on Rivers and Canals. 645 
this laft cafe I have found, by 'experience, that by the 
method I here propofe the difference of heights may 
very eafily be found, and that very near to the truth. 
For this end it may be proper to premife a few necef- 
fary coniiderations and precautions to be obferved in 
making ufe of the method I here propofe. They would 
eafily occur to any one who confiders the principles 
whereon it is grounded ; but to fave trouble I ill all put 
them down in a few words. 
81. The firft is, that a particular attention muft be 
had to the quantity of water adtually in the river at the 
time of the operation, fo that according as the greater or 
lefs quantity thereof may augment or diminifh the ve- 
locity of the current, allowance may be made conforma- 
ble thereto in determining the quantity of declivity from 
the degree of velocity. 
2dly, Obferving this precaution throughout the whole 
river, or all that part of it wherein we want to find the 
difference of elevations, we muft next endeavour to de- 
termine, as near as poffible, by the principles laid down' 
in the laft fedtion, all the variations of declivity from the 
variations of velocity zvithin thofe limits , and alfo the 
exaci length and quantity of each. 
3dly, The fame attention muft be had in taking the 
difference of heights by canals, while their fluices and 
Vol. LXIX. 4P 
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