f&zo Mr. mann’s Treatife 
fuch time as the heavier fluid fhall have communicated 
its motion to all the parts of the lighter. But the lighter 
fluid will not lofe its former motion and direction at 
once, but in ■ a decreafing feries, the law whereof will vary 
.according to the greater nr lefs difference of fpecific gravity 
in the two fluids , until the whole of the lighter has ac- 
quired the velocity and direction of the heavier which 
buoys it up. 
The time and fpace required for a greater current of 
fait water to communicate its motion and direction to an 
appofite one of frefh water will be but very little, fince 
they differ in fpecific gravity only - 3 T parts that the fait is 
heavier .than the frefh. It would require match greater 
between water and oil, and ftill much more between 
quickfilver and oil, and fo on. The elements for deter- 
mining them in every cafe might be found by a proper 
number of experiments. 
56. Let the two currents be equal or unequal in 
force and velocity but nearly of the fame fpecific gravity , 
if we fhould fuppofe at the fame time that their furfaces 
are not upon a level, but that the one is higher than the 
other (as is conflantly the cafe in all fluices that open to 
the fea, except at the moment when the furface of the 
tide is upon a level with the furface of the water in the 
canal behind the fluice); this circumftance entirely 
changes both the cafe and the effects. It is certain, on 
this 
