Pascal formulated the law in the 17th century - pressure exerted in a fluid acts equally in all directions.The apparatus is two vertical cylinders joined together with a fluid inside. The fluid moves freely from one cylinder to the other. The cylinders are unequal - one has an area of 1 sq.in. and the other 5 sq.in.. Pistons are placed in the cylinders and with 500lb on the larger and 100lb on the smaller there is balance. The pressure is 100psi.
If the 500lb weight is pushed down 2 ins, the smaller weight is raised 10 ins (2 x5), and vise- versa - this is mechanical gain - the basis of hydraulic jacks etc.
What is not clear from the many diagrams I have seen is if there was (say) no friction present would the 500lb weight rise back and assume its former balance. Again with no friction would the addition of another 500lbs raise the small weight and keep it there. Are we balancing the weight with the weight of the volume of the fluid.

Yes, they will tendback to equilibrium. If you think about a point in the fluid midway between the two pistons, for that pount to be in equilibrium the pressure from both sides must be equal. The pressure is equal to the weight divided by the piston area, PLUS the pressure due to the head of fluid. Now, if you're talking about fluid at 100 psi due to the pistons and weights - compare that to the pressure difference caused by 2 inches of head more on one side than the other, which is only about 0.1 psi. So the equalizing effect is relatively small, but it's there.

Thanks for the rapid reply.
I see that the thing should balance.
With an additional weight added (500+500) is this amount of fluid weight displaced up to balance the added weight.

U need to understand all of this before u look it up

And make sure that u know about the fluid and air difference try fitting a short piece of tubing onto a 5ml syringe and fill both weil water excluding any trapped air connect to a 20ml. Syringe also containing water but with the plunger fully pushed in .

But u should always try these things first right