
Then ^ 
21 c 
X 
4 <2 4 a 
required VII. 
Or ^ — 1 x — is a rule, by which the fall may 
21 c\ 
be readily computed. 
Here a = 16,0899 feet and 4 a = 64,3596. 
Example I. For London- Bridge. 
By the obfervations made by Mr. Labelye in 1 746, 
The breadth of the Thames at London-bridge is 
926 feet; 
The fum of the water-ways at the time oi the 
greateft fall is 236 feet ; 
The mean velocity of the ftream taken at its fur- 
face juft above bridge is 3^ feet per fecond. 
Under almoft all the arches there are great num- 
bers of drip-fhot piles, or piles driven into the bed 
of the wateF-way, to prevent it from being wafhed 
away by the fall. Thefe drip-fhot piles confide- 
rably contrad the water-ways, at leaft £ of their 
meafured breadth, or about 39^ feet in the whole. 
Vo L. 50. S f f So 
