636 Mr. 'mann’s ^rea life 
This quantity of three fathoms four feet and eight 
inches of declivity in 20,000 fathoms of extent, gives 
the proportion of the declivity to the extent as 1 to 
5292, which is one line or twelfth part of an inch in 6| 
fathoms, and two feet feven inches in one French league 
of 2283 fathoms. Now the meafured Englilh mile con-- 
taining 5280 feet, this proportion of approaches fo 
very near to one foot of declivity in every meafured mile 
of extent, that I fhall every where reduce what I call the 
mean declivity to that quantity, as a ftandard wherewith 
to compare the reft. 
74. In canals, all whole fluices and vents have been 
kept lliut a fufficient time to render the water ftag- 
nant throughout their whole length, there cannot be al- 
lowed above an inch or two of declivity for each mile in 
length, on account of the water that unavoidably runs 
off through the chinks of the doors of fluices, drains, &c. 
75. According to the obfervations of the AbbeCHAPPE 
d’auteroche (m \ the floor of the Hall of the Royal Ob- 
fervatory at Paris is forty-five fathoms three feet and five 
inches French above the level of the fea at the mouth of 
the Seine. According to the Abbe nollet, this fame 
floor is forty-fix fathom above the level of the Ocean, and 
only forty-five fathoms above the level of the Mediter- 
( m ) See Relation de fon Voyage en Siberie, tom. II. p. 406, 407. 444. 
ranean 
