on Rivers and Canals . 621 
this fuppofition, that the overplus of velocity and eleva- 
tion in the higher current, though it fhould be the leffer, 
•will make the waters in the lower and greater current 
reflow upon themfelves until they come to a level and 
equilibrium with thofe in the upper current; fince 
thefe, by the laws of univerfal gravitation, cannot flow 
back from a lower to a higher level, but mull defcend 
according to the declivity of the furfaces. If the currents 
are of very different Specific gravities , they will come to 
an equilibrium according to the law laid down above (N° 
55); but their greater or leffer quantity and velocity 
will produce little or no effect in this cafe. 
57. Now as the running of two currents in oppofite 
directions, after their rencounter, and beyond the limits 
laid down above (N° 5 5 .), is incompatible with, and con- 
tradictory to, the laws of nature, and confequently im- 
poflible ; we may draw this ufeful conclufion, which be- 
comes important during inundations, and efpeeially 
during the annual overflowing of the low grounds in 
flat countries; to wit, that if the fluices next the fea 
againft which the tide flows be_fhut only a quarter of 
an hour before the flood has rifen to the level of the 
water in the canal, not a drop of fait water can enter 
the faid canal, nor even into the fluice itfelf; becaufe 
both the progreflive motion of the point of rencounter of 
Vol. LXIX. 4M the 
